Usually, I have Skype pinned to the taskbar on Windows 7 and its icon prints normally. For a few days, this icon has seemed broken/not found :    If I un-pin the program from the taskbar, the icon shows as usual :    But if I re-pin it (or even simply right-click on it), the icon is broken again.  The program itself doesn't bug for the moment, but I wonder why this happens, and how I could fix this.  EDIT : My question has been marked as a possible duplicate (see comments), but the fixes provided to the other thread don't work for me, which is why I need other methods of fixing this.  UPDATE : After magicandre's and user1167442's answers, the icon now looks like this :    UPDATE 2 : When I right-click on the pinned Skype "icon", then right-click again on the program's name to see its properties (see screenshot) ; when I click on "Change icon", a dialog box says      File not found : %SystemRoot%\Installer{6A0549A9-...-B19}\SkypeIcon.exe.     I also noticed the link's target is joined with the /secondary argument, which is still there even when I erase it and apply.

I have a program pinned to the taskbar. After I upgraded the program (I think) the icon broke. What I get now is what you see below; that ugly default application icon thing.    If I unpin it, I get the correct icon. If I then right-click on it, the icon breaks again. If I pin it again, still the broken icon. Unpin again and back to good icon. Very annoying. How can I fix this?


Consider a Lagrangian $L(q, \dot{q}, t)$ for a single particle. The variation of the Lagrangian is given by: $$\delta L= \frac{\partial L}{\partial q}\delta q + \frac{\partial L}{\partial \dot q}\delta \dot q\tag1$$ When using Hamilton's Principle ($\delta S = 0$): $$\delta S = \delta \left[ \int L(q, \dot q, t) dt \right] =\int \delta [L(q, \dot q, t)] dt = 0\tag2$$ and the right-hand side of equation (1), $$\int \left( \frac{\partial L}{\partial q}\delta q + \frac{\partial L}{\partial \dot q}\delta \dot q \right)dt = 0\tag3$$ We then swap the order of time derivative and variation on the second term in the integrand to get: $$\int \left( \frac{\partial L}{\partial q}\delta q + \frac{\partial L}{\partial \dot q}\frac{d}{dt}(\delta q) \right)dt = 0\tag4$$ Where we then integrate by parts and get the E-L equations. My question is how do we justify trading the order of time differentiation and the variation on q? What is the argument for the identity $\delta \dot q = \frac{d}{dt}(\delta q)$, i.e. to rewrite equation (3) as equation (4).

(Just some recalls)  We have an action on which we want to apply Least action principle.  $$ S=\int_{t_i}^{t_f} L(q,\dot{q},t)dt$$  We assume that $t \mapsto q(t)$ is the function that will extremise the action, thus if we replace :   $$q(t) \rightarrow q(t)+\delta q(t)$$  with $ \delta q(t_i)=\delta q(t_f)=0$  We will have at first order in $\delta q$ the variation of the action that will be zero.  $$ \delta S = \int_{t_i}^{t_f} \frac{\partial L}{\partial q} \delta q + \frac{\partial L}{\partial \dot{q}} \delta \dot{q} = 0  $$  And we say that because : $ \delta \frac{dx}{dt} = \frac{d \delta x}{dt} $, we can integer by part (the boundary term vanish because of (*)): $$ \delta S = \int_{t_i}^{t_f} (\frac{\partial L}{\partial q} - \frac{d}{dt} \frac{\partial L}{\partial \dot{q}} )\delta q  = 0  $$    My question :  It makes sense to me to say that, as :  $$q(t) \rightarrow q(t)+\delta q(t)$$  then by derivation I will have :  $$\dot{q}(t) \rightarrow \dot{q}(t)+\frac{d}{dt} \delta q(t)$$  We immediately find that $ \delta \frac{dx}{dt} = \frac{d \delta x}{dt} $, so it seems to be very general for any transformation.  But my teacher said it could'nt be the case for any transformation. I don't see why ??  What's more, just to understand :  In a Lagrangian, we consider $q(t)$ and $\dot{q}(t)$ as independant variables, because at a given point, if I have a value of a function, its derivative can take any values.  But the functions $q$ and $\dot{q}$ are not independant functions (they are linked by derivative operation).  So, we always have $\delta q(t)$ and $\delta \dot{q}(t)$ that are independant, and to proove that  $ \delta \frac{dx}{dt} = \frac{d \delta x}{dt} $, we use the "trick" that, the functions associated are not independant.  In a sense, we "go" in the function space where functions are not independent and then we go back in "variable" spaces where they are independent.  Am I right ? And why did my teacher said that $ \delta \frac{dx}{dt} = \frac{d \delta x}{dt} $ is not always true, was him wrong?


I don't know what year my old bike is.(like is it a 1970's etc). I think it may be a 1970's, or something like that due to research but I'm still not sure. I know it's a French old make, I believe Ravenna. But what year is this bike?

This is a canonical question that will hopefully encompass all of the questions we get asking us to determine what year a bicycle was manufactured (how old is my bike, how old is my frame, etc.). Each "answer" should address a different way to determine the manufacturing year of a bicycle. To some extent, these will also help you narrow down the model as well as it will tell you what distinguishing features to look at.  The span of answers would include things such as:      Skiptooth cranks Cartridge bottom brackets (BB30/90 etc) Elliptical chainring Quick releases versus contemporary through-axles How the steerer attaches to the fork (quill vs. threadless)  Braking system (especially with mountain bikes: v-brake, direct pull, etc.)   Shifting ramps and pins on cassettes/sprockets and chainrings (versus flat sprockets) Frame material (e.g., shift from tubular steel to use of CF and hydroformed aluminium)  Frame tubing type (e.g. Reynolds 531 tubing introduced around 1934, still used today; Reynolds 501 tubing introduced around 1983). So a frame with an original 501 decal can't be from the 70', 531 decal doesn't help as much because of the range of years produced. Also, see Decals.   *Decals: style/design, amount, placement, color, etc. can help not only determine models, but also year of manufacture. Many changed decal design for the same model bike often yearly, with special/limited editions even more specific. This applies only if decals are original or have been replaced with the identical design. Tubing decals (Reynolds, Columbus, etc.) also changed designs for same tubing made in different years, although the changes aren't made as often, and manufacturers didn't always put tubing Brand decals on every model. Earlier bikes tended to have less fancy, less colorful, less quantity of decals.   Headbadges: most early bikes had actual "badges", often quite detailed and fancy, made of metal, then plastic, and finally just using decals on the headtube. Most bikes lost their metal badges in the 60's and early 70's, although some brands still have actual badges to this day, but in general a real badge indicates an earlier model bike. Dropout style (including the later inclusion of lawyers lips) Presence or absence of CPSC reflectors (USA only) Manufacturing method (shift from lugs to TIG welding, for example)  Number of gears (shift from 2x5 ten-speed to 3x8, 2x9, 1x11, etc.)  Inspection of lug design: pantographs, cut-out, stampings, custom brazing/filing, chrome, fork crow n sweep, etc. Frame/Fork Braze-ons. Race bikes tended have less braze-ons the earlier the year of manufacture. Most 60's racers and earlier had few or none with cable guides/stops, shifters, bidons, etc. being clamp-on. By the 80's if it was clamp-on, it was a sign of very cheap models and most frames had braze-ons for all cables, with multiple bottle and accessory bosses depending on style and use.   and so forth. Please edit this question if you have an answer that isn't in the index above.   A couple of caveats:   A previous owner could have replaced or upgraded components on a bicycle, so this guide only applies to original equipment There is a delay between the introduction of a technology (such as indexed shifting) and when manufacturers start selling products that use it. Low-end manufacturers may continue using technology that is severely outdated (such as one-piece cranks, freewheels, or quill stems) because it is cheaper to do so. A number of bikes are built in a Retro style, but with modern components.  Dutch bikes, trendy coffee cruisers, and beach cruisers may be modern but appear to be 1950s styling.  These tend to have modern rims and brakes as a giveaway.     Note that bicycles.stackexchange does not do valuations of bicycles. How much your bicycle is worth depends on the location of the buyer and seller, how much the buyer wants the bike, etc. etc. etc.


Is it possible in pdo to have a query with a operator like + - * or / ?  The solution is to put spaces between the ? and the operators  this will not work  $table01.Amount - ?*?   This works  $table01.Amount - ? * ?   the query  UPDATE account SET account.Amount = account.Amount - 1000000*0.03 WHERE account.Id = 23;   throws an error but putting this query into the database clients gives me no error  $sqlAccountUpdate="UPDATE $table01                 SET                  $table01.Amount = $table01.Amount - ?*?                 WHERE                 $table01.Id = ?;"; try  { $stmtAccountUpdate = $pdo->prepare($sqlAccountUpdate); $stmtAccountUpdate->execute([$_POST['new_account_initial_deposit'], $deposit_fee, $accountid]); } catch(Exception $e)  { echo 'Exception -> '; var_dump($e->getMessage()); }   the error I get is caused by this this is the line that gives the error:  $table01.Amount - ?*?   I know I can work around it by calculating the values before the prepared statement and using 1 variable instead of doing the calculation in the prepared statement.  this works  $initalFee=$_POST['new_account_initial_deposit']*$deposit_fee; $sqlAccountUpdate="UPDATE $table01                 SET                  $table01.Amount = $table01.Amount - ?                 WHERE                 $table01.Id = ?;"; try  { $stmtAccountUpdate = $pdo->prepare($sqlAccountUpdate); $stmtAccountUpdate->execute([$initalFee, $accountid]); } catch(Exception $e)  { echo 'Exception -> '; var_dump($e->getMessage()); }

Why can't I pass the table name to a prepared PDO statement?  $stmt = $dbh->prepare('SELECT * FROM :table WHERE 1'); if ($stmt->execute(array(':table' => 'users'))) {     var_dump($stmt->fetchAll()); }   Is there another safe way to insert a table name into a SQL query? With safe, I mean that I don't want to do  $sql = "SELECT * FROM $table WHERE 1"


Suppose I have: $$x^2 + y^2 = 1$$ Then, $$d/dx(x^2) + d/dx(y^2) = d/dx(1)$$ $$2x + 2y*y' = 0$$ $$y' = -2x/2y = -x/y$$  What I don't get is when we do $d/dx(y^2)$, where do I get the $y'$ from? In $d/dx(x^2)$, its simply $2x$ ... for $d/dx(y^2)$, why isn't it just $2y$

I want to differentiate   $x^2 + y^2=1$ with respect to $x$.  The answer is $2x +2yy' = 0$.  Can some explain what is implicit differentiation and from where did $y'$ appear ? I can understand that $2x +2yy' = 0$ is a partial derivative but then it becomes multi calc not single.  This is in a chapter about chain rule so I assume there is some use of the chain rule here but I can't see any composite functions here. We can express y in terms of x but y is not composite.  P.S: I am NOT looking for HOW to solve the problem, I am looking for the WHY as stated above.


what type of software do you use for drawing diagrams (user flows) in a faster manner? I tried draw.io as an online tool because I wanted defined shapes in order to make the flow faster. I do not want to use Sketch, Illustrator, Photoshop type of thing because I have to create my own shapes which is time consuming, projects are different from one another.

What tools (ideally affordable ones) would folks recommend for creating a website wireframe? What tools have you used, and did you find them better than pen and paper?


I need to deliver a report using Latex. Since the beginning I have had a lot of difficulties with it, that thankfully were resolved when I started using the Texmaker tool. Unfortunately the references still do not work and I have already wasted too many hours trying to figure it out.  I have a file where I define all my references which is called refbase.bib . As an example, this is one of my entries in it:   @Article{belkin:1990,  title                    = {The cognitive viewpoint in information science},  author                   = {Nicholas J. Belkin},  journal                  = JIS,  year                     = {1990},  pages                    = {11--15},  volume                   = {16} }   For me to call these references to my the reference section of my report, I have the file references.tex and have just the following command:  \bibliography{./refbase}   My sources told me that this should call the bibliography list and make my reference page, however nothing is happening. The file is completely ignored.  Also, whenever I call a citation during the report, I will get this in it's place "(??)". The way I make the citation is as follows (also according to what the source websites tell me to do:  bla blabla blabla bla \cite{belkin:1990}.   Anyone have a clue as to what is going on and how to fix it?  Thank you very much.

I've browsed the forums and found a number of posts that have addressed this issue, but none of the solutions seem to work for me. I have the following script that I just copied from the bibtex home page to get familiar with it. Instead of the citation number I get a question mark. I compile using Latex+Bibtex+Latex+Latex+PDFLatex+ViewPDF just as has been previously suggested and the problem persists.   \documentclass[11pt]{article} \usepackage{cite}  \begin{document}  \title{My Article} \author{Nobody Jr.} \date{Today} \maketitle  Blablabla said Nobody ~\cite{Nobody06}.  \bibliography{mybib} \bibliographystyle{plain} \end{document}   My bibliography (Bib.bbl)  @misc{ Nobody06, author = "Nobody Jr", title = "My Article", year = "2006" }    Looking at previous posts one thing that is concerning is that my .bbl looks empty as shown below. Further, I don't have a .blg  \begin{thebibliography}{}  \end{thebibliography}


I'm currently doing dissertation work which requires me to average 95 pieces of tidal data for a month. I have all my data, but averaging them is turning out to be a tedious pain.   I have 95 points I want to average together, then moving on to the next day's data. So, at the moment, my equations are like such =SUM(D12:D107)/95, then =SUM(D108:205)/95, and so on.   Is there a way to make Excel automatically clump the next 95 pieces of data rather than having to manually select the group of cells? I have to do this for about twenty different locations, so having a quicker way of doing it would be useful!  I know I probably haven't explained it very well...

In column A I have a set of numbers (over 1,000), and I want to get an average of a block of them at a time, (e.g., 10, so A1:A10 then A11:A20 and so on) and write these averages into column B, such that B1 contains the average of the first block, B2 the second block and so on.  Then, in C1 I would like to have a value which defines the number of rows each average should consider (e.g., 10 = A1:A10,A11:A20... and 25 = A1:A25,A26:A50... and so on).  When I change the value of C1 I want column B to update automatically to average using the new block sizes.  How should I go about doing this?


Is there a list somewhere, that explains which questions are appropriate in each of the Stack Exchange sites?  Frequently, I'd ask a question on /// et al., based on the site's similar questions, only to be notified by the site moderators that my question is inappropriate for their site.  How can I learn what can be asked about so that I can ask questions that won't be closed as off-topic?

I frequent Stack Overflow a lot and there are many questions that get downvoted because they don't fit in the "Code only" question format. There are many users who will immediately downvote people for asking the question in the wrong place, but whoe never provide suggestions about WHERE they should be asking that question.  My question is: Is there a good description of what types of questions are acceptable in each of the Stack Exchange family of sites?    I know there is an FAQ, but this really does not address which types of question is fit for each site.


I want to output my circuit from -5..5v into 0..10v. My circuit generate  -5..5v. What should i do to generate that signal? what analog circuit i should use? thanks

I have a front end module that generates an (ECG) signal that varies from +/-2.5 V. I want to shift this signal to 0 - 5V. What is the best way to do this? Would a summing amplifier like the below circuit good enough? With R1 = R2 and V1 = 2.5V, V2 = my signal, V3 = V4 = GND


I have one of those compact mini computers that is just a small box, about 6 inches wide. Lately, it has been making a whirring sound. How can I find out if it is the fan that is going bad, or the hard drive?

On my new computer, my fan(s) is/are plain LOUD! Using SpeedFan, I find that my temps are all under 32 degrees C under my normal use (IDK how accurate that is but the CPU itself says 27 C).  The fan doesn't sound unbalanced and it isn't making sounds in rhythm (Not the usual rrrrrrrRRRRRR... rrrrrrrRRRRRR... rrrrrrrRRRRRR... rrrrrrrRRRRRR), just a constant fan. I am thinking about oiling the fans, but I can't decide if it is the PSU fan or a case fan or the CPU fan. If it's the CPU fan I will most likely just replace that with a quiet fan. How can I tell which fan is making the noise (if not multiple)?  One thing that I am thinking may be the problem is my CPU fan isn't PWM so would swapping that out help? There is no "linear voltage" or etc thing in BIOS so I think it may be running at full speed. Also, I could have too much airflow because it is also making a whistling noise that you can hear close to it that sounds like when I accedentially put part of the side cover over my floor vent to get it out of the way.


I just got an iPhone5S and want to use an mp3 I have in My Music as a ring tone.  How can I do this. The manual only seems to talk about purchasing tones.  EDIT I have made a 40 second m4r file and it is in My Music on the iPhone, but I can't find it in Ringtones.  I cannot find  "Sync Ringtones" in the phone sync options.

The iPhone has a very limited selection of default ringtones.  How do I create a new ringtone for my iPhone?


I was just reading through some comments on  and noticed the comments overlap the right-sidebar links slightly.

On , the comments are overlapping the related questions sidebar. Is this something to do with the weird formatting of the first comment?    Browser: Mozilla/5.0 (Windows NT 6.0; rv:16.0) Gecko/20100101 Firefox/16.0


Assigned to enhance image quality for a (let's say) viewfinder, a developer is puzzled when the display seems to show movement when no one else is in the room.  Figures out that the device is seeing through walls.  Refines device and still gets unexpected imagery.  Discovers that the device can display images from any distance and time in the past.  Realizing the consequences of the technology, he produces a few and goes to elaborate lengths to hide his identity, selling them in consignment shops. Instructions said to point in the desired direction and adjust DISTANCE knob appropriately, but always leave TIME knob set on zero.  The device becomes ubiquitous, displaces many other technologies, eliminates crime; advanced computer tracking coupled with the viewer solves ancient mysteries and reveals the identity of the developer after his death.

When I started reading science fiction in 1987, I read a lot of “best of” collections and believe I found this story in one. To the best of my recollection, the story involves a man who invents a screen to watch history unfold. He watches the jfk assassination and can view all angles and sees a man on the grassy knoll.  As he updates his machine, people use it to watch life unfold in real time. A man sees his wife entering a motel with another man and the husband tracks her down to shoot them.


Let $f_k : \mathbb R \to \mathbb R$ be an equicontinuous function sequence. If for every $k$, $f_k(0)=0$ then the sequence $\langle f_k \rangle$ has a convergent subsequence.  $\langle f_k \rangle$ may not necessarily have a uniformly convergent subsequence (consider the counterexample $f_k(x)= \frac x k$) so the question is whether even in this case there is a pointwise convergent subsequence.

Let $\langle f_n \rangle$ be sequence of equi-continuous real-valued functions on $\mathbb R$ such that $f_n(0)=0$ for every $n$. Does $\langle f_n\rangle$ have a converging subsequence?   I found that even equi-continuity condition and the value at $x=0$ does not guarantee the uniform convergence, as there is a counter-example of $f_n (x) = x/n$, which does not converge uniformly, but do the conditions of the question guarantee pointwise convergence of some subsequence?


I'm doing trap to kill subprocesses of script and it's working perfectly: myscript.sh  trap "kill 0" EXIT  ./someProcessA &  ./someProcessB &  the problem that I don't understand why the following doesn't work if I run from my current shell : tail -F file& kill 0  the tail command doesn't exit , so why kill 0 works only in the script ?

In the man page, it says:  kill [ -s signal | -p ] [ -a ] [ -- ] pid ...  pid... Specify the list of processes that kill should signal.  Each pid can be one of five things:           0      All processes in the current process group are signaled   And I tried like this in bash:  $ man kill & [1] 15247 $ [1]+  Stopped                 man kill $ kill 0 $ ps 15247 pts/41   00:00:00 man   Here 0 is used as pid. As I understood, kill 0 will kill all processes in the current process, which includes pid15247. However, it didn't do anything in this example. Does anyone have ideas about how to use it?


I'm looking for inmate records for Bodmin Gaol jail in the UK circa 1835-1839. My 4th great-grandfather Phillip Stephen Rule (born 1777) was an inmate there based on records I have found on ancestry.com, but all I can find is his release year of 1839. I cannot find when he was imprisoned or what he was in there for to begin with.

In the  there is a record that belongs to my 4th great grandfather William Hobbs.    The Session held at Bodmin names his wife as Jane (nee Courtenay) and also Jane's sister Philippa.  It indicates that William would be spending the next 12 months at  (presumably not the former  in London).     QS/1/7/190, 191   William Hobbs, lab., Philippa Courtenay, spinster,   and Jane, wife of W.H., all of Truro, indicted for assault on William   Jolly: Jane Hobbs acquitted. P.C. fined 1s. paid in court and one   month in house of correction. W.H. fined 1s. paid in court and 12   months in bridewell, and to enter into recognizance of £50 to keep the   peace, particularly to W.J. for 7 years.   Are there prison records available for Bodmin Bridewell for that period (1802-1803)?  Such records seem to be available for 20 years later:


The relevant part of Hexblade's Curse states (XGtE p.55):  Starting at 1st level, you gain the ability to place a baleful curse on someone. As a bonus action, choose one creature you can see within 30 feet of you. The target is cursed for 1 minute. The curse ends early if the target dies, you die, or you are incapacitated. Until the curse ends, you gain the following benefits:  You gain a bonus to damage rolls against the cursed target. The bonus equals your proficiency bonus. [...]   Typically when features add extra damage they clarify that the extra damage only applies to one damage roll, or the added damage just applies to the damage as a whole, like the various cleric subclasses' Potent Spellcasting:  Starting at 8th level, you add your wisdom modifier to the damage you deal with any cleric cantrip.  Or the Evocation Wizard's Empowered Evocation (PHB p.117):  …you can add your Intelligence modifier to one damage roll of any wizard evocation spell you cast.  Does this mean the wording of the Hexblade's Curse would apply to all of the dice rolled for the attack? If when the warlock gains Pact of the Blade at 3rd level, could they pick a great sword as there pact weapon, which deals 2d6 damage, and add their proficiency bonus twice to the damage?

I'm playing a Shadow sorcerer/ and I am rather confused about the Hexblade's Curse feature (Xanathar's Guide to Everything, p. 55). It says, on the first bullet point of the curse details:     You gain a bonus to damage rolls against the cursed target. The bonus   equals your proficiency bonus.   What exactly is a damage roll? Is it the cumulative rolling of all dice, is it a roll of one or more of the same die type, or is it a single roll of a damage die?


In the terminal, how can I show history while typing?  Is this possible? what is the library that I have to install? I think this will boost my productivity while using the terminal,  currently I search commands by pressing up-down arrows, but sometimes it take time to reach the command I want..  Something like this:

I use Ubuntu on a daily basis, recently I've seen that the terminal on Kali Linux behaves differently from the terminal on Ubuntu... I can see the end of the command that I'm typing based on the history of commands that I've typed like the following:  I know I can use autocompletion by pressing tab, but this feature is something else since I can see the command before pressing tab... That's not a Kali related question too since I just want to replicate this feature on Ubuntu 20.04. Is it a feature that I can easily install on Ubuntu by installing some tool with apt install? Or is it a configuration that I need to do?


In people search result while hovering over all result item, in the hover panel in the middle I can see a message "We weren't able to find additional information".  I saw a msdn post for the same question in this   but there it is said that "If the person has no authored documents, then it will display this in the hover panel: We weren't able to find additional information."  But for my case the authored document is also shown.  Please find in the screen shot below marked in red:    Also to mention this not a duplicate question of another question which i have posted earlier - one says hover panel is not shown for first result item and another says in the shown hover panel an error message is shown. Please find the link of the other question -   Did I missed something?

When i key in a search keyword and give a people search.  I am seeing a couple of people results.  But on the first result item i am not able to see the hover panel when i hover over that item.  same is the case when move to other pages of the result. Only for the first item the hover panel is missing.  screen shot when mouse hovered on the first result:  screen shot when mouse hovered over the second result item:   Am i missing something?


I installed tomcat and removed the comments of cgi script in web.xml in conf folder Now iam not able to understand where to place my cgi script  I have read many resources and i placed it in a Webapps/Myfolder/WEB-INF/cgi/hello.cgi here hello.cgi is my sample cgi script I also typed the following in Webapps/Myfolder/WEB-INF/web.xml  `    <web-app>    <servlet>     <servlet-name>cgi</servlet-name>     <servlet-class>org.apache.catalina.servlets.CGIServlet</servlet-class>     <init-param>       <param-name>debug</param-name>       <param-value>0</param-value>     </init-param>     <init-param>       <param-name>cgiPathPrefix</param-name>       <param-value>WEB-INF/cgi</param-value>     </init-param>      <load-on-startup>5</load-on-startup> </servlet>   `  Now iam not understanding where should i map my cgi script here I am not getting any idea what exactly this web.xml does Can any one provide a solution how can i do this.

im trying to run a cgi script (.cgi) with tomcat. I am getting the below error and cant find out whats wrong. I know i should really use apache and mod proxy but this really isnt my area of expertise so im taking the easy way out!  Thanks for any help.   java.io.IOException: Cannot run program "perl" (in directory "C:\Java\tomcat\webapps\my_app_name\WEB-INF\cgi"): CreateProcess error=2, The system cannot find the file specified     java.lang.ProcessBuilder.start(ProcessBuilder.java:459)     java.lang.Runtime.exec(Runtime.java:593)     java.lang.Runtime.exec(Runtime.java:431)     org.apache.catalina.servlets.CGIServlet$CGIRunner.run(CGIServlet.java:1705)     org.apache.catalina.servlets.CGIServlet.doGet(CGIServlet.java:597)     javax.servlet.http.HttpServlet.service(HttpServlet.java:627)     javax.servlet.http.HttpServlet.service(HttpServlet.java:729)     org.tuckey.web.filters.urlrewrite.UrlRewriteFilter.doFilter(UrlRewriteFilter.java:738)     org.apache.struts2.dispatcher.FilterDispatcher.doFilter(FilterDispatcher.java:416)


Who knows a collection with limit and ability to remove old items before add new, if limit is reached?   Old are the entries, which were placed at the beginning.

Please don't say EHCache or OSCache, etc. Assume for purposes of this question that I want to implement my own using just the SDK (learning by doing). Given that the cache will be used in a multithreaded environment, which datastructures would you use? I've already implemented one using  and , but I'm curious if any of the new concurrent collections would be better candidates.  UPDATE: I was just reading through  when I found this nugget:     If you need constant-time access and want to maintain the insertion order, you can't do better than a LinkedHashMap, a truly wonderful data structure. The only way it could possibly be more wonderful is if there were a concurrent version. But alas.   I was thinking almost exactly the same thing before I went with the LinkedHashMap + Collections#synchronizedMap implementation I mentioned above. Nice to know I hadn't just overlooked something.  Based on the answers so far, it sounds like my best bet for a highly concurrent LRU would be to extend  using some of the same logic that LinkedHashMap uses.


It seems that an equation factor must be “squared” as a convenient approximation to balance out some other series of factors invisibly . Might $E=mc^{1.95}$  work as well as $E=mc^2$? Might $A^2+B^2=C^2$ also work if all the factors used the same but different power and be even more accurate? Does anyone know of such a triangle that the current formula comes out to an even number.

I mean, why is $F=ma$? Why not $m^{0.123}$, $a^{1.43}$ or some random non-integers or irrational?  I hope you understand that my question isn't limited just to force, energy, velocity, etc.; it also extends to the area of a square, circle, etc. and all other formulas.  I think the whole thing starts with direct proportionality.  Most of them tell about the area of a circle, $A = πr^2$, where π is 3.14159..... an irrational number! It's not about the constant. I am talking about the power of a physical quantity.  I know why it has pi. It's because we chose the constant for area square to be one. If we chose it to be 1 for the circle then the area of a square will have a constant, 1/pi.   I've edited the question to 'rational exponents' since all are giving me examples of decimal non-integers.


I have a HashMap<Object, Student> where the Object is the ID of the Student, and the Student is an object from Student.  How can I resort the HashMap by the Students name, student->getName()?

I am relatively new to Java, and often find that I need to sort a Map<Key, Value> on the values.  Since the values are not unique, I find myself converting the keySet into an array, and sorting that array through array sort with a custom comparator that sorts on the value associated with the key.  Is there an easier way?


I have been taught that $0.\overline9=1$.  Now if we think of the finite series' in turn, each closer in turn to $1$ than the previous:  0.9  0.99  0.999  etc.  If we called the difference between this series and $1$ e then this difference approaches zero as the series is extended.  Next, I imagine constructing a rectangle of length l and height e.  Clearly the height approaches zero as the length approaches 1.  Would this mean that the limit of a two dimensional shape (a rectangle in this case) is a one dimensional shape (an interval)?  I would have though that for any e the shape would be a rectangle, but that a rectangle of zero width is a line segment.

I'm told by smart people that $$0.999999999\ldots=1$$ and I believe them, but is there a proof that explains why this is?


Quite frequently as a user, I see posts like this:     On Linux systems (either 32- or 64-bit), what is the size of pid_t,   uid_t, and gid_t?   Source:   It could probably be argued that this post does not meet the guidelines; for example, it shows no evidence of research.  Yet this question was useful to me and a number of other people.  I've posted questions like this in the past - some of the questions are well received, some are voted down or flagged for closing.  It seems that some people are too ready to vote down or close a question without really reading the question and trying to understand whether the question may be useful to other people.  It seems that some users have gotten too pedantic with closing and voting down answers?  Maybe a badge is needed for people who try to offer "helpful" comments when they close or vote down a question?

Note: If you're looking for a simple explanation as to why comments aren't required on downvotes, see .   I used to get "upset" (though that is too strong a term) when I got downvoted without comment. If my answer isn't good enough then I'd like to know why. Not only does it improve the answer for the OP but it improves my knowledge too.  Where the down-vote has been explained I've found it useful & it has improved my answer, or forced me to delete the answer if it was totally wrong.  So is there any way we can encourage people to leave a comment? Perhaps they don't lose rep if they explain their down-vote?  I must admit though that I haven't always explained my down-vote either so you could call me a hypocrite. I've also grown a thicker skin over the months of using SO (it seems to have come with the higher rep score ;-)), so I'm less bothered about this now.


How to prove that one of $2,3,6$ is a square modulo every prime $p$?  I am thinking in terms of quadratic reciprocity but not getting any clue.

To find when $6$ is a square I divided $6$ in to $2$ and $3$ by The Legendre symbol.  $$\frac{6}{p} = \frac{2}{p} \times \frac{3}{p}$$  I know that we can multiply the cases of when $2$ and $$ are squares because then it is (1)(1) by Legendre.   $2$ is a square when $p \equiv 1 \pmod{8}$   $3$ is a square when $p \equiv 1 \pmod{12}$  so $6$ is a square when $p \equiv 1 \pmod {24}$ ($96$, but the $lcm(8,12)= 24$)   but how do I find the cases when $2$ and $3$ are both NOT squares? because then $6$ would be a square then as well, $(-1)(-1) = 1$ by Legendre.


When we do su John I switch as user John. When we do su - John I switch as user John having user John's environment. As far as I can see the first option is useless. What is the use of switching to another user and not have that user's environment? Am I wrong? What is the difference in use cases between these 2 options?

I don't understand why su - is preferred over su to login as root.


I understand what negative velocity means, but how does negative velocity and negative acceleration works together. Although it depends on frame of reference, how negative acceleration with negative velocity make it more negative. (I understand the math but want an intuitive explanation.)

Why is speed and acceleration negative when $V_1$ of an object is say 150m/s, $V_2$ is 0 m/s and $\Delta d=0.50\,\rm m$? I found the time it takes which is 0.0033s and the acceleration to be 90909.09 m/s$^2$.   My questions:   What do negative velocity and negative acceleration mean?  Is it negative because it is slowing down? Why is the acceleration so big? Is that even reasonable?


I'm using PostgreSQL 11.3 (compiled by Visual C++ build 1914, 64-bit) and I want to access a table in another Postgres database using postgres_fdw but since the original table has an SP-GiST index on a geometry column, the query on the foreign table fails with "ERROR:  cache lookup failed for type 0", even if the table is empty.    Does anyone knows how to query the foreign table without throwing the error and still keeping the SP-GiST index on the remote table?  Here's a sample SQL to reproduce the error:     Create remote database:  CREATE DATABASE remote     WITH      OWNER = postgres     ENCODING = 'UTF8'     LC_COLLATE = 'English_United States.1252'     LC_CTYPE = 'English_United States.1252'     TABLESPACE = pg_default     CONNECTION LIMIT = -1;  Create local database:  CREATE DATABASE localdb     WITH      OWNER = postgres     ENCODING = 'UTF8'     LC_COLLATE = 'English_United States.1252'     LC_CTYPE = 'English_United States.1252'     TABLESPACE = pg_default     CONNECTION LIMIT = -1;  Setup remote database (connect to 'remote' database before running the following SQL):  CREATE EXTENSION postgis; CREATE EXTENSION postgres_fdw;  CREATE SCHEMA remote;  CREATE TABLE remote.locations (     id bigserial NOT NULL UNIQUE,     name character varying,     lonlat geometry NOT NULL,     PRIMARY KEY (id) )  WITH (     OIDS = FALSE ); CREATE INDEX remote_locations_lonlat_idx     ON remote.locations USING spgist     (lonlat);  Setup local database (connect to 'localdb' database before running the following SQL):  CREATE EXTENSION postgis; CREATE EXTENSION postgres_fdw;  CREATE SCHEMA localdb;  CREATE SERVER remote FOREIGN DATA WRAPPER postgres_fdw OPTIONS (host 'localhost', dbname 'remote', port '5432', extensions 'postgis'); CREATE USER MAPPING FOR postgres SERVER remote OPTIONS (user 'postgres', password 'YOUR_POSTGRES_USER_PASSWORD');  IMPORT FOREIGN SCHEMA remote FROM SERVER remote INTO localdb OPTIONS (import_not_null 'true');  Querying the foreign table...  SELECT * from localdb.locations;  ...results in:  ERROR:  cache lookup failed for type 0 CONTEXT:  remote SQL command: SELECT id, name, lonlat FROM remote.locations SQL state: XX000

I've been noticing something like this popping up every 15 seconds in my server logs:  2020-01-30 21:10:30 UTC::@:[24969]:ERROR: XX000: cache lookup failed for type 0 2020-01-30 21:10:30 UTC::@:[24969]:CONTEXT: automatic vacuum of table "myschema.mytable" 2020-01-30 21:10:30 UTC::@:[24969]:LOCATION: get_typlenbyval, lsyscache.c:2036   When I manually run a VACUUM ANALYZE myschema.mytable; it runs without any error, and the error in my logs goes away. However, inevitably, it returns again at some point.   The table in question is defined as follows:  tfs_dev=> \d myschema.mytable;                                                    Table "myschema.mytable"    Column    |   Type    | Collation | Nullable |                                  Default -------------+-----------+-----------+----------+---------------------------------------------------------------------------  level       | text      |           | not null |  id          | integer   |           | not null | nextval('myschema.mytable_id_seq'::regclass)  text_id     | text      |           |          |  name        | text      |           |          |  geom        | geometry  |           |          |  valid_dates | tstzrange |           |          | tstzrange(NULL::timestamp with time zone, NULL::timestamp with time zone)  adjacent    | integer[] |           |          |  valence     | integer   |           |          |  color       | smallint  |           |          | 0 Indexes:     "mytable_pkey" PRIMARY KEY, btree (level, id)     "mytable_text_id_key" UNIQUE CONSTRAINT, btree (text_id)     "mytable_gix" gist (level, geom, id, text_id, name)     "mytable_spgix" spgist (geom)     "mytable_text_id_ix" btree (text_id, level, id, name) Referenced by:     TABLE "otherschema.othertable" CONSTRAINT "othertable_mytable_fk" FOREIGN KEY (level, level_id) REFERENCES myschema.mytable(level, id)     TABLE "otherschema.othertable2" CONSTRAINT "othertable2_mytable_fk" FOREIGN KEY (level, level_id) REFERENCES myschema.mytable(level, id)   Many other posts with a similar error seem to be about a foreign data wrapper, which is present in this database, but not with this table/schema.


Suppose that the discrete random variable X can only assume values in the interval [0,1]. Among all such random variables, which has the largest variance?

How can I prove that a random variable taking values in $[0,1]$ has variance no larger than $\frac{1}{4}$? If it matters, discrete and continuous proofs are both welcome.


I am assuming that there is a maximum level in Fallout 4 and because taking perks is tied to leveling, there is a presumable maximum number of perks that you can take.  Is there a maximum level in Fallout 4?  And by extension, is there a maximum number of perks that you can take?

Building a character in Fallout 4 gives you 28 points in total to assign to your SPECIAL attributes during the initial character build - compared to 40 points in Fallout 3/ Fallout New Vegas.  In Fallout New Vegas you could increase your SPECIAL attributes over the course of the game via temporary buffs but you could also permanently increase your attributes using perks and implants. Using all available perks and implants there was a maximum of 61 SPECIAL points available (reference:  )  I realise it's day one and we've all got a long way to go but hopefully someone else can answer this question before I can:  How many permanent SPECIAL points are available in Fallout 4?


My Dockerfile looks like this:  FROM php:5-apache  RUN docker-php-source extract \     && docker-php-ext-install mysql mysqli pdo pdo_mysql \     && docker-php-source delete   Unfortunately, when I click on a cgi file I see the actual cgi script rather than the HTML file inside the browser:   #!/usr/bin/perl -w print "Content-type: text/html\n\n"; ...    How is it possible to configure cgi?  Thank you in advance,

I would like to configure Apache 2 running on Kubuntu to execute Perl CGI scripts. I've tried some steps that I came across by googling, but nothing seems to work.  What is the right way of achieving this?


After fixing this error:  I received  the error:  Cannot evaluate expression because the current thread is in a stack overflow state. Here is where I initially get the error (As you can see, it's the same line as the first error):    UPDATED: When I add the ignore statement to my CreateMap<>(); declaration:   CreateMap<Pages, PagesViewModel>()             .ForMember(m => m.PageTypes, x => x.Ignore())             .ForMember(m => m.SiteMap, x => x.Ignore())             .ForMember(m => m.Row, x => x.Ignore())             .ForMember(m => m.Tracks, x => x.Ignore());   I received this message: System.Reflection.TargetParameterCountException: Parameter count mismatch. So i removed the Ignore statement and the CreateMaps are as follow:  CreateMap<Pages, PagesViewModel>();  CreateMap<PagesViewModel, Pages>();   the error does one of the following (all three have happened and hard to reproduce a specific one):   The first one is the application just keeps running and eventually times out.   The program '[5256] iisexpress.exe' has exited with code -2147023895 (0x800703e9).  moves on to another part of the code (my PagesViewModel)   (UPDATED):  private string PagesURL; [Required] [Display(Name = "Page URL")] [DataType(DataType.Url)] [StringLength(100, ErrorMessage = "The {0} must be at least {2} characters long and no longer than {1} characters.", MinimumLength = 2)] public string pagesURL {     get { return PagesURL; }     set { PagesURL = HttpUtility.UrlEncode(value); } }   When this third one comes up, I get the same error: Cannot evaluate expression because the current thread is in a stack overflow state.  I have tried separating out the code to see where and why it's doing this by trying the following:   List<Pages> getPages = db.Pages.ToList(); List<Pages> getPagesOrdered = getPages.OrderBy(o => o.pageOrder).ToList(); List<PagesViewModel> convertPages = new List<PagesViewModel>(); foreach (Pages item in getPages) {     PagesViewModel pagesViewModel = new PagesViewModel();      pagesViewModel.pageDescription = item.pageDescription;     //...loops through all other properties      convertPages.Add(pagesViewModel); }   It gets all Pages from the database context, orders it and declared the PagesViewModel fine BUT when it goes inside the for loop, it declares the pagesViewModel variable fine and then right after it, before it even jumps to the pagesViewModel.pageDescription = item.pageDescription; line the application crashed The program '[5176] iisexpress.exe' has exited with code 0 (0x0).  MrChief, in the answer I linked at the top, suggested it is how my models connect with each other that is causing this issue. For example, PagesViewModel connects to public virtual PageTypeViewModel PageTypes { get; set; } and PageTypesViewModel connects back to PagesViewModel public ICollection<PagesViewModel> Page { get; set; }  My model requires those be connected and it works fine without auto mapper (with the exception of me trying to set each variable manually which cause the application to crash). I'm stuck and would appreciate some insight.     UPDATE:  After going here:  a co-worker of mine was able to get us the symbols for AutoMapper so we can trace into that source code. We added a trace breakpoint to output what was actually causing the issue. Here is what it output a few hundred times:  From "Pages"."PageTypes" to "PagesViewModel"."PageTypes"  So MrChief's prediction was correct. Now how to fix this, I don't know.   This is where the PageViewModel links to the PageTypes view model: public virtual PageTypeViewModel PageTypes { get; set; } and then in PageTypesViewModel is links to PageViewModel: public ICollection<PagesViewModel> Page { get; set; }  This is how EF6 generated my model. I went about this doing model first.   I might have forgotten to include some important information so let me know if there is anything missing. Thank you in advance for the help.

I know java and would normally put in getter/setter methods. I am interested in doing it in C# with the following code, but it throws a StackOverflow exception. What am I doing wrong?  Calling Code  c.firstName = "a";   Property Code    public String firstName; {     get     {         return firstName;     }     set     {         firstName = value;     } }


I have 2 questions regarding security issues related to initialization vectors used in AES CBC   Consider that I'm using a sequence number (which increments with every encryption) as an initialization vector. Would this be a security issue? Is there some way that an attacker who knows the sequence numbers and in turn knows the IV, be able to decrypt even some part of the data, if not the whole?    I'm asking this question because some of the comments on the 1st answer in the following link seem to suggest that using a counter for an IV would not be a good choice.   If it is possible to perform such an attack, can you provide an explanation of how this would be possible? I tried searching. But, none of the sources I found seem to answer why the IV cannot take values from a counter. They only talk about safety in terms of storage.   The other question is related to transmission of IVs. I searched quite a bit and everyone says that the IV need not be secret. But, if IV is exposed, isn't it possible for a man in the middle to change the IV so that the end result at the decryption end will fail?

I just learned that using CBC encryption with an IV which is predictable is not secure.  From what I understand, using certain plain texts, and then guessing the IV that it uses, the attacker can verify if the IV he guessed was right. How does this qualify as an attack?


This is a pretty easy question, but I can't quite figure out the answer. I have:   The initial position of a projectile. The gravity vector. The (stationary) target I want to hit. A maximum launch speed of the projectile.   I want to calculate the initial velocity the projectile must be launched at to hit the target, assuming it follows a ballistic trajectory.

If I have a point which I want to hit at the end or during a parabolic arc, how would I calculate the needed x and y velocity?


I would be mostly obliged if you can help me:  I would like to insert a graph like this:     In my case, I need 3 inputs, 4 hidden layers and 2 outputs.  I know that there are similar topics, but I do not understand how to modify the code.   Thank you.  EDIT  For example,  \documentclass{article} \usepackage{tikz} \usetikzlibrary{matrix,chains,positioning,decorations.pathreplacing,arrows}  \begin{document}      \begin{figure}[htp]     \centering     \begin{tikzpicture}[ plain/.style={     draw=none,     fill=none, }, net/.style={     matrix of nodes,     nodes={         draw,         circle,         inner sep=8.5pt     },     nodes in empty cells,     column sep=0.6cm,     row sep=-11pt }, >=latex ] \matrix[net] (mat) {     |[plain]| \parbox{1cm}{\centering Input\\layer} & |[plain]| \parbox{1cm}{\centering Hidden\\layer} & |[plain]| \parbox{1cm}{\centering Output\\layer} \\     & |[plain]| \\     |[plain]| & \\     & |[plain]| \\     |[plain]| & |[plain]| \\     & & \\     |[plain]| & |[plain]| \\     & |[plain]| \\     |[plain]| & \\     & |[plain]| \\ }; \foreach \ai [count=\mi ]in {2,4,...,10} \draw[<-] (mat-\ai-1) -- node[above] {I\mi} +(-1cm,0); \foreach \ai in {2,4,...,10} {\foreach \aii in {3,6,9}     \draw[->] (mat-\ai-1) -- (mat-\aii-2); } \foreach \ai in {3,6,9} \draw[->] (mat-\ai-2) -- (mat-6-3); \draw[->] (mat-6-3) -- node[above] {O1} +(1cm,0); \end{tikzpicture}  \end{figure} \end{document}   I do not understand how to add or remove units. Most importantly, what does \ai or \aii stand for?

I need draw some picture, but I don't know how do that.   First, the simple graph with basic shapes, arrow,....   And then, draw a picture with math symbol or equation in math environment. Like that:   Please help me draw two picture. And if you don't mind, please show me some tools which can help me draw easily and generate to latex code, or some document help me learnt quickly. Thanks for your help


If $90! = (90)(89)(88)...(2)(1)$,  then what is the exponent of the highest power of $2$ which will divide $90!$ ?   How would I apply one of the easiest method from ?  I need help on applying the link to this question.   I do not understand which one explains my case, and how I can solve using the method.  I would appreciate if someone showed how it is applied to this

How does one find the highest power of a prime $p$ that divides $N!$ and other related products?  Related question: How many zeros are there at the end of $N!$?    This is being done to reduce abstract duplicates. See   and  for more details.


We are on our Honeymoon for 6 months.  We spent two months in South America and then came to the states.  Our 90 days is nearly up and after it is;  we are planning on spending the last three weeks of our honeymoon in Vancouver Canada.  The problem is that our flights home go back through America.  So we have spent 90 days in the states under the Visa Waiver Program. Then three weeks in Canada; and we will have to come back through America to get home. Our problem is that America seems to count going to Canada as part of the 90 days:  From the CBP website:  When traveling to the U.S. with the approved ESTA, you may only stay for up to 90 days at a time - and there should be a reasonable amount of time between visits so that the CBP Officer does not think you are trying to live here. There is no set requirement for how long you must wait between visits.  We only want to come back to the States for 1 day in transit to get our flight home but are worried that the border patrol won't let us back in because we have only been to Canada and not back to our country of residence.   Do you think this is a possibility?  Will they let us back in if it is just to fly home?  We have tried to change our flights to avoid coming back through the States but have been unable to do so.

According to numerous online reports (e.g. ), a VWP national who enters the US by air solely in transit to Canada or Mexico, and then spends over 90 days in Canada/Mexico, unless a resident of Canada/Mexico, will be in violation of the VWP, refused re-entry to the US and barred from future travel under the VWP. Is this really correct? It would imply a VWP national wishing to spend e.g. 4 months in Mexico would need a tourist/transit visa for the US if both the trip to and from Mexico takes place with a flight connection in the US. This would frankly be ridiculous. My understanding is instead that stays in Canada/Mexico do not count towards the VWP admission duration on an automatic, strictly binding basis (as the reports suggest); rather, if the stay in Canada/Mexico is sufficiently short that it can reasonably be considered a mere side trip from the US, the CBP is likely not to grant a new 90-day admission period, but only the remainder of the original 90 days.


Although group of order $p^2$ are well known, the rings of order $p^2$ may not be so well known; I was feeling that there could be more than two rings of order $p^2$. I have two questions related to this, and I don't know whether the questions I am posing are trivial. $\mathbb{Z}_{p^2}$ and $\mathbb{Z}_p\oplus\mathbb{Z}_p$ are obvious examples of rings (with unity) of order $p^2$.  Question 1. Are there more than two rings (with unity) of order $p^2$? Question 2. If there are more than two rings of order $p^2$ (with unity), is the group of units of these rings known?

I'm trying to classify unital commutative rings of order $p^2$, where $p$ is a prime. At first, I happened to neglect the 'unital' and 'commutative' requirements, and after an arduous route I managed to show that there are $11$ such rings up to isomorphism. Some are non-commutative and non-unital, and the journey to that result is a bit ugly for a class on commutative algebra.  This should be easier if we only consider unital rings of order $p^2$. (It's easy to show that if it's unital, then it's commutative in this case). But so far, I haven't found a solution that doesn't use the fact that I found representations for the $11$ rings.  I am certain that a much less technical and messy method to find unital commutative rings of order $p^2$ is possible. For reference on the context, this question comes amidst a review of the Chinese Remainder Theorem, the Structure Theorem on Modules over a PID, tensor products, and algebras. I heavily suspect that if I were more fluent in applying the CRT, I would be able to get there.  Do you have any ideas?  By the way, I believe there are $4$. They should look like $\mathbb{F}_{p^2}, \mathbb{Z}_{p^2}, \mathbb{Z}_p [x]/(x^2),$ and (I don't know a convenient name for the fourth, but e.g., the Klein 4-group with standard ring structure on top, which I'm inclined to designate $\mathbb{Z}_{p \times p}$). In a more convenient designation, 1 is built on the additive group $C_{p^2}$ and 3 are built on $C_p \times C_p$. I would be content if I could enumerate how many rings are on each additive group, up to homomorphism, instead of actually classifying them.


Let $a,b\in G$, with finite $G$. Assume the order of element $|a|$, $|b|$ is finite, then what i want to know is $|ab|$ finite?    First what i know is if $ab=ba$, $i.e$, $G$ is abelian, $|ab|$ is finite.   For $(|a|,|b|)=1$, $|ab| = |a| |b|$, and general case i notice that  $|ab| = \textrm{lcm}(|a|,|b|) = \frac{|a||b|}{gcd(a,b)}$    I want to relax this by neglecting abelian condition.  Then is $|ab|$ finite?, If so how one can prove this?

Let $G$ be a group and let $a,b$ be two elements of $G$. What can we say about the order of their product $ab$?   says "not much":     There is no general formula relating the order of a product $ab$ to the orders of $a$ and $b$. In fact, it is possible that both $a$ and $b$ have finite order while $ab$ has infinite order, or that both $a$ and $b$ have infinite order while $ab$ has finite order.   On the other hand no examples are provided. $(\mathbb{Z},+), 1$ and $-1$ give an example of elements of infinite order with product of finite order. I can't think of any example of the other kind! So:   What's an example of a group $G$ and two elements $a,b$ both of finite order such that their product has infinite order?     Wikipedia then states:     If $ab = ba$, we can at least say that $\mathrm{ord}(ab)$ divides $\mathrm{lcm}(\mathrm{ord}(a), \mathrm{ord}(b))$   which is easy to prove, but not very effective. So:   What are some similar results about the order of a product, perhaps with some additional hypotheses?


Whenever I write 23842899791470069 number into my browser's console it becomes 23842899791470068. Observed this in chrome and firefox. Can anyone confirm and explain the reason why is that so. Thanks in advance.

Is this defined by the language? Is there a defined maximum? Is it different in different browsers?


It is well-known that there is "no" nowhere vanishing continuous tangent vector field on $S^2$, by the so-called Hairy-ball theorem. But then, is there a continuous tangent vector field on $S^2$ which vanishes at only one point?

I need to find a vector field as described in the title.  I was given a couple hints, and this is what I have so far. Let $\varphi:S^2\setminus\{N\}\to\mathbb{R}^2$ be stereographic projection ($N$ is the north pole). Let $Y$ be a smooth, non-zero vector field on $\mathbb{R}^2$ that dies off to $0$ "at infinity." Since $\varphi$ is a diffeomorphism, we can push $Y$ forward along $\varphi^{-1}$ to get a smooth vector field $(\varphi^{-1})_*Y_{\varphi(p)}$ on $S^2$. But this vector field is not zero at the north pole... it's undefined. So can I just define my vector field on $S^2$, call it Z, to be the pushforward of $Y$ (as above) away from $N$ but $0$ at $N$? And if so, do we still have smoothness?


Let $G×M→G$ be a properly discontinuous action of a group $G$ on a differentiable manifold $M$. Prove that the manifold $M/G$ is orientable if and only if there exists an orientation of $M$ that is preserved by all the diffeomorphisms of $G$. (Each group element $g∈G$ gives a diffeomorphism of M, namely the map $\phi_g:M→M$ sending $x∈M$ to $g⋅x$. These are the diffeomorphism the exercise is talking about) My attempt: since the action is properly discontinous it induces a smooth manifold structure on $M/G$.Let $π:M→M/G$ be the quotient map. If $M/G$ is orientable, then I can choose an oriented atlas $(U_i, x_i)$ on $M/G$. $π^{ -1}(U_i)$ is a cover of M. And here I'm stucked.

I'm trying to solve the following exercise in Lee's book.  Suppose M is a connected, oriented smooth manifold and Γ is a discrete group acting freely and properly on M. We say the action is orientation-preserving if for each γ ∈ Γ, the diﬀeomorphism $x\rightarrow γx$ is orientation-preserving. Show that M/Γ is orientable if and only if Γ is orientation-preserving.  Assuming $\Gamma $ to be orientation-preserving, I tried taking a commutative diagram taking an orientation of M to one of M$/\Gamma$ and vice-versa and tried using local diffeomorphism arguments to prove the orientability of M$/\Gamma$. However, I can't make this proof explicit. Nor can I proceed for the other direction.   Can someone please provide a proof for both if and only if parts? Thanks in advance.


I want to change the colors of root@MyComputer:~#and when i type the ls command show me the result as colors!  I write a simple BB code to display what i want:  [color=red]root@MyComputer:[/color][color=blue]~[/color]#    I had this command but it does not apply whan open a new terminal window  PS1='${debian_chroot:+($debian_chroot)}\[\033[01;31m\]\u@\h\:\[\033[01;34m\]\w\$ '   thank you in advance

My current prompt is colorized and edited as so -  #!/bin/bash  # GIT Prompt help if tput setaf 1 &> /dev/null; then     tput sgr0; # reset colors     bold=$(tput bold);     reset=$(tput sgr0);     txtund=$(tput sgr 0 1);     black=$(tput setaf 0);     blue=$(tput setaf 33);     cyan=$(tput setaf 37);     green=$(tput setaf 64);     orange=$(tput setaf 166);     purple=$(tput setaf 125);     red=$(tput setaf 124);     violet=$(tput setaf 61);     white=$(tput setaf 15);     yellow=$(tput setaf 136); else     bold='';     reset="\e[0m";     black="\e[1;30m";     blue="\e[1;34m";     cyan="\e[1;36m";     green="\e[1;32m";     orange="\e[1;33m";     purple="\e[1;35m";     red="\e[1;31m";     violet="\e[1;35m";     white="\e[1;37m";     yellow="\e[1;33m"; fi; ORIG=$PS1 HOST=$HOSTNAME PS1="\[${txtund}${green}\]${HOST}\[\[${reset}\]"; PS1+="\$(prompt_git \"\[${white}\] on \[${violet}\]\")"; PS1+="\[${reset}\]"; PS1+="\[ at - ${orange}\W\]"; PS1+="\[${reset}\]"; PS1+="\[ - \u \]"; PS1+="\n\$ ";   I know that the line  PS1+="[ - \u ]"; will show me my current user. However, I want that section to be red if it is root. All other users should be the default color of gray. Is there a way to change the color in that section based off of current user or should I just declare a variable and use an IF statement to insert that section or a modified section with red as a color?  My expected output is gray named user for all normal users. Root should be red. This is BASH.


I'm looking at selling my MacBook for parts.  It's completely broken: it won't turn on or charge.  Is it possible to obtain files from it? If so, what security measures should I be taking?  Should I be removing the hard drive or do I not have to worry about it?

My Mac won't turn on but I'd like to sell it for spare parts. How can I make sure there's no personal information stored on the hard drive for the buyer to retrieve?


I have a landing page that I'm wanting to get my logo centered. Struggling to figure how to get it exactly in the center and stay there on device scale. I have got the video background to scale accordingly, but I cannot get my logo to go exactly into the center.  Here is a fiddle   shows a black box and in the square I'm wanting to get centered within the landing div.   This is the specific div needing to be sorted out.  .main_logo{     background:#000;     position:relative;     float:left;     width:300px;     height:200px;     z-index:2000;     display: block;     margin: auto; }

I have a div 200 x 200 px. I want to place a 50 x 50 px image right in the middle of the div.   How can it be done?  I am able to get it centered horizontally by using text-align: center for the div. But vertical alignment is the issue..


I have this question:  Let $\omega_1$ the least uncountable cardinal, and for all $n \in \omega$, $n \geq 1$. Let $\omega_{n+1}$ the least cardianal greater than $\omega_n$. Show that $$\bigcup_{n \in \omega}\omega_n$$ is a cardinal.  My try: Let $f_n : \omega_n \rightarrow A_n $ a bijectionfor all $n \in \omega$. And let  $$F_n :\bigcup_{n \in \omega}\omega_n \rightarrow \bigcup_{n \in \omega}A_n$$. $F(x) = f_n(x)$, where $n = min\{ n: x \in A_n\}$. An d prove that $F$ is a bijection. But I'm not have sucess.

I have to prove the following:  Let $\omega_1$ be the least uncountable cardinal and, for each $n \in \omega$ (here, $\omega$ is the set of natural numbers denoted as an ordinal/cardinal), $n\ge1$, let $\omega_{n+1}$ be the least cardinal that is greater than $\omega_n$. Show that $\bigcup_{n\in\omega}\omega_n$ is a cardinal.  I suppose the proof has something to do with transfinite induction/recursion, but I don't know exactly how I'm supposed to use it. Any thoughts?


I am working on a development where I should invoke my testcases through the Method Reflect API. Currently, I am able to invoke the method and I am also able to get the output after invoking a specific method. But after the method invocation, the invoke method is throwing NullPointerException. I have tried to add a try and catch so that, I can know exactly which part of my code is throwing the NullException. Based on my observation this code is the culprit here. passed = (boolean) payload.invoke(testcase);  Here's my code surrounded with try and catch: public void execute() {     try {         Matcher m = Pattern.compile("([^()]*)[(]([^()]*)[)]").matcher(getPayloadString());         m.find();         boolean passed;                 if (m.group(2).isEmpty()) {             System.out.println("m.group(2) is empty!!!!\n"); //This line of code is being executed. Because my argument is empty.             passed = (boolean) payload.invoke(testcase);             System.out.println("Boolean is: " + passed + "\n");          }else {             System.out.println("Inside else!!!\n");             passed = (boolean) payload.invoke(testcase, ObjectCreator.convertParameters(m.group(2)));             System.out.println("Boolean2 is: " + passed + "\n");          }         //If already invoked the method, this line should be executed. But now it didnt print out this debug message below.         System.out.println("Already exited the convertParameters()\n");         String output = passed ? "PASS" : "FAIL";         TestCase.printResult(testcase.getClass().getName(), output, "", "", "");     } catch (Exception ex) {         System.out.println("Exception in execute is : " + ex.toString() + "\n");     } }  I am not really sure why is it throwing NULLPointerException.

What are Null Pointer Exceptions (java.lang.NullPointerException) and what causes them?  What methods/tools can be used to determine the cause so that you stop the exception from causing the program to terminate prematurely?


I have been looking around on Google and StackOverFlow and I have not been able to find a good SIMPLE definition for:  @Override when it is used.  Can someone help me find a very simple definition for this?  Thanks.

What are the best practices for using Java's @Override annotation and why?     It seems like it would be overkill to mark every single overridden method with the @Override annotation.   Are there certain programming situations that call for using the @Override and others that should never use the @Override?


I want to iterate through an enum so I can call a method with each value of that enum. How can I do that?  enum Base { ANC, BTC, DGC };  XmlDocument doc;  doc = vircurex.get_lowest_ask(Base.ANC) doc = vircurex.get_lowest_ask(Base.BTC) doc = vircurex.get_lowest_ask(Base.DGC)   I want it instead to be something like  foreach (var val in values)    doc = vircurex.get_lowest_ask(....)   Is there a way to do this?

How can you enumerate an enum in C#?  E.g. the following code does not compile:  public enum Suit {     Spades,     Hearts,     Clubs,     Diamonds }  public void EnumerateAllSuitsDemoMethod() {     foreach (Suit suit in Suit)     {         DoSomething(suit);     } }   And it gives the following compile-time error:     'Suit' is a 'type' but is used like a 'variable'   It fails on the Suit keyword, the second one.


I can access my user profile, and view all the tabs execpt for the inbox one. I get the "Oops! Something Bad Happened!", and a cute panda falling off a slide. The url bar shows this:  It isn't just the profile page link that is broken, it also the Chrome notifications - I get them but clicking on them takes me to the error page as well.

I can't access my global inbox.  When I try to , I get Oops! Something Bad Happened!. I am currently part of one private beta, that may be related.  I have also noticed that sometimes (but not always), the drop-down inbox link at the top shows old items only.  That may or may not be related.


I have Ubuntu 15.04 installed on Lenovo Z5070 laptop. For Nvidia 840m I have nvidia-346 installed. While waking up from sleep I get the following error on a black screen  drm: hsw_uncalimed_reg_clear Error unknown claimed register before writing to c7204 drm: hsw_unclaimed_reg_check   I sshed into my system in that state. Here's the tail of xorg.0.log  [    70.924] (II) modeset(G0): EDID vendor "CMN", prod id 5579 [    70.924] (II) modeset(G0): Printing DDC gathered Modelines: [    70.924] (II) modeset(G0): Modeline "1920x1080"x0.0  136.62  1920 1964 1992 2070  1080 1082 1086 1100 -hsync -vsync (66.0 kHz eP) [    70.924] reporting 3 4 37 280 [    71.376] reporting 3 4 37 280 [    73.380] (II) XKB: reuse xkmfile /var/lib/xkb/server-7C75F152E85183199599C3E0B919739C0EE668AA.xkm [    76.529] reporting 3 4 37 280 [    77.733] reporting 3 4 37 280 [    77.834] (II) modeset(G0): EDID vendor "CMN", prod id 5579 [    77.834] (II) modeset(G0): Printing DDC gathered Modelines: [    77.834] (II) modeset(G0): Modeline "1920x1080"x0.0  136.62  1920 1964 1992 2070  1080 1082 1086 1100 -hsync -vsync (66.0 kHz eP) [    77.834] reporting 3 4 37 280 [    77.857] reporting 3 4 37 280 [   110.090] reporting 3 4 37 280 [   121.436] reporting 3 4 37 280 [   167.116] reporting 3 4 37 280 [   178.272] reporting 3 4 37 280 [   217.841] reporting 3 4 37 280 [   243.717] reporting 3 4 37 280 [  1345.073] reporting 3 4 37 280 [  1533.403] reporting 3 4 37 280 [  1537.357] (WW) NVIDIA(GPU-0): Failed to enter interactive mode.  [  1537.357] (II) NVIDIA(0): Setting mode "NULL" [  1537.358] (EE) NVIDIA(0): Failed to allocate primary buffer: error [  1537.358] (EE) NVIDIA(0):  *** Aborting ***

My laptop diagnostic shows several pre-fails and has other issues so I am urgently shopping for a new laptop, my second using Ubuntu. I need a laptop with good graphics capabilities and have come across a couple with the Nvidia GeForce 840M graphics card. In other words, I do not have a problem now and am hoping to avoid one.  My research on Ask Ubuntu and elsewhere shows that there have been some bugs with Ubuntu 14.04 and Nvidia drivers (not just for the 840M driver) but that fixes were made or a least suggested. But I have seen nothing definitive, e.g. the Ubuntu Certification for laptops is barely starting with 14.04.  I am about to spend a 1000 dollars and would love a little more assurance before I proceed -- my understanding is that a Live CD cannot perform a full simulation. Are there easy-to-recognize concrete factors which make compatibility (more) predictable, such as specific models of computer and their processors?  I am a considering a  with an Intel i5 4200M  and an  with an Intel i7 4700HQ. Both use an Nvidia 840M graphics card.


I was just looking through the itertools documentation, looking for a way to get rid of a nested for loop like this:  for a in b:     for c in b:         <statement>   However, I couldn't find anything. Is there not a function for this? Should I just keep the nested loops?

Is there a simple way to flatten a list of iterables with a list comprehension, or failing that, what would you all consider to be the best way to flatten a shallow list like this, balancing performance and readability?  I tried to flatten such a list with a nested list comprehension, like this:  [image for image in menuitem for menuitem in list_of_menuitems]   But I get in trouble of the NameError variety there, because the name 'menuitem' is not defined.  After googling and looking around on Stack Overflow, I got the desired results with a reduce statement:  reduce(list.__add__, map(lambda x: list(x), list_of_menuitems))   But this method is fairly unreadable because I need that list(x) call there because x is a Django QuerySet object.  Conclusion:   Thanks to everyone who contributed to this question.  Here is a summary of what I learned.  I'm also making this a community wiki in case others want to add to or correct these observations.  My original reduce statement is redundant and is better written this way:  >>> reduce(list.__add__, (list(mi) for mi in list_of_menuitems))   This is the correct syntax for a nested list comprehension (Brilliant summary !):  >>> [image for mi in list_of_menuitems for image in mi]   But neither of these methods are as efficient as using itertools.chain:  >>> from itertools import chain >>> list(chain(*list_of_menuitems))   And as @cdleary notes, it's probably better style to avoid * operator magic by using chain.from_iterable like so:  >>> chain = itertools.chain.from_iterable([[1,2],[3],[5,89],[],[6]]) >>> print(list(chain)) >>> [1, 2, 3, 5, 89, 6]


I'm currently using precise. I need to upgrade a certain package (namely  and it's dependencies) to a newer version, which is currently found in raring (alfa at the time of writing this). How to do it ?  What will happen if I just add to my sources.list:  deb http://archive.ubuntu.com/ubuntu raring main   Will the next apt-get update / apt-get upgrade, upgrade most of my system to raring then ?

I need these packages with the latest upstream version backported for ubuntu/natty   libccid_1.4.5-0ubuntu1_amd64.deb libpcsclite1_1.8.1-0ubuntu1_amd64.deb libpcsclite-dev_1.8.1-0ubuntu1_amd64.deb libusb-1.0-0_1.0.9-0ubuntu1_amd64.deb libusb-1.0-0-dev_1.0.9-0ubuntu1_amd64.deb opensc_0.12.2-1ubuntu1ppa1~natty1_amd64.deb pcscd_1.8.1-0ubuntu1_amd64.deb pcsc-tools_1.4.18-0ubuntu1_amd64.deb   I tried to build them from source with update but it failed. I am not an expert at packaging and there are so many tutorials and recipes out there.  Do you know a good and up to date tutorial for backporting Ubuntu packages?  How do I get packages from other releases without having to build them myself?


When iterating over an object's members with a for .. in loop, the enumeration order . But, assuming the object is not modified, is iterating over it twice in the same browser during the same page visit guaranteed to give the same order?  Here is sample code that tests the property I'm talking about. The question boils down to whether or not a browser may throw "not the same":  a = [] b = [] for (var e in obj)     a.push(e); for (var e in obj)     b.push(e);  // Are a and b's contents now the same? // Or is the same-ness implementation defined? for (var i = 0; i < a.length; i++)   if (a[i] !== b[i])     throw "not the same";   I'm interested in the answer both according to spec and in practice on existing browsers.  To be clear, I am not asking . I am asking if the order is consistent across multiple enumerations of the same object, within the context of a single visit to a web page and when the object is not being modified.

Does the "for…in" loop in Javascript loop through the hashtables/elements in the order they are declared? Is there a browser which doesn't do it in order? The object I wish to use will be declared once and will never be modified.  Suppose I have:  var myObject = { A: "Hello", B: "World" };   And I further use them in:  for (var item in myObject) alert(item + " : " + myObject[item]);   Can I expect 'A : "Hello"' to always come before 'B : "World"' in most decent browsers?


I am a Javascript beginner. i am creating function expressions dynamically and calling parameters with JSON from text file. the functions are being created correctly with the correct parameters and looping is incrementing fine. i.e it is all working. my issue is that when i call the dynamically generated function with an onclick event nothing happens and no errors. this is the function:  function imgurl(arr) {  for(var i = 1; i < arr.length; i++) {  arr[i].fname = function() { addImage(arr[i].image, 0.5, 0.75); };   /*$scope.addImage1 = function() {  //this is a sample of the function  addImage('image.png', 0.5, 0.75);  };*/  } };    here is the onclick event  <button type="button" class="btn image1" ng-click="addImage1()">Image</button>   the code imports an image into canvas when the button is clicked. i tested the dynamically generated functions by invoking them immediately with () and as they were invoked when the page loaded the function looped and imported all the images into the canvas perfectly i.e the function expressions work as expected. but when i try the onclick event i get nothing, not even an error.   then i manually wrote out a test function expression and the onclick event works. from the code above:  $scope.addImage1 = function() {  addImage('image.png', 0.5, 0.75);  };   so when i manually write out the function expression then the onclick event works but when i try with the dynamically generated expressions then the onclick does not work. i just want to mention again that the dynamically generated functions do work when i invoke immediately, i just cannot call with the onclick event.

How would you explain JavaScript closures to someone with a knowledge of the concepts they consist of (for example functions, variables and the like), but does not understand closures themselves?  I have seen  given on Wikipedia, but unfortunately it did not help.


So, once this is calculated (10 plus modifiers), who rolls (assuming a d20???) to see if something is seen/heard/etc...and since modifiers push the number higher for the character, I would assume the roll needs to be under that number for the character to have success? If I am wrong, how does this work?

Total noob question, but the passive perception is confusing. I don't understand how to calculate it or use it, and I need help.


I am planning a visit to London for a couple of days. I will be staying at a hotel near Heathrow airport.  I am planning to use the underground as follows   Upon my arrival, from terminal 5 to Hounslow West, where my hotel is situated in From Hounslow West to terminal 5, to meet a colleague Then from Hounslow West to somewhere in central London, to see Big Ben and London's Eye(perhaps I will do this route once more, so 2 in total) From central London to terminal 5   What's the best way to organize the tickets? Shall I buy a card? Is it better to use simple paper tickets?  In case it is better to buy a card, where to buy it from? Is there a possibility to get such a card in the airport? How much money should I load it with?  Note that I will be having Euros and not British Pounds, so I was wondering if Euros are accepted both for cards or paper tickets?

I'm visiting London for a couple of days, and planning to use public transport while there.   What kind of ticket(s) would it make most sense to use? Do I get the most value out of single tickets, day travelcards, an Oyster card, or something else? (I'm guessing the Oyster card is more geared towards residents of London, but I'm not sure.)  I'd mostly move around in central London (within ) with accommodation being located a little further away (). Most probably I'd take several trips each day (some of which might be during rush hour).  Bonus question: while I would surely be using the tube the most, are the same tickets generally valid for buses too?


I know that the answer involves Bezout's theorem in some way. I tried this: Let $(a, b) = d$ Let $a = dk_1 and b = dk_2$ So, $ax + by = d$ becomes $dk_1x + dk_2x = d$ Dividing by d, we get: $k_1x + k_2x = 1$ So, $(x,y) = 1$ Is this sufficient?

Suppose $a$ and $b$ are positive integers, and that $d=\gcd(a,b)$. Suppose we have found integers $x$ and $y$ such that $ax+by=d$. Prove that $x$ and $y$ are relatively prime.


First of all, I do not think this is a duplicate of . This question was about several answers that referred to the same question.    If I ask several questions or a question containing different parts, what should I do if different answers refer to different questions?  Consider a question about a math task that consists of two subtasks. IMHO there is no sense in creating two different posts for each subtask, if they are directly related. So what do I do if person A answers the first, and person B answers the second subtask in a separate question?  Of course I could (and should) upvote both of them, but I really think that awarding only one answer by accepting it isn't fair to the people who spent their time trying to help me.  This is why I would vote for a new feature that allows accepting more than one answer, if the provided answers are equitable.

Or at least have maybe the ability to set some other answers as assisted answer. I find myself finding an acceptable answer unusable while other answers underneath it might be more suitable. Would it be safe to say that the accepted answer should only make sense to the asker whether or not the question is important to others?


sudo sh -c "echo 'deb http://download.opensuse.org/repositories/home:/Horst3180/xUbuntu_15.10/ /' >> /etc/apt/sources.list.d/arc-theme.list" sudo apt-get update sudo apt-get install arc-theme   After this I am getting this error in Synaptic:  E: Type 'dep' is not known on line 1 in source list /etc/apt/sources.list.d/arc-theme.list E: The list of sources could not be read. Go to the repository dialog to correct the problem. E: _cache->open() failed, please report.   I can't even carry out any updates.

Every once in a while I see users having issues to update due to errors like this:  E:Type 'ain' is not known on line 1 in source list /etc/apt/sources.list.d/some-ppa.list'   with varying types/line numbers/source list files (often after removing a PPA).  How can such an error be fixed?


I want to learn cryptography. I just wanted to ask people who are experienced about the field where I should start. Any recommendation would be something for me.   Thank you in advance.

What mathematical fields of knowledge would be required in order to get a good understanding of encryption algorithms?   Is it basic algebra, or is there a "higher education" mathematical field dedicated to encryption? I know there is the cryptography field, but what is the subset of knowledge required for cryptographers?


I'm pretty sure that this was an entire book, written before the 90's, the protagonist was a teenage male, and his family was emigrating to one of the moons of Jupiter. When they got there they found that the younger sibling couldn't take the lack of pressure, so had to live in a sealed room with higher pressure. They were farmers, and he had this plow which broke up the stony surface. There was a crisis, and the sealed room broke and the child inside died. I think the rest of the family went back or left, but the boy stayed to continue the colonization.

A magazine connected with (Boy) Scouting, possibly "Boys Life", in the early 1950's ran a short story in which a group of interplanetary explorers on an extraterrestrial planet, which may have been postulated to have a breathable atmosphere, encountered a functional alien transport designed to travel across rough terrain.   The machine was half hidden or buried when a member of the team discovered it and when he reported back was asked to "roll it out." He replied, "It, 'er, walks."   The alien inventors turned out to be long gone. The machine resembled a large multi-legged insect. There may have been a drawing accompanying the story.  The link I'm attempting to add here may not work, but if it does it's the only source to the Boy's Life image of the alien walking machine that's currently accessible. Boy's Life issues from 1950 are not to be found. If the link works, open Boy's Life November 1950 issue and scroll down to page 21.


I have written a python code, to solve a puzzle. It was not working as expected. So while debugging I saw something very weird. Row is a list of lists. The append 1 to single row[n] appends to all lists inside row!   def trap( height):         row = []          for index, i in enumerate(height):             if i == 0 and len(row) == 0:                 continue             else:                 if(i > len(row)):#time for a new row                   #to old rows append 0 below                     for j in range(0, len(row)):                         row[j].append(0)                   row = row + [[0]] * (i - len(row))                             else:                   for j in range(0,i):                      row[j].append(0)                   #PROBLEMATIC CODE START                     for jo in range(i,len(row)):                         if(index == 1):                             print("jays are",row)                             print("jo is",jo, row[jo])                         row[jo].append(1)                   #PROBLEMATIC CODE END (I GUESS?)           print(row) #print this, it gives an idea   trap([2,0])   When I try this independently, it works fine, see:  row = [[0],[0]] index = 1  for jo in range(0,len(row)):       if(index == 1):                   print("jays are",row)                   print("jo is",jo, row[jo])                   row[jo].append(1) print(row)    The problematic code has been marked in the python as comment #PROBLEMATIC CODE START and #PROBLEMATIC CODE END. Call the trap() with [2,0]  row variable should be [[0,1],[0,1]] but row variable is coming [[0,1,1],[0,1,1]]  I have been at this for many hours! I just don't understand why does .append() appends to all the lists inside row, but when I try the smaller code it works. Please help me and guide me

I needed to create a list of lists in Python, so I typed the following: my_list = [[1] * 4] * 3  The list looked like this: [[1, 1, 1, 1], [1, 1, 1, 1], [1, 1, 1, 1]]    Then I changed one of the innermost values: my_list[0][0] = 5  Now my list looks like this: [[5, 1, 1, 1], [5, 1, 1, 1], [5, 1, 1, 1]]    which is not what I wanted or expected. Can someone please explain what's going on, and how to get around it?


so i'm a newbie in blender and i don't know what happened to my scene, but my hdri doesn't show up in the final render while it works just fine on the viewport. Please help me, Thank you. viewport:  render:  additional information please contact me right away

I'm trying to make a test render of my model. But everytime I render it the render looks completely different from the viewport. The viewport is in render mode so it should look something like that, but this doesn't come even close.   I'm using cycles renderer. And my world note is just the standard one so nothing installed there.  There are 2 sun's in the scene both set to 0.5.  I've tried different hdris but they all have the exact same outcome. I've pressed alt+h to see if there was anything hiddden in my scene that blocked the background and nothing shows up so I don't think that's the problem. I've also copied it to a new Blend file to see if there was anything wrong with the file itself but it also happens there so it must be something in my settings I guess. But I just can't find out which one    this is how it looks in the viewport render.    And this is how it looks when I render it.  I really don't know how to fix this so I hope someone can help me here


I recently asked this question:   I just now updated the question, and honestly I don't know if it's an acceptable question even now, but it's one of those things where I simply don't have anywhere else to go for help so I hope that with my update it will be acceptable "enough". Although that's not why I'm writing this...   I received feedback that I needed to update the question, but it was closed before I was able to update it (I was still trying some other things and wanted to wait until I could update with results of all my own troubleshooting). Now that I've updated, will it possibly be reopened or should I just create a new question?

To reopen a closed question,  must agree that the question is suitable for the site and cast votes to reopen the question. But:  How does one actually vote to reopen? Is there a reputation level that you must have before you can see this function? Or do I just leave a comment along the lines of, "I vote to reopen"? What happens if someone with less than 3000 reputation leaves a comment that they wish to vote to reopen? How can one draw attention to their closed questions? Is it done via a posting new question? Or do you edit the closed question?   Related:  For more information, see "" in the .


I messed up my new laptop while trying to set up a dual-boot with Ubuntu and Windows 8.I did a whole bunch of stuff trying to get it to work and now when I try to boot I get stuck in GRUB's recovery mode with no errors.   My question is: Can I boot from Ubuntu on a USB drive to erase both Windows 8 and Ubuntu to just install Ubuntu? Will this get ride of the GRUB rescue mode problem.

I installed Windows 7, which ate Ubuntu's boot file. When starting up the computer, it now goes straight to Windows, without giving me the option of booting Ubuntu.   How can I get Ubuntu back?


Please settle an argument for me! Is it correct to say 'How long of a sentence [can you make]' as opposed to 'How long a sentence can you make'   Which is correct?   Thanks!

Quite a few phrases in English are constructed like so:     How [adjective] a [noun]...?   This is the question form of the construction, which is often answered with the negative:     Not that [adjective] a [noun].   or the positive:     Quite [adjective] a [noun].   However, from time to time I'll hear the word 'of' inserted before the 'a', e.g.:     Not that [adjective] of a [noun].   This usually sounds wrong to me, with the exception of the case where the adjective 'much' is used.  So, this sounds fine to my ear:     Not that much of a problem.   whereas this doesn't:     Not that loud of a noise.   Why is it that 'much' should be used with 'of', and other adjectives not?  Is it because 'much' is seen as measuring a quantity (of something), whereas other adjectives that may be used in this construction are seen as measuring the quality of a whole thing?


I have this sentence: "Kaieteur Falls, which/that is 741 feet tall, is located in Guyana. My English teacher told me "that" would be correct. However, I strongly believe "which" would be correct. Could someone explain which one is correct?  Also, I am aware of threads such as  or others, and I have looked through those. I am fairly certain that "which" would be correct by reading these threads, but my English teacher thinks different.

When is it appropriate to use that as opposed to which with relative clauses?


I have an account at Stack Exchange which I have used to log in into the Stack Exchange app. My problem is that it does not show my name. Instead it shows "user153722" as my name. How to change my name to "Aman Palariya".

How can I change (or remove / reset ) my name on a site in Stack Exchange network?


I want to remove duplicate domains in a large list of URLs in c#, For example if the list was: https://example.com/example.php/login/ https://2example2.com/example/ https://example.com/register.php/ https://example.com/info/ https://example.com/example.php/login/ https://2example2.com/register/  I need to remove all of the duplicate domains except the first one so this would be the end result: https://example.com/example.php/login/ https://2example2.com/example/  Can anyone help me? I know how to separate the domain from the rest of the URL but I'm not sure how to keep the first one.

I am playing with LINQ to learn about it, but I can't figure out how to use Distinct when I do not have a simple list (a simple list of integers is pretty easy to do, this is not the question). What I if want to use  on a list of an Object on one or more properties of the object?  Example: If an object is Person, with Property Id. How can I get all Person and use Distinct on them with the property Id of the object?  Person1: Id=1, Name="Test1" Person2: Id=1, Name="Test1" Person3: Id=2, Name="Test2"   How can I get just Person1 and Person3? Is that possible?  If it's not possible with LINQ, what would be the best way to have a list of Person depending on some of its properties in .NET 3.5?


I am working on an assignment question, and am having trouble moving ahead. The question is as follows:     Let the total number of bit strings with three consecutive zeros be $t_n$. Prove for $n \ge 4$ that $t_n= t_{n-1} + t_{n-2} + t_{n-3} + 2^{n-3}$   So, I started to check if the base case is true with n=4 $t_4 = t_3 + t_2 + t_1 + 2^1 = 1 + 0 + 0 + 2 = 3$  But isn't the total number of combinations for n =4 only 2? 1000 or 0001   I might just be going about this the wrong way. Any guidance would be appreciated.

I have $F_n$ number of bitstrings that have $000$, How would I prove that for $n \ge 4$ , $a_n = a_{n-1} +a_{n-2}+a_{n-3}+ 2^{n-3}$?  Now there are many  ways to go about this but if I choose starting a $n-3$ bit string and append it with $000$ how would I show that?


The following just happened to me :   I posted a comment on a question:     Related: [Link to other question]  I made up my mind and decided that the related question was not only related but a duplicate. I deleted my comment, figuring that a comment would be generated when I voted to close as a duplicate. I voted to close as a duplicate. No automatic comment was generated. I manually generated a “possible duplicate” comment.   Going by the comments and answers to , the comment is not generated, if the duplicate question is already linked in an existing comment. This is okay by me, but this search should not take into account deleted comments, because then we end up with the duplicate being unintentionally linked nowhere.  Depending on how exactly the comment generation works, excluding deleted comments might also address .

I posted a comment to a duplicate question on . But I decided to delete it. Afterwards I voted as duplicate and thought a comment would be generated like     possible duplicate of ...   But it didn't. My guess is that the duplicate-comment won't be generated if a comment exists with that link, even if deleted.


I was playing with the calculator recently and I noticed that we seem to have, as $k\to\infty,$ the limiting value of $$\sum_{j=1}^{\infty} {\frac{1}{j^k}}$$ to be $1.$  Is this in fact true? If so how can one see it? In particular, is there a proof for this somewhere?  Thanks.

I need to prove the following:  $$\lim_{s \to \infty} \sum_{n = 1}^\infty \frac 1 {n^s} = 1$$  This is my attempt:  \begin{align} \lim_{s \to \infty} \sum_{n = 1}^\infty \frac 1 {n^s} & =  \lim_{s \to \infty} \left( 1 + \sum_{n = 2}^\infty \frac 1 {n^s} \right) \\ & = 1 + \sum_{n = 2}^\infty \lim_{s \to \infty} \frac 1 {n^s} \\ & =  1 + \sum_{n = 2}^\infty 0 \\ & = 1 \end{align}  I need help justifying the interchange of the sum and the limit though. Is it enough to show the limit exists and the sum is convergent? I read that I need to prove uniform convergence, how can I do this?


I got this snippet of code and if you see in the return statement of NewNode after new in bracket i have buff. What does this buff in bracket is doing in new statement.   struct Node     { int key; }      Node* NewNode(int key)     {char *buff = allocate();     ...     return new (buff) Node(key)}

Has anyone here ever used C++'s "placement new"? If so, what for? It looks to me like it would only be useful on memory-mapped hardware.


I have an circuit with six high torque servos connected parallel to power brick that provides 5V @5Amps. Everything is running fine. But I wanted to add few LEDs to circuit but I dont know how do I do it ?  Connecting LEDs  parallel to other components will I think burst LEDs as current is too high from source. All servos needs to powered on all the times too. Current drawn by servos are very dynamic.  Any ideas?  Thanks

I'm trying to work out what value resistor to use in a LED circuit. The equation I'd use to do this is:  $$ R = \frac{V_{cc} - V_f}{I_f} $$  Seems logical, and makes complete sense. The answers to the question  confirm this too.  I have the following LEDs:   \$ V_f = 3.3V \$ \$ I_{f_{typ}} = 20mA \$   Using a 5V power supply:   \$ V_{cc} = 5V \$   Plugging these into the above equation gives:  $$ \begin{eqnarray} R & = & \frac{V_{cc} - V_f}{I_f} \\   & = & \frac{5V - 3.3V}{20mA} \\   & = & 85\Omega \end{eqnarray} $$  All good so far.  However, if I use the calculator at , that gives me 100Ω. If I use the ElectroDroid app on my phone, that gives me 85Ω.  So, I assume that the linear1 calculator is using a different method of calculating this resistor value; is there some better way of doing this?


I have a table products structure like this    product_id   product_name   product_created_date   product_expiry_date  availability   ----------   ------------   -------------------    -------------------  ------------     1           x             12-NOV-13                14-JUN-14               No     2           y             13-DEC-13                   null                Yes     3           z             14-DEC-14                14-DEC-15              Yes     1           p             12-NOV-13                14-JUN-14              Yes      ----        --             -----                     ----               ----         I need a query like :  select product_id from products where (product_expiry_date>='12-DEC-14' or product_expiry_date is null) and availability='Yes'   how to do this using EntityFieldQuery?

I have never had the need to do this before today, but it doesn't seem like you can make OR queries with EntityFieldQuery, since db_or is used for select queries.  An example would get to all entities that has a date field where the value is null or after today.  Am I missing something or some trick or is this simply not supported?


Say I have a USB flash drive I found lying on the ground. I want to see if it has any information that could help me return it to its owner, but I live in an urban society. It could easily contain malware and be lying on the ground in wait for its next victim.  How can I browse the contents of this USB flash drive safely?

I work at an embedded software company. This morning I found a USB stick in the parking lot in front of the building. With all the stories of "dropped USB stick attacks" in mind, I'm obviously not going to just plug it into my laptop. OTOH, I'm curious to know whether this was actually an attempt to compromise our systems, or it's really just an innocent case of somebody accidentally losing a USB stick. How do I safely inspect the USB stick without risking exposure?  I'm worried not just about malware and crafted file system images; there's also stuff like power surge attacks:.  EDIT: Many of the answers seem to assume I want to keep the drive and use it afterwards. I have no interest in that at all, I know USB sticks are cheap, and that it wouldn't be mine to keep anyway. I only want to know whether this was indeed a semi-targeted attack, partly out of curiosity whether this actually happens in real life and not just in security papers, but also so that I could warn my coworkers.  I want to know how I would figure out whether the stick contains malware. And that's not just a matter of looking at the drive contents and seeing a suspicious autorun.inf or a carefully crafted corrupt file system - I very much also want a way to inspect the firmware. I sort-of expected that there were tools for extracting that and comparing to known-good or known-bad binaries.