I am doing an experiment for a school project which consists of blowing into 6 different bottles to create different notes/harmonics. Each bottle is filled with different volumes of water to create a different sound. I have to calculate the theoretical frequency of each, but I do not know how.

I'm sure you have tried sometime to make a sound by blowing in an empty bottle. Of course, the tone/frequency of the sound modifies if the bottle changes its shape, volume, etc.   I am interested in understanding how the tone/frequency of this sound can be (approximately) calculated from the shape of the bottle. Is there some formula?  If you have any references about this please post them in your answers.  Being a mathematician I keep hearing that the principal tone is given by the first eigenvalue of the Laplace operator, and I'd like to understand the physical explanation.


I just came across a term called the hessian and read that it represents the local curvature of a function at a point.  So, if it represents local curvature, then is there any way the Hessian can be calculated in terms of the Ricci Curvature?

According to ,  It describes the local curvature of a function.   AFAIK, for one-variable function $f(x)$, its local curvature is  $$\kappa = \frac{|f''|}{(1 + f'^2)^{3/2}},$$ and its Hessian matrix is  $$\mathcal{Hess}(f) = [f''],$$ right? And here is my problem, I think the local curvature is not just described by its Hessian matrix, because $f'$ also has its role in it, doesn't it?  And furthermore, for 2-variable function $f(x,y)$, its Hessian matrix is  $$\mathcal{Hess}(f) = \left[ \begin{array}{cc} f_{xx}'' & f_{xy}'' \\ f_{xy}'' & f_{yy}'' \end{array} \right].$$ How does it relate to the local curvature of $f(x,y)$?


Recently upgraded to 2020 MacBook Pro running Catalina. I no longer seem to be able to use 2010 iMac as second screen. (I was connecting with a Thunderbolt to thunderbolt cable. then pressing Command + F2). Is there any way that I can continue to use my 2010 iMac as second screen? It is not good for anything else! Thanks, Miles

I have a Surface Pro 4 and an iMac (late 2019). I would like to connect my Surface Pro to the iMac to use the iMac screen as a second monitor.  Is that possible?  What are the requirements to use an iMac as a second screen?


If someone claim that chemical formulation for the water isn't $H_2O$, how we can prove for he/she that water has constituted of $H_2O$ molecules. Generally based on what evidences we sure that water is $H_2O$ (has one oxygen and two hydrogens per each molecule) and isn't anything else?

Who first determined the structure of water (two hydrogen atoms stuck to an oxygen atom at approx 105 degrees), and, more importantly, how was this done?


I have dual booted lapy with ubuntu and windows,the problem is I am unable to start windows. Why this is happening??

I installed Windows on my computer, followed by an installation of Ubuntu. However, now I'm unable to boot into my Windows install.  What can I do to fix this?


The subject of span and linear independence is still new to me and im unsure if what I did is considered true or not.  what I did was, I put all of the coeffecients of the polynomials in a matrix, and reached this echelon form: \begin{bmatrix}1&2&1\\0&2&3 \\ 0&0&2\end{bmatrix} which means that the polynomials are all independent so I can write all the polynomials up to the power of 2 as a linear combination of these three polynomials.  Is this the way to proof it? are the steps true?  am I supposed to do something with what i got after reaching this matrix?  The second questions is asking if the polynomials: $x^3$, $(x+1)^3$, $(x+2)^3$, $(x+3)^3$ span all the polynomials of power to the power of 3 and less. will it be solved the same way? or im missing something that makes it easier?  any tips or corrections or answers are really appreciated, thanks in advance!

I'm having trouble understanding this homework problem.   Suppose four polynomials are defined by the following:   $ p_{1}(x) = x^3 - 2x^2 + x + 1 \\ p_{2}(x) = x^2 - x + 2 \\ p_{3}(x) = 2x^3 + 3x + 4 \\ p_{4}(x) = 3x^2 + 2x + 1 \\ $  Does the set $S = $ { ${p_{1}, p_{2}, p_{3}, p_{4}}$ } span $P_{3}$ (the space of all polynomials of degree at most 3)?  So if I start with the polynomial $y = ax^3 + bx^2 + cx + d$ I understand(I think) that in order for $S$ to span $P_{3}$ then $y$ must be a linear combination of $S$ but I'm not sure where to go from there.   EDIT:   $$ A =  \begin{bmatrix} 1 & -2 & 1 & 1\\  0 & 1 & -1 & 2\\  2 & 0 & 3 & 4\\  0 & 3 & 2 & 1 \end{bmatrix}  \sim  \begin{bmatrix} 1 & 0 & 0 & 0\\  0 & 1 & 0 & 0\\  0 & 0 & 1 & 0\\  0 & 0 & 0 & 1 \end{bmatrix} $$  So $\operatorname{rank}(A) = 4$ which means the vectors in the set are linearly independent because there are only 4 column vectors in the matrix (AND there is only the trivial solution to the matrix) and therefore we span $P_{3}$?


My program compiles perfectly, but whenever I try to run it throws a NullPointerException. I tried searching this up, and found that the error relates to some value being null while the program is trying to use it, but I re-checked everything and came up blank.      import hsa.Console;     import java.awt.*;     import java.util.* ;     public class PlatformAndBall2     {         static Console g ;        public static void BallLoop() throws InterruptedException     {     do {        int height = g.getHeight();        int width = g.getWidth();        int xpos,ypos ; xpos= 0; ypos =0 ;        int radius ;  radius = 20 ;        int addx, addy ; addx = 3; addy = 3;         xpos += addx ;        ypos += addy  ;        g.setColor(Color.red) ;       g.clear();       g.fillOval(xpos,ypos,radius,radius) ;        Thread.sleep(15); // sets frames        if ( xpos < 0 )         addx += 3 ;         if ( xpos > width - radius)          addx += -3 ;            if ( ypos < 0 )         addy += 3 ;            if ( ypos > height -radius )          addy += -3 ;     }while (true);        }        public static void main(String[] args)throws InterruptedException {          Console g = new Console() ;         BallLoop();        }}

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?


Suppose that I have a random variable $X\sim\mathcal{N}\left(0,1\right)$.  I know that $$\mathbb{E}\left[X\right]=0$$ $$\mathbb{E}\left[X^{2}\right]=\operatorname{var}\left(X\right)+\left(\mathbb{E}\left[X\right]\right)^{2}=1-0=1$$ But then how would I calculate $\mathbb{E}\left[X^{4}\right]$?  (By the way, I do not have any background in moments and moment generating functions, so could I request for more explanation if your response involves moments and moment generating functions please?)  Thank you!

Let there be a random variable $X$ and $N(0, \lambda ^2)$ given. I am to find $E[X^n]$ knowing that $\Gamma \big(\frac{1}{2} \big) = \frac{\pi}{2}$.     I started with the definition: $$E[X^n] = \frac{1}{\sqrt{2 \pi}\lambda}\int \limits_{- \infty}^{\infty}x^ne^{\frac{-x^2}{2 \lambda^2}} \mbox{d}x$$ I figured out that: $$E[X^n] = \begin{cases} 0 &\text{for } n = 2k + 1; k \in \mathbb{N} \cup\{0\}\\\text{something}&\text{for } n = 2k; k \in \mathbb{N} \end{cases}$$    So now: $$ \frac{1}{\sqrt{2 \pi}\lambda}\int \limits_{- \infty}^{\infty}x^ne^{\frac{-x^2}{2 \lambda^2}} \mbox{d}x =  \frac{2}{\sqrt{2 \pi}\lambda}\int \limits_{0}^{\infty}x^ne^{\frac{-x^2}{2 \lambda^2}} \mbox{d}x$$ I substitute: $$t = \frac{x}{\lambda}$$ $$\mbox{d}x = \lambda \mbox{d}t$$  Thus: $$\frac{2}{\sqrt{2 \pi}\lambda}\int \limits_{0}^{\infty}x^ne^{\frac{-x^2}{2 \lambda^2}} \mbox{d}x = \frac{2 \lambda^{n}}{\sqrt{2 \pi}}\int \limits_{0}^{\infty}t^ne^{\frac{-t^2}{2}} \mbox{d}t.$$ By integrating the formula above by parts I do get:  $$\frac{2 \lambda^{n}}{\sqrt{2 \pi}}(n-1) \int \limits_{0}^{\infty}t^{n-2}e^{\frac{-t^2}{2}} \mbox{d}t$$ I don't really know where to go from here. And unfortunately I don't see how can I use the Gamma Function, which is given by this formula: $$\Gamma (z) = \int \limits_{0}^{\infty} x^{z-1}e^{-x} \mbox{d}x,$$ isn't it?


I'm Newbie on Ubuntu :) I've tried to install ubuntu with Wubi ,after downloading the OS it prompts "Reboot Now" after rebooting ubuntu starts to install while installing the Mouse cursor Appears and hangs and suddenly My Monitor turned off and on again and again i cant proceed with the installation I'm stuck, I'm in running Windows 7 2gb ram,500gb hd,amd-apu 3400..

I am trying to boot Ubuntu on my computer.  When I boot Ubuntu, it boots to a black screen.  How can I fix this? Table of Contents:


I am using GNOME Terminal but, I am not able to make it transparent. Am I missing some packages?  I am using the latest gnome 3.18, so I dont think its a DUP issue.

I recently installed ubuntu GNOME and I have done all the updates and upgrades. When I try to add transparency to my terminal it don't work. It only goes dark-black to white.  Transparency set to none:    Transparency set to maximum    However, when I have added transparency to the terminal and I go into Help -> About, the terminal window applies the right transparency, (very strange).   Transparency with about box open    I would really appreciate some assistance with this issue.


This is question from yesterday's IMO.  The real numbers $a$, $b$, $c$, $d$ are such that $a ≥ b ≥ c ≥ d > 0$ and $a + b + c + d = 1$. Prove that $$(a + 2b + 3c + 4d)a^a b^b c^cd^d < 1$$

I have just learnt about homogenization, however I don't really understand it and I can't really utilize it ( this is where I learnt about it). Could you please explain to me how to use it optimally? Moreover I found a question on IMO 2020, which I believe can be solved using homogenization. It goes as follows: Let the real numbers $a, b, c, d$ be such that $a\ge b\ge c \ge d >0$ and $a+b+c+d=1$. Prove that: $(a+2b+3c+4d)a^ab^bc^cd^d<1$ I attempted to solve it by saying that RHS is equal to $(a+b+c+d)^a$, but then I realized that this doesn't help and it doesn't homogenize the inequality either. Then I tried using AM-GM on LHS and getting something which can be homogenized, however I did not succeed there either. Could you please explain to me how to use homogenization optimally and how this question could be solved using it?


using the code form  how could I redirect the page after the countdown ends?  var timeleft = 10; var downloadTimer = setInterval(function(){   if(timeleft <= 0){     clearInterval(downloadTimer);     document.getElementById("countdown").innerHTML = "Finished";   } else {     document.getElementById("countdown").innerHTML = timeleft + " seconds remaining";   }   timeleft -= 1; }, 1000);

How can I redirect the user from one page to another using jQuery or pure JavaScript?


I am aware that Boole's inequality can be proved by induction. How do we extend these results to an infinite set? (since induction cannot be applied in this case)  P($\bigcup\limits_{i=1}^{\infty} A_i$) $\leq$ $\sum \limits_{i=1}^{\infty} P(A_i)$

I am trying to prove Boole’s inequality  $$P\left(\ \bigcup_{i=1}^\infty A_i\right) \leq \sum_{i=1}^\infty P(A_i).$$  I can show it of any finite $n$ using induction. What to do for $\infty$ ?


I have a project that needs to detect the DISPLAY in shell () variable to be able to display some  stuff on the local machine.  Or a better solution ( ?) to open  stuff in non interactive shell without trying to figure out DISPLAY and XAUTHORITY.  I can set DISPLAY=:0 but that will fail if a user uses another session.  As far as I'm not in interactive mode, what I tried (works well, but only as root) is:  strings /proc/$(pidof Xorg)/environ | grep -Eo 'DISPLAY=:[0-9]+(:[0-9])*'   or as user:  ps uww $(pidof Xorg) | grep -oE '[[:blank:]]:[0-9]+(:[0-9])*\b'   But I don't know if it's reliable on any Linux (Unixes?)  Is there a more reliable/portable way?

Once upon a time,  DISPLAY=:0.0 totem /path/to/movie.avi   after ssh 'ing into my desktop from my laptop would cause totem to play movie.avi on my desktop.  Now it gives the error:   No protocol specified Cannot open display:    I reinstalled Debian squeeze when it went stable on both computers, and I guess I broke the config.   I've googled on this, and cannot for the life of me figure out what I'm supposed to be doing.   (VLC has an HTTP interface that works, but it isn't as convenient as ssh.)  The same problem arises when I try to run this from a cron job.


Stormvirux provided this nice script in another thread.  It puts column headings on the output of an ls -l listing:  #!/bin/bash { echo Permissions links Owner Group Size Last_modMonth Last_modTime Last_modTime Filename; ls -l ; } | column -t | awk 'NR!~/^(2)$/' ls -l | head -1  Just running it on the command line works great.  I like it and want to use it as an alias for, say lsl on a Linux system running bash.    But I can't for the life of me figure out how to make the delimiters work so it is interpreted correctly.  When I try just using the alias command on the command line (which I do before I commit it to a configuration file like .bashrc), it won't work.  I get various errors when I try various ways to package it so it's interpreted correctly.    Can someone tell me how to get this working as an alias, or point me to a good reference that can clearly tell me what I need to do to get something like this working as an alias?

Most of my my aliases are of this form: alias p='pwd'  I want to alias git commit so that it does git commit -v  But trying to create an alias with a space gives an error:  $ alias 'git commit'='git commit -v' -bash: alias: `git commit': invalid alias name


Hello I used the following command to change my password sudo usermod abdurrehman --password thinkp@dt440 and after running this command, my password is not working i logged in via the fingerprint pass on my pc but I still can't access my sudo or root. Any one know what i am missing

I am superadmin of a server and like to change password of an existing user . How can I do that ?  I tried   usermod -p 'new-password' john   but it didn't worked ?


I have the following query to get me a list of sysadmins in the current server:  select 'ServerRole' = SUSER_NAME(rm.role_principal_id),         'MemberName' = lgn.name ,        'Status'= Case when lgn.is_disabled=0 then 'Enabled'                                               else 'Disabled'                   end  from sys.server_role_members rm  INNER JOIN  sys.server_principals lgn          ON rm.member_principal_id = lgn.principal_id  where rm.role_principal_id = SUSER_ID('sysadmin')      Should it be a concern that the accounts starting with NT SERVICE are sysadmin?  the sql server agent service is running under the account shown above in the item number 10, so maybe the number 7 is even redundant, how can I find it out?  There is a similar question:

I'm not sure if I'm googling for the wrong thing but I can't find an answer. We're running SQL 2014. The SQL Server and SQL Agent processes are both running under Domain Managed Service Accounts.  When SQL was installed NT SERVICE\MSSQLSERVER, NT SERVICE\SQLSERVERAGENT, NT SERVICE\SQLWriter, NT SERVICE\Winmgmt are all setup as SysAdmins.  If I am running SQL and SQL Agent as a Domain user, do I still need these NT Service accounts, and/or do they need to be SysAdmin?


Determine all groups G such that the set $H:=${$(g,g^2): g\in G$} is a subgroup of the Cartesian product $G\times G$  Is the answer to this not just any group G containing $g$?

The Question: Determine all groups G such that $ \{ H=(g,g^2): g \in G \}$  is a subgroup of $ G\times G$   I am confused what the question is asking for. I assume there is some small finite number of groups that satisfy this property i.e groups with 4 or less elements.  Things I know:   The subgroup test states that if H is a subgroup then it must contain the identity element of G(say $e$) but since $e^2=e$ then by the criterion of H every cartesian product $G \times G$ contains $(e,e^2)$ Right? If $(g,g^2)$,$(h,h^2)$ $\in$ H then $(gh,(gh)^2)$ $\in$ H. Not sure how to proceed with this. And if $(g,g^2)$ $\in$ H then $(g^{-1},(g^2)^{-1})$ $\in$ H.   Any hints/clarifications would be appreciated.


Hello I'm somewhat new to the world of programming. So I tried creating an interface for a test object. As soon as I create the object I get a StackOverflowException.  Program.cs       class Program {      static void Main (string[] args) {         TestObject test = new TestObject("test");     }  }   TestObject.cs  public class TestObject : IObject {      public string Name {get{return Name;}set{Name = value;}}      public TestObject (string name) {         Name = name;     }  }  interface IObject {     string Name {get;set;} }

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 just install steam from the terminal as you can see:    But when I try to open steam as you can see:    And finally, if I continue with it I get this:    I have already install the Canonical packages from software update, I have tried use the .deb file from steam website but it also didn't work, it always shows me the same. Also, if I click on steam icon it shows me the same image 3.  So, what should I do?

Alright so my computer is 64-bit and I only want to use Linux to install Steam, play Team Fortress 2 and get that penguin item but It's not working out for me. The version of Ubuntu I installed was the "Wubi.exe" which let's me use it along with Windows. But when I get it up and download the Steam .deb file, it tells me "Wrong Architecture 'i386'". What do I have to do to run Ubuntu alongside windows without using a CD/USB stick?


What is the easiest way to extract an audio track from a video file and save it as an MP3 file?

Is there an application that allows me to extract the audio from a MP4 or FLV video file and store it as an MP3 file for use in a portable media player?


I'm just getting into string theory and I'm wondering if it has made any unique predictions that have been verified by experiment?

Possible Duplicate:          We carefully observe things, observe patterns and then build theories that predict.   String theory is frequently criticized for not providing quantitative experimental prediction.   What are the problems that prevent this theory from producing quantitative experimental predictions?   Is there no experiment suggested by string theorists to verify validity of their theory?  Is the problem mathematical? (or it just requires many dimensional equipments...just kidding).  I am not criticizing the theory because to do that I should understand it first, but I haven't studied it. I just want to know.


I want to translate the page based on the accept-language headers, do I need to redirect to /$lang or having different content(actually same content but translated) in the same url is ok?

When I set up a site in multiple languages, how should I set up my URLs for search engines and usability?  Let's say my site is www.example.com, and I'm translating into French and Spanish. What is best for usability and SEO?  Directory option:  http://www.example.com/sample.html http://www.example.com/fr/sample.html http://www.example.com/es/sample.html   Subdomain option:  http://www.example.com/sample.html http://fr.example.com/sample.html http://es.example.com/sample.html   Filename option:  http://www.example.com/sample.html http://www.example.com/sample.fr.html http://www.example.com/sample.es.html   Accept-Language header:  Or should I simply parse the  header and generate content server-side to suit that header?  Is there another way to do this? If the different language versions don't have different urls, what do I do about the search engines?    UPDATE 2011-12-06  Google has new recommendations for meta tags for explicitly pointing to other language content: .  UPDATE 2012-05-25  Related but not precisely:   UPDATE 2013-06-12  includes discussion of several URL schemes directly relevant to the question.


Please suggest me a single word for a person who can speak many languages.

A polyglot is a person who is fluent in many languages, but what do we call a person who is fluent in only two or three languages?  Is bi-glot a proper term for this?  I don't think the words bilingual / trilingual fit the bill, because I am looking for a -glot word.


At my terminal, I can't cd to the folder containing mysql and call it.   TimsMacBookAir:/ tim$ cd /Applications/MAMP/Library/bin/ TimsMacBookAir:bin tim$ mysql -bash: mysql: command not found   If I execute the file by using the whole path, it works  TimsMacBookAir:bin tim$ /Applications/MAMP/Library/bin/mysql Welcome to the MySQL monitor.  Commands end with ; or \g. Your MySQL connection id is 7 Server version: 5.5.38 Source distribution   Why is that?

Let's suppose i have an application in /Applications/MacPorts/KDE4/myapp.app/Contents/MacOS/myapp.  When i use cd to reach the directory 'MacOS' and then issue the command myapp it says command not found.  But when i issue the command with the full path like /Applications/MacPorts/KDE4/myapp.app/Contents/MacOS/myapp it works.  I had the same problem with mysql inside xampp.  Why is that?


I'm stuck trying to figure out how to prove that $(\mathcal P (\mathbb N) \space , \space \triangle)$ is an abelian group? I know the definition, but I'm confused how to incorporate the fact that I'm dealing with a powerset instead of just the integers or natural numbers, so I'm seeking help to understand how proofs on sets works.  Also, I'm using $\triangle$ to mean symmetric difference.

$G$ is the set of all subsets of a set $A$, under the operation of $\triangle\;$: Symmetric Difference of sets.  $A$ has at least two different elements.  I need to check if this is a group, and if it does to show if the group is abelian and/or finite.  Associativity - easy from Symmetric Difference. Identity element - empty group. Inverse element - each element is inverse to itself. Am I right?  abelian? finite?


i have met a girl online from rusia. we have been sending massages through my email address.it has now been six weeks since her first message.she uses the internet cafe. she was short $516 american for her airplane ticket. so i sent her $820. canadian. after arriving at the airport, she said the lady in the airport cannot give her the tickets unless she has $2070. in her account to prove she can afford to visit Canada. i think that i am being scammed.

On Travel.SE we often see questions about a person (usually a love interest) asking for money to pay for airfare, visas, permits and other expenses. For example        What general advice can be given in such situations to the person asking the question? Is it all a scam or can there be legitimate situations in which an online friend could be asking for money?


I'm trying to write the prime factorisation of 60 in latex. Using underline and vertical bar, I was able to create something similar to what I intended. Is there a better way to do so? $5| \underline{60}$         $3| \underline{12}$               $2| \underline{4}$                                               $\color{white}{--}2$

Is there any way that I can create the following in LaTeX which renders using MathJax:


I forgot the description, but it's about the mc being summoned by a king of a kingdom, then kicked out to live in a forest infested by an army of the demon. He later purifies the whole Forest. I read this about a year ago, and the last time I checked, it had more than 10+ chapters, probably 15-30.

This is what I remember from the manga. It starts off with the main character getting transported to another world, however if I remember correctly, he wasn't the one that was supposed to be summoned, or so thought the people who summoned him. The story progresses and the MC becomes super powerful, and the demon king starts to come to his house and tries to makes friends with him because he knows how powerful the MC is. The MC also runs into a a white demon which he defeats, and it turns into the form of a human woman and starts to follow him around, and they are pretty much a couple. I also remember that there was a super powerful demon locked up in the basement of the castle, if I remember correctly, and it gets released and no one can stop it. However, it harms his girlfriend/wife, and he pretty much goes berserk and attacks the super powerful demon and wins. After that, the demon starts to follow him around and I believe calls him master.


I'm building a trading site and have these tables  Example:  The First one is fruit.   +----+--------+ | ID |  Name  | +----+--------+ |  1 | Apple  | |  2 | Orange | |  3 | Banana | +----+--------+   The Second one is the trading  +-----+-------+--------+-------+ | tID | first | second | third | +-----+-------+--------+-------+ |   1 |     2 |      1 |     3 | |   2 |     3 |      1 |     2 | +-----+-------+--------+-------+   I want to join them into a table show that whats the first/second/third means , like  +-----+-------+--------+--------+-------+-------+--------+ | tID | first |   n1   | second |  n2   | third |   n3   | +-----+-------+--------+--------+-------+-------+--------+ |   1 |     2 | Orange |      1 | Apple |     3 | Banana | |   2 |     3 | Banana |      1 | Apple |     2 | Orange | +-----+-------+--------+--------+-------+-------+--------+   Here is my sql   SELECT trade.tid ,trade.first , fruit.name AS n1,trade.second ,      fruit.name AS n2,trade.third , fruit.name AS n3 FROM trade  INNER JOIN fruit  ON trade.first=fruit.id INNER JOIN fruit  ON trade.second=fruit.id INNER JOIN fruit  ON trade.third=fruit.id   And I got the response is   Error Code: 1066. Not unique table/alias: 'card'   What should I do ? Is my sql wrong or the database should not store data like this ?

I have 2 tables. One (domains) has domain ids, and domain names (dom_id, dom_url).  the other contains actual data, 2 of which columns require a TO and FROM domain names. So I have 2 columns rev_dom_from and rev_dom_for, both of which store the domain name id, from the domains table.  Simple.  Now I need to actually display both domain names on the webpage. I know how to display one or the other, via the LEFT JOIN domains ON reviews.rev_dom_for = domains.dom_url query, and then you echo out the dom_url, which would echo out the domain name in the rev_dom_for column.  But how would I make it echo out the 2nd domain name, in the dom_rev_from column?


Let $F$ be a field such that $char F=p\neq 0$, let $n>0$ and $m=p^n$ prove that $a^m+b^m=(a+b)^m$ for all $a,b\in F$   Attempt:  Since $a,b\in F$ then $a^m=(1\cdot a)^m=1^ma^m=0\cdot a^m=0$  and $b^m=(1\cdot b)^m=1^mb^m=0\cdot b^m=0\implies a^m+b^m=\color{red}0$   $(a+b)^m=[1\cdot (a+b)]^m=(1)^m(a+b)^m=0\cdot (a+b)^m=\color{red}0$

If $p$ is prime, then $(x+y)^p=x^p+y^p$ holds in any field of characteristic $p$. However all the proofs I have seen use induction and some relatively nasty algebra despite how fundamental this fact seems.  What is the nicest, "highest level proof" you know?


There are lots of Q&A site on Stack Exchange.   I asked a question on Programmers. But it was migrated to Stack Overflow.  There are many questions about data structures and algorithms asked on Stack Overflow, but some are downvoted, and I don't know why.  Is there a specific Stack Exchange site to post algorithm (data structure) questions? Or, more precisely, do we have stack exchange site for people who face interviews?  Which Stack Exchange site is meant for what?

Is there a Stack Exchange site for both technical and behavioural interview questions? Where I could ask questions about, for example:   Sample questions and solutions, experiences, or seeking advice for interviews A place where I could ask interview questions and what the best way to answer it (not just a correct way or the most efficient).   What to do in a certain situation such as negotiation, unfair interviews, etc.   Experiences interviewing at so-and-so company.


Let U and w be two subspaces of the same vector space V.Is U union W necessarily a vector space?

I'm having a difficult time understanding this statement. Can someone please explain with a concrete example?


Can anyone explain what's going on here? My calculator insists that the result of equitation is 13201.    double test = 132.01/0.01D; System.out.println(test); <- 13200.999999999998   Probably, this is very simple question, but I really don't understand. Most of questions in SO on similar topic involves Integer and Double.

Consider the following code:   0.1 + 0.2 == 0.3  ->  false   0.1 + 0.2         ->  0.30000000000000004   Why do these inaccuracies happen?


I have a scene where I want to use Objects with a nice Neon glare, but without actually emitting Light.  Is there a way in cycles to do that?   I use an Emissive Shader and put in the glare with the compositor. This is actually working fine already, but as I mentioned my scene gets lighten up by the emissive objects too. How can I change that     ((In Eevee it is already working, but I want cycles))

I am trying to make a computer monitor. This means I have to have an image texture for the screen. My image texture is currently connected to an Emission BSDF shader. Unfortunately, when the emission is bright enough to emit light, the image is white-washed. I tried adding a Brightness/contrast node to the image before pushing it into the Emission BSDF, but it seems to directly counter the Emission's strength.  The desired light level   The desired image brightness


Humans have approximately 25000 genes. Why are these genes on 46 chromosomes? Why not 40 or 50?

In humans, each cell normally contains 23 pairs of chromosomes, for a total of 46. Monkeys, chimpanzees, and Apes have 24 pairs (twenty-four pairs), for a total of 48.   What caused humans to have 46?  EDIT: @TomD is right, I was asking why we have one less chromosome pair than chimpanzees (for example) [23 pairs instead of 24].


I want to install KDE desktop.If i install KDE then will i get  UBUNTU security updates and newer versions of UBUNTU? OR, Will be blocked from getting  getting them?

I have read this  from  .It says that other Ubuntu flavors of LTS get only 3 years of support. Is this true?   This question is not a duplicate of the linked post as I want to know support time for only other Ubuntu flavors.


Solving the equation $x^2=2^x$ algebraically.  I solved it graphically, however, upon solving this equation algebraically, I realized that I was not equipt with the necessary tools to solve it. I was not able to reduce it to anything solvable within the scope of my knowledge.

We know that $2^4 = 4^2$ and $(-2)^{-4} = (-4)^{-2}$. Is there another pair of integers $x, y$ ($x\neq y$) which satisfies the equality $x^y = y^x$?


I have recently started using collectingAndThen and found that it is taking a bit long time comparatively to the other coding procedures, which i used for performing the similar tasks.  Here is my code:          System.out.println("CollectingAndThen");         Long t = System.currentTimeMillis();         String personWithMaxAge = persons.stream()                                         .collect(Collectors.collectingAndThen(                                                                 Collectors.maxBy(Comparator.comparing(Person::getAge)),                                                                 (Optional<Person> p) -> p.isPresent() ? p.get().getName() : "none"                                                 ));           System.out.println("personWithMaxAge - "+personWithMaxAge + " time taken = "+(System.currentTimeMillis() - t));         Long t2 = System.currentTimeMillis();         String personWithMaxAge2 = persons.stream().sorted(Comparator.comparing(Person::getAge).reversed())                                                     .findFirst().get().name;         System.out.println("personWithMaxAge2 : "+personWithMaxAge2+ " time taken = "+(System.currentTimeMillis() - t2));   And here it the output:  CollectingAndThen personWithMaxAge - Peter time taken = 17 personWithMaxAge2 : Peter time taken = 1   which is showing that collectingAndThen is taking a greater time comparatively.  So my question is - Should I keep on collectingAndThen or there is some other suggestions?

How do you write (and run) a correct micro-benchmark in Java?  I'm looking for some code samples and comments illustrating various things to think about.  Example: Should the benchmark measure time/iteration or iterations/time, and why?  Related:


My phone is automatically sending sms to number 6592324290 and i have been charged heavily by my service provider. I don't remember that i have installed any app which can send such sms. please advice so that at lest further sms can be stopped.

I normally have "Block Premium Messaging" set up on the phones in my family. This is a verizon content block the prevents you (or family members) from sending messages to premium services that charge.  Recently, I had to replace a phone that broke and when the new phone was activated the blocks I had set up were cleared. On my bill was a $9.99 charge for a text to one of these premium services.   I know nobody sent the SMS to the service to sign up for this. I suspect some application on the phone sent the SMS message. Is there a way I can find out which app on my phone is sending SMS messages to premium services? Can I view all the apps that have permission to send SMS messages?  -- update -- well, I ran RL Permissions and no downloaded apps have the Send SMS permissions. I must have already deleted the offending app or something else is going on.


I have a current version of VirtualBox 4.3.20, but after the latest Windows 7 update on Dec. 11th, 2014, it cannot open any virtual machine. First, it would not open the program at all. After a reboot, it would say:   STATUS_OBJECT_NAME_NOT_FOUND Driver is probably stuck stopping/starting. Try 'sc.exe query vboxdrv' to get more information about its state. Rebooting may actually help. (rc=-101)  The log file says: b28.146c: supR3HardenedVmProcessInit: Opening vboxdrv stub... b28.146c: Error opening VBoxDrvStub:  STATUS_OBJECT_NAME_NOT_FOUND b28.146c: supR3HardenedWinReadErrorInfoDevice: NtCreateFile -> 0xc0000034 b28.146c: Error -101 in supR3HardenedWinReSpawn! (enmWhat=3) b28.146c: NtCreateFile(\Device\VBoxDrvStub) failed: 0xc0000034 STATUS_OBJECT_NAME_NOT_FOUND (0 retries)  I've tried installing an older version (4.3.18) as well, reinstalling the current version several times, rebooting, uninstalling antivirus software, but nothing helped.

In the last 18 hours my Windows 7 Pro 64-bit machine has rebooted in response to the Windows Update applying patches.  Now I find when I try to start the VirtualBox Manager (not the actual VM's which the manager manages, I don't get that far) I get the message "\.\GLOBALROOT\SystemRoot\System32.dll is either not designed to run on Windows or it contains an error"    When I search for this file at %systemroot%\System32\crypy32.dll it doesn't exist.  It seems that the patching has wiped this file out ?  I have followed the advice from the dialog box of re-installing VirtualBox (both the version I was originally running and several later versions) but none of that helps (well actually I get different errors from the later versions but I would rather address this issue than wade into a other VirtualBox errors).  How can I most easily get a relevant copy of crypt32.dll ? I only have a recovery partition not the "original installation media".    EDIT: Good advice from @Bob. Unfortunately, it seems, I've used up all my System Restore points in trying to fix this earlier. I've put up another question asking what options I have :  .    EDIT: Here are the updates most recently applied to the machine in question:


The hot questions feed (the lightning bolt icon on the left just under my profile icon) doesn't accept taps on the large hot questions at the top, so I can't go to them from there.  The smaller hot questions below the large top row work fine.   App Version: 1.6.0.10 Device: iPad Air 2 (Cellular) OS Version: Version 10.0.2 (Build 14A456)

App Version: 1.6.0.10 Device: iPad mini 2G (Wi-Fi) OS Version: Version 9.3.5 (Build 13G36)   Just updated (to 1.6.0.10) and the hot question links no longer work. They highlight as if selected but do not open the question.  "Recent" question links still work.  I've tried    closing and reopening the app; force rebooting the iPad (hold home/power until second apple logo displays);   deleting and reinstalling the app upgrading from iOS 9 to iOS 10   No joy.  This bug does not occur on an iPhone 4s running iOS 9.3.5 and the SE app 1.6.0.10.  Prior to tapping on a hot question    After tapping on the leftmost hot question-note the color change but the question does not open.


I'm getting a date from mongodb like 2015-07-09T22:04:51.974Z, I want to format this string in Python to something like "Wed, July 7, 2015"  Does python have any libraries for working with this type of date and formatting?  I can't seem to find anything specific to this.

I've got a huge list of date-times like this as strings:  Jun 1 2005  1:33PM Aug 28 1999 12:00AM   I'm going to be shoving these back into proper datetime fields in a database so I need to magic them into real datetime objects.   This is going through Django's ORM so I can't use SQL to do the conversion on insert.


I am training a deep neural network for classification (specifically, a convolutional neural network for object recognition). I use mini-batches for my training, because I cannot fit the entire training set onto my GPU. The total size of my training data is 10,000 images, and each minibatch is of size 100. For each training epoch, the training data is randomly shuffled, and then mini-batches are chosen one after the other until all training data has been used for that epoch. For each mini-batch, I record the training error for that mini-batch, before the mini-batch is processed, together with the testing error, which is computed over all 1,000 testing images.  What I am finding, is that during the first epoch, the training error for each mini-batch is around 10% lower than the testing error. As the first epoch progresses, both the training and testing error decrease, but generally with a 10% margin between the two. Of course, training error is typically lower than error, but this only makes sense when those same data have already been used in training. In my case, I am seeing this behaviour in the first epoch, such that when I compute the training error for a mini-batch before it is processed, the system has never seen this data before.  What could be the reason for this? I have tried different random shuffles of the data, and the same thing occurs. My only explanation is that the training data generally is easier to separate than the testing data, but I  would be surprised if this is the case to such an extend, given that the training-testing split is random itself....  Any thoughts? Thanks!

I found two questions  and  about this issue but there is no obvious answer or explanation yet.I enforce the same problem where the validation error is less than training error in my Convolution Neural Network. What does that mean?


Please I got this error message when trying to delete some rows a table on my database :  message 9002, level 17, state 2, line 48 The transaction log for database 'NewHopeHospial' is full due to 'log_backup'.  Please what do I do

This one seems to be a common question in most forums and all over the web, it is asked here in many formats that typically sound like this:  In SQL Server -  What are some reasons the transaction log grows so large? Why is my log file so big? What are some ways to prevent this problem from occurring? What do I do when I get myself on track with the underlying cause and want to put my transaction log file to a healthy size?


I want to use the property of "caller" for a function which is defined   It works fine for this style of function declaration      function g() {       alert(g.caller.name) // f }     function f() {       alert(f.caller.name) // undefined       g() }  f()     But then my function declaration is something like       g = function() {       alert(g.caller.name) // expected f, getting undefined     }      f = function() {      alert("calling f")      alert(f.caller.name) // undefined      g()    }   f()   and I am getting undefined (basically not getting anything)     Is there any way that I can use the caller property without having to rewrite my code? Also, I hope I have not made any mistakes in usage and function declaration since I am quite new to using JS.

I've recently started maintaining someone else's JavaScript code. I'm fixing bugs, adding features and also trying to tidy up the code and make it more consistent.  The previous developer used two ways of declaring functions and I can't work out if there is a reason behind it or not.  The two ways are:  var functionOne = function() {     // Some code };     function functionTwo() {     // Some code }   What are the reasons for using these two different methods and what are the pros and cons of each? Is there anything that can be done with one method that can't be done with the other?


The list of users one can click to log in from the login screen has disappeared. This is what I have done so far:  My C:\Users folder was in reality a NTFS Junction that pointed to D:\Users, so when D:\ was unavailable due to a temporary hardware problem, I got into some problems with my setup.  After    removing the junction pointing to the other volume copying all the profiles back into C:\Users mucking around in the Registry in the HKLM.../Windows NT/CurrentVersion/ProfileList and reverting any profiles that had gotten a .bak postfix setting the "Flags", "RefCount" and "State" values to 0 in the same key (I only did this on my own user accounts - not all)   logging in finally worked as before.   With one difference.   The login screen no longer showed the list of available users! Previously I could just select an icon, but now there is only an icon for the last logged in user, as well as "Other user". I have tried many tips so far, most of which involve fiddling with config settings available through gpedit.msc. Any tips on how I could restore the easy login that is the standard?  I wonder if I could delete any of the other register keys in ProfileList. Any clues?

I recently had a profile problem with my Windows 7 PC. My original profile in the registry had ".bak" appended to it and a new profile was created. I was unable to login with the new profile. I fixed this immediate problem by logging on in safe mode. This enabled me to restore my original profile.   However, since that moment the login screen now operates differently. Instead of showing icons for all the users with accounts on the PC, it now only shows two icons. The first icon is the last user who logged on and the second icon always shows "Other User". I have tried several different solutions recommended by other people with similar problems, but none of them have fixed the problem. I think the person who started this  has the same problem, but none of the proposed solutions helped him either.   Any help much appreciated.


How do I view the code behind an .exe program on Linux? I want to see the programs code because I want to see how it works. I am just learning coding, so I thought the program would be cool to look at!

I would like to know where to find the source code of Ubuntu. I'd like to see how far it is "open source".


I have a COC level 36 account and I'm playing on my tablet. I downloaded bluestack to my computer and installed COC so that I can play with both my computer and tablet. However, after I linked my level 36 account to the new level 2 account, I lost my level 36 account. I have used my tablet and computer with 1 Google account. How do I get back my level 36 account?

I got Clash if Clans on my laptop and started a village. Then a couple of weeks later it said "This app was not installed properly". So I went and redownloaded it but it started a new account. How do I reclaim my old one back?


Let's say that there is a question asked in a bad manner, in a way that the question is missing. For example the user shows the code and doesn't explain very well which is the problem.    Usually someone starts digging in the OP mind posting comments and asking him for details. It may happen that the question gets edited and the OP edits it to a valid question.    Do you usually give the OP time to edit the question or you immediately vote to close it?

Since it's currently impossible to delete a "vote to close",* when I see a poorly asked question, should I immediately vote to close, or should I comment, and give the OP a chance to improve his question?  Waiting increases the chance that I'll forget, and never vote to close, potentially leading to a cluttered site.  Voting to close immediately increases the chance that the question will be closed, and the author will be forced to re-post his (hopefully improved) question, which leads to a cluttered site. :)  *It is now possible to , but that doesn't change the fundamental nature of this question.


How can I prove that ${2^n-1\choose k}$ and ${2^n-k\choose k}$ always returns odd numbers? It is possible to prove this by congruence?  by the way : $0 \leq k \leq (2^n-1)$

By Newton's Formula:  $$(a+b)^n = \sum_{k=0}^n \binom{n}{k} a^{n-k}b^k $$ Proof that every $\dbinom{n}{k}$ is odd if and only if $n=2^r-1$.   I have already shown that if $n$ is of the form $2^r-1$, having used the property  $$\binom{n-1}{k} = \binom{n}{k}-\binom{n}{k-1}+ \binom{n}{k-2} - \cdots \pm \binom{n}{0}.$$ But I have not been able to demonstrate "$\Rightarrow$".   Please, help me! Thanks.


I recently just obtained my visa to enter the US. I'm a Mexican with a Mexican passport. My name on my passport is as follows: First Name: Maria Second Name: de Lourdes.   My name on my visa is: First Name: Maria Second Name: Lourdes.   My question is if I will have trouble entering the US because one says "Maria de Lourdes" and the other document says, "Maria Lourdes". The last names appear the same, my issue is only with the 2nd name.

The old passport with a still valid US visa on it has Given Names: XXX YYY and Surname: YYY  Due to this my visa has the same combination i.e. full name reading XXX YYY YYY (surname twice at the end).   On my new passport this was corrected to Given Name: XXX Surname: YYY  Will this cause an issue during travel? The new passport has the old passport number officially recorded in the alterations page.   I would like to clarify that the name change in the new passport was correcting the old mistake (a clerical error). It is not an official name change with legal documents to back it.


I am trying to create a script that resets the viewport display color of all selected objects that have a material. I'm talking about this value:  Any help would be appreciated!

How can I define the different properties of an Cycles/Eevee material? Such things as glossy, glass, diffuse, RGB-color, emissive via python scripting?


Why is below happening and how can i use this under the EnglishHomepage ?  my code(not working) :  public class EnglishHomepage {      WebDriver driver = null;       public Actions action = new Actions(driver);      By login_button = By.xpath("//*[@id=\"wrapper\"]/div[1]/header/div[2]/div[1]/div/a[15]");       //constructor of EnglishHomepage     public EnglishHomepage (WebDriver driver) {          this.driver = driver;      }      public void clickFogotPassword() {         driver.findElement(login_button ).click();     }       public void clickDropdownValue() {          action.doubleClick(driver.findElement(xpath2)).perform();         } }   code (working)  public class EnglishHomepage {      WebDriver driver = null;       By login_button = By.xpath("//*[@id=\"wrapper\"]/div[1]/header/div[2]/div[1]/div/a[15]");       //constructor of EnglishHomepage     public EnglishHomepage (WebDriver driver) {          this.driver = driver;      }      public void clickFogotPassword() {         driver.findElement(login_button ).click();     }       public void clickDropdownValue() {          **Actions action = new Actions(driver);**         action.doubleClick(driver.findElement(xpath2)).perform();         } }   I can use public Actions action = new Actions(driver); under the clickDropdownValue method and test is working fine as expected.   however,  when i move public Actions action = new Actions(driver); under the class, could see java.lang.NullPointerException.   exception      at java.util.Objects.requireNonNull(Objects.java:203)     at   org.openqa.selenium.interactions.Actions.(Actions.java:65)  at   pages.EnglishHomepage.(EnglishHomepage.java:12)     at   testcases.Tc4_Market_status.click(Tc4_Market_status.java:27)  at   sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)   at   sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)     at   sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)     at java.lang.reflect.Method.invoke(Method.java:498)     at   org.testng.internal.MethodInvocationHelper.invokeMethod(MethodInvocationHelper.java:133)     at org.testng.internal.TestInvoker.invokeMethod(TestInvoker.java:584)     at   org.testng.internal.TestInvoker.invokeTestMethod(TestInvoker.java:172)     at   org.testng.internal.MethodRunner.runInSequence(MethodRunner.java:46)     at   org.testng.internal.TestInvoker$MethodInvocationAgent.invoke(TestInvoker.java:804)     at   org.testng.internal.TestInvoker.invokeTestMethods(TestInvoker.java:145)     at   org.testng.internal.TestMethodWorker.invokeTestMethods(TestMethodWorker.java:146)     at   org.testng.internal.TestMethodWorker.run(TestMethodWorker.java:128)     at java.util.ArrayList.forEach(ArrayList.java:1257)     at   org.testng.TestRunner.privateRun(TestRunner.java:770)     at   org.testng.TestRunner.run(TestRunner.java:591)    at   org.testng.SuiteRunner.runTest(SuiteRunner.java:402)  at   org.testng.SuiteRunner.runSequentially(SuiteRunner.java:396)  at   org.testng.SuiteRunner.privateRun(SuiteRunner.java:355)   at   org.testng.SuiteRunner.run(SuiteRunner.java:304)  at   org.testng.SuiteRunnerWorker.runSuite(SuiteRunnerWorker.java:53)  at   org.testng.SuiteRunnerWorker.run(SuiteRunnerWorker.java:96)   at   org.testng.TestNG.runSuitesSequentially(TestNG.java:1180)     at   org.testng.TestNG.runSuitesLocally(TestNG.java:1102)  at   org.testng.TestNG.runSuites(TestNG.java:1032)     at   org.testng.TestNG.run(TestNG.java:1000)   at   org.testng.remote.AbstractRemoteTestNG.run(AbstractRemoteTestNG.java:115)     at org.testng.remote.RemoteTestNG.initAndRun(RemoteTestNG.java:251)     at org.testng.remote.RemoteTestNG.main(RemoteTestNG.java:77)

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?


A duck is in the center of a square pond and cannot fly.  A fox is in the corner of the pond and cannot swim.  If the duck can swim with speed 1, and both play optimally, what is the minimal fox movement speed $v$ such that the fox can prevent the duck from escaping the pond?  I expect there is a closed form solution, and here is my thinking about how to solve it. The combined positions of the duck and the fox form a point in three dimensional space.  At the critical speed $v$, there should be a critical set of positions such that the duck can approach arbitrarily close to the set, but if the duck reaches just beyond it, the duck can escape.  Analysis of the puzzle, or perhaps numerical simulation of the perfect play, should reveal the geometry of this set, after which the problem may become straightforward.  For the circular pond (of radius $r$), the solution is well-known:  Near the critical speed, the duck stays opposite of the fox until the duck gets to distance $r/v$ from the center, and then the duck continues on the straight path (tangent to the radius $r/v$ circle) to the edge of the pond, just barely escaping at $v=4.603...$.  For the square pond problem, the lack of rotational symmetry makes the game state three-dimensional and thus harder.  Update:  See  for the generalization to a regular polygon and some good but (at least as of this writing) suboptimal strategies.  I am looking either for an exact solution, or an accurate numerical simulation of the perfect play, including a graph showing the perfect play at the critical speed (minus epsilon, thus allowing the duck to escape). Despite the simplicity of the problem, the continuous nature of the play makes efficient accurate simulation tricky.

This is related to   Suppose there is a boy in the center of a regular $n$-gon. The teacher is on the edge of the $n$-gon (but cannot leave the edge) and wants to capture the boy. At the beginning he is on a vertex. The teacher is $v(n)$ times faster than the boy. Which is the maximum $v(n)$ such that the boy can escape? (By escaping means he reaches the edge of the $n$-gon and the teacher is not there)  From the linked question we know $3 \le v(4) < 6$, and for $n= \infty$ (a circle) then I know a strategy such that $v(\infty) = \pi + 1$ suffice; I don't know if this is optimal. I also put convergence in the tags because my wild hypothesis is that the maximum $v(n)$ will converge to some value and it would be interesting to know which one!  Any cool way to solve this?


Given any sequence whatsoever of real numbers (a_r), there is a smooth function $f: \mathbb{R} \to \mathbb{R}$ such that $f^{(r)}(0) = a_r$.  Pugh's hint says to try $f=\sum \beta_k(x)a_kx^k/k!$, where $\beta_k$ is a well-chosen bump function.  I'm working with a few other people and we're trying to define a bump function $\beta_k(x)$ = bump $*b_k*x$ where from 1/2 to 1, $(exp(\frac{1}{|x|-1}))(1-exp(\frac{-1}{|x|-1/2}) + C_k$ but have no idea how to define

Do you have some reference to a proof of the so-called Borel theorem, i.e. every power series is the Taylor series of some $C^{\infty}$ function?


My confusion is-  1) While taking limits, tension at end B of rod is taken 0, The function comes showing maximum Tension at end A.  But it seems to me that that at end A, T must be 0 as x=0 and at end B it must be max. Why?

A rod of length $L$ & mass $M$ is rotating in a circle about one end then calculate tension in the rod at a distance $x$ from the support. For its solution why should we take mass of $L-x$ portion of rod instead of taking mass upto $x$ distance from support as we have the formula, $T = m{\omega}^{2} x$ I am in confusion....


I have a question about using WolframAlpha, but I'm not sure which site I should ask on. My question is "Does Wolfram Alpha support ?".  Would it be more appropriate to ask about WolframAlpha on , , , or ?  All four sites have tags for WolframAlpha, meaning that certain questions could be at least somewhat on-topic for each site.

There seems to be more than a few computer science/programming Stack Exchange networks (is that the correct term?). Stack Overflow, being the first, has by far the most users, questions, and answers.  What is the reasoning for creating the others, and are there clear guidelines for which kinds of questions should be posted? I can see a large amount of potential overflow, many cases of people not getting a good answer to their question, because the person who has the answer isn't browsing that particular network at the moment.  I understand that they were probably created for organizational purposes, but wouldn't it almost make more sense to just have them as categories under Stack Overflow, keep them separated but still connected, instead of making people have to create multiple 'account's, one for each network?  I am sure there was a very good reason to break them up, but as someone that is new to SE, it can be somewhat intimidating to decide which one to post in to ensure you get a good answer. For example, if I am a computer science student, my first instinct might to be to post in the computer science network, until I see that it literally has 1% of the users as the Stack Overflow network, which still seems to be for programming/computer science related questions.  So my second instinct would then to be to post it into the Stack Overflow site on the basis that I have a significantly higher chance of my question being seen and getting a good answer. Then I notice that there is also a 'programmers' network, and I don't even begin to know where that fits in. I, and I am assuming most people, will probably just post on Stack Overflow to be safe.  My question is, other than the short little description blurb of each one, is there a clear cut set of guidelines which what each network is intended for, what kinds of questions should go to each one, and is anything being done to encourage people to post in these newer, smaller (more specialized?) networks as opposed to just posting in big daddy Stack Overflow?  Now again, this is for computer science/programming/"why isn't this code doing what I want?" related questions, I am not saying that if I had a question about Linux or WordPress or something I would have the same confusion.


If I have two cipher texts lets say $C_1$ and $C_2$ of the same length encrypted through stream cipher technique using the same keystream. Let's say they are:  $$C_1: \texttt{96 C6 A1 08 E7 F2 33 3B 3F 5C AB}$$  $$C_2: \texttt{90 C6 A1 1E E6 F3 31 2B 37 4A B6}$$  $C_1$ is encrypted as ($P_1 \oplus \text{Keystream}$) and $C_2$ by ($P_2 \oplus \text{Keystream}$) where $P_1$ and $P_2$ are corresponding plaintexts.    I am asked to tell how can I differentiate between corresponding plain text $P_1$ and plain text $P_2$ from $C_1$ and $C_2$ as an attacker without knowing the keystream?   So, I think the answer would be since both ciphers are encrypted through the same key stream, they would have similarities where the same plain text and keystream value exists. In this way, I can differentiate the other parts of the plain text. Is there anything more to it?  Thanks.

Suppose Alice wants to send encryptions (under a one-time pad) of $m_1$ and $m_2$ to Bob over a public channel.  Alice and Bob have a shared key $k$; however, both messages are the same length as the key $k$.  Since Alice is extraordinary lazy (and doesn't know about stream ciphers), she decides to just reuse the key.  Alice sends ciphertexts $c_1 = m_1 \oplus k$ and $c_2 = m_2 \oplus k$ to Bob through a public channel.  Unfortunately, Eve intercepts both of these ciphertexts and calculates $c_1 \oplus c_2 = m_1 \oplus m_2$.     What can Eve do with $m_1 \oplus m_2$?     Intuitively, it makes sense that Alice and Bob would not want $m_1 \oplus m_2$ to fall into Eve's hands, but how exactly should Eve continue with her attack?


I have to create a cascade of elements dynamically.  Prix:
      <select  name="prix" id="prix">         <option value="0">Type de prix</option>         <option value="1">Gratuit</option>         <option value="2">Montant</option>         <option value="3">Echange</option>         <option value="4">Prix à discuter</option>         <option value="5">Prix n'est pas d'application</option>     </select>

   When I choose option value="2", a checkbox have to be inserted into div id="select_prix"  So I use jquery:  $('#prix').on('change', function() {     if ($(this).val() < 2) {         $("#select_prix").empty();     }        var stroffre  ='Offre ?                                  ';     stroffre +='
';     stroffre +='';     stroffre +='<input class="sr-only" id="chkoffre" type="checkbox" value="">';     stroffre +='Autoriser les  offres';     stroffre +='
';      if ($(this).val() == 2) {         $("#select_prix").append(stroffre);     } });   Ok. The checkbox appears and what I get is:     checkbox ' Autoriser les offres'   Now, when I select checkbox , I want to insert a new line input into div id="select_offre"  I made for that a new jQuery code:  $("#chkoffre").change(function () {     alert( 'CheckboxOffre' ); // or $(this).val()     if($(this).is(":checked")) {      }else{      } });   Unfortunately, I don't get the alert message.

I have a bit of code where I am looping through all the select boxes on a page and binding a .hover event to them to do a bit of twiddling with their width on mouse on/off.  This happens on page ready and works just fine.  The problem I have is that any select boxes I add via Ajax or DOM after the initial loop won't have the event bound.  I have found this plugin (), but before I add another 5k to my pages with a plugin, I want to see if anyone knows a way to do this, either with jQuery directly or by another option.


I'm wondering how to allow non-identified developers' apps since I updated my MacBook Pro to macOS Sierra.     This option seems to have disappeared.   Can someone help?

In previous versions of the OS in the security pane, you are provided an option of:    But in this version there is no option anywhere:    Is there a way that I can turn on anywhere? (via Terminal maybe?)


I'm having trouble validating email addresses with JS. Here is my code:  var regEmail = /^([a-zA-Z0-9_.+-])+\@(([a-zA-Z0-9-])+\.)+([a-zA-Z0-9]{2,4})+$/i; var result = regEmail.test('username'); alert(result);   So no matter what kind of email address I type, it always shows "false". I copied the regex expression from somewhere on stackoverflow, but I think it should actuylly work...  EDIT  $(document).ready(function() {          $('#submitBtn').click(function() {          //Überprüfe ob der Benutzername leer ist         var username = $('#username').val();         if (username == ""){             alert ("User name cannot be empty!");         }          //Email validation          var regEmail = /^([a-zA-Z0-9_.+-])+\@(([a-zA-Z0-9-])+\.)+([a-zA-Z0-9]{2,4})+$/i;         var result = regEmail.test('username@example.com');         alert(result);          });     });

Is there a regular expression to validate an email address in JavaScript?


Given the finite group $(G, \cdot)$ with $2n$ elements, $n \in N^*$ and also H a subgroup of G with $n$ elements.   Prove that if $x \notin H, y \notin H$ then $xy \in H$.  I'm not really sure how to do this since I've barely started working with subgroups. Could I have some hints on how to approach this type of exercise? Thank you.

Let $G$ be  a group and $H$ be a subgroup of $G$ of index $2$ .Then show that if $a,b\notin H$ then $ab\in H$.   $a,b\notin H\implies aH,bH\neq H\implies $ $aH=bH$  since $H$ has only two distinct left cosets in $G$.  $\implies a^{-1}b\in H$  How to show that $ab\in H$ from here?


Gmail has a nice feature that permanently deletes e-mails after they've been in the Trash folder for 30 days. This is pretty clever, because if I don't need a trashed message back after 30 days it's very likely that I won't ever need it again.  I want to do the same for my Trash folder in OS X to make the best use of my disk space. Is there an easy way to automatically delete trashed files after 30 days?  Meta note: I don't think this is a duplicate of  because there the trigger is "disk space drops below X GB" and the effect is "the entire Trash folder is emptied". I only want to autodelete files older than 30 days.

Sometimes my hard drive gets low on space, and various things don't work properly until I empty the Trash and free up some space. I know there are applications that empty the Trash after a set interval—but what I want is an app or script (or other means) to empty the Trash whenever there's e.g. less than 2GB of free space on a given hard drive.


I am going to school. I am going to the school. I'm going to my school.   Clearly, number 2 is grammatically wrong but has its exceptions.  What are they?  Also, students would number one.  Do teachers say number 2?  Parents say number 2 when they are off the pick up their child or going to the school for whatever reason.  What does a university student say? 1. I am going to the university. 2. I am going to university. 3. I am going to my university.  What about college? 1. I am going to the college. 2. I am going to college. 3. I am going to my college.   I am going to the temple. / I am going to my temple. I am going to church. / I am going to my church. I am going to the mosque. / I am going to my mosque.   The church is a social institution but are the temple and mosque not?  Is my wrong?   I am going to the apartment. I am going to my apartment. I am going to the apartment over there.   Number 2 is most common and grammatically correct.  Is number 1 grammatically wrong?  Why?  When do you use "the"?  Number 3 is correct and purposely vague.   I am going to the classroom. I am going to my classroom.   Both are correct but does number 2 mean the student's homeroom?  Does "the" sometimes imply not yours?   I am going to the concert. I am going to a concert.   Number 1 is wrong but why is "the concert" wrong?   I am going the doctor. I am going to the doctor's office.   Can both of their locations be in the hospital?  Do they mean the same thing?  I know the doctor's office can possibly be in the hospital but also it can be at their facility not having to do with the hospital.  But can you explain if there is a difference.   I'm going to bed. I'm going to my bed.   Are they both the same?  Thanks in advance!!!

Why do British speakers omit the article in constructions like "go to hospital" or "go on holiday"? Pretty much all American speakers would rephrase those as "go to the hospital" and "go on a holiday", I think. Is there any good reason, or forgotten sense behind those words that might explain why the articles are ommitted?  Are there other common constructions other than those two that the British use that drop the article?  EDIT: I just realized per Kosmonaut's comment that Americans do much the same thing with a few nouns, so this isn't all that special.   Do grammaticists designate nouns that can have their article dropped with anything, i.e. do they have anything in common?


Consider the following:  <head>     <script src="somejsfile.js"></script>     <script>         var firstScript = document.getElementsByTagName("script")[0];         firstScript.innerScript = firstScript.innerScript.replace("foo","test");     </script> </head>   somejsfile.js:  console.log("foo");   Now, this (pseudo)code is for illustration only; it does not work, because there is no innerScript element. So, is there a way to get/manipulate console.log("foo"); using JavaScript DOM manipulation, in a similar way to iframe.innerDocument?  (EDIT: If you would like to know what I need it for:  I am trying to debug whether the script file is loaded or a 404 is occurring. My script contains console.log in the first line, and that line is not executed inside my WP8 xap cordova application.)

If I have a script tag like this:  <script     id = "myscript"     src = "http://www.example.com/script.js"     type = "text/javascript"> </script>   I would like to get the content of the "script.js" file. I'm thinking about something like document.getElementById("myscript").text but it doesn't work in this case.


I have been trying to get a program called "passenger-memory-stats" to run as the super user. The program is installed in the following directory: /usr/local/rvm/rubies/ruby-1.9.3-p327/bin/ruby . I know this because i installed phusion passenger using ruby gems:  $ gem env RubyGems Environment:   - RUBYGEMS VERSION: 1.8.24   - RUBY VERSION: 1.9.3 (2012-11-10 patchlevel 327) [x86_64-linux]   - INSTALLATION DIRECTORY: /usr/local/rvm/gems/ruby-1.9.3-p327   - RUBY EXECUTABLE: /usr/local/rvm/rubies/ruby-1.9.3-p327/bin/ruby   - EXECUTABLE DIRECTORY: /usr/local/rvm/gems/ruby-1.9.3-p327/bin   - RUBYGEMS PLATFORMS:     - ruby     - x86_64-linux   - GEM PATHS:      - /usr/local/rvm/gems/ruby-1.9.3-p327      - /usr/local/rvm/gems/ruby-1.9.3-p327@global   - GEM CONFIGURATION:      - :update_sources => true      - :verbose => true      - :benchmark => false      - :backtrace => false      - :bulk_threshold => 1000   - REMOTE SOURCES:      - http://rubygems.org/  /usr/local/rvm/gems/ruby-1.9.3-p327/bin $ ls -l | grep passenger-memory-stats -rwxr-xr-x 1 guarddoggps rvm 416 Dec 14  2012 passenger-memory-stats   So obviously the program is there. I then add it to the root user's PATH environment variable:  $ sudo su export PATH=$PATH:/usr/local/rvm/gems/ruby-1.9.3-p327/bin echo $PATH /usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/usr/local/rvm/gems/ruby-1.9.3-p327/bin   So now it has been added to PATH environment variable. I exit from sudo user to my regular user with sudo privilege and I run the following:  $ sudo passenger-memory-stats sudo: passenger-memory-stats: command not found sudo -E passenger-memory-stats sudo: passenger-memory-stats: command not found   Why does it tell me command is not found? Didn't I add it to my sudo user's path? So I check:   sudo su  echo $PATH /usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games   ANd to my surprise, the directory is no longer in the path. What happened?

for normal user I get :     /usr/lib/lightdm/lightdm:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/home/monty/google_appengine   which is actually the content of /etc/environment  For root I get:     /usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin   what's the reason behind this and which file contains this line?


Suppose $a \in \mathbb{R}$, $v \in V$, and av = 0. Prove that $a = 0$ or $v = 0$.   Clearly if $a = 0$, then $av = 0v = 0$.  If $v = 0$, then $av = a0 = 0$.  Is this rigorous enough to be an acceptable proof answer? I feel like I am missing something here.

Is there an example of a vector space $V/F$ such that $0 \ne c \in F$, $\vec{0} \ne v \in V$, but $c \cdot v = 0 $?  I couldn't think of an example.


When I went to the home page of MSE today, this is what I see:  It seems as though the "latest activity" text is sticking to the tags. Did a recent change cause this, or is this intended behavior? To reproduce this bug, go to the home page of any SE site or it's per-site meta. It only happens on the "Active questions" tab, so clicking on "Questions" in the side bar won't repro the bug. The only thing I can think of that could cause this was  Note that this is network wide:

In the  (homepage), the asked/modified X mins ago text was left-aligned next to the tags. Previously the text was right-aligned. Seems some alignment issue introduced on that page recently. Screenshot for reference:      Configuration Version     Operating System Windows 10 Pro   Mozilla Firefox 89.0 (64-bit)   Google Chrome 91.0.4472.101 (Official Build) (64-bit)     Update: I see the same alignment issue in all other Stack Exchange site's "Top Questions" page too.


I have this PHP code below:   $value = -.9; for ($i = 0; $i <= 6; $i++) {     echo "value = $value , ";     $value += .3; }   Output:  value = -0.9 , value = -0.6 , value = -0.3 , value = -1.11022302463E-16 , value = 0.3 , value = 0.6 , value = 0.9 ,    Why am I getting -1.11022302463E-16 and not 0? And what can I do to correct the problem? Thanks again.

Consider the following code:   0.1 + 0.2 == 0.3  ->  false   0.1 + 0.2         ->  0.30000000000000004   Why do these inaccuracies happen?


Q. Let $T: C[0,1] \rightarrow C[0,1]$ be a mapping defined by $$ T f(x)=\int_{0}^{x} f(s) d s $$ Show that: $T$ is NOT a contraction but $T^{2}$ is a contraction. It seems to me that $\begin{aligned}\left|\int_{0}^{x}(f(s)-g(s)) d s\right| & \leq \operatorname{sup}_{[0,1]}|f(s)-g(s)| \int_{0}^{x} d s \\ & \leq \sup _{[0,1]} |f(s)-g(s) |\end{aligned}$ What point I have missed here?

Let $\psi: C([0,1]; \mathbb{R}) \mapsto C([0,1]; \mathbb{R})  $, be such that if $x \in [0,1]$ then $$\psi(f)(x) = \int\limits_{0}^{x} f(t) dt $$  I am asked to show that $\psi$ is not a contraction but $\psi^{2}$ is. And then conclude that $\psi$ has a unique fixed point.   I´ve managed to prove a statment that helps me conclude the last part but I´m having trouble proving that the function is not a contraction. This is what I've got.   If $f,g \in  C([0,1]; \mathbb{R})$  then $$ |\psi (f) - \psi(g)| \leq \int\limits_{0}^{x} |f(t)-g(t)| dt \leq \sup\limits_{x\in [0,1]} |f(x)-g(x)| \int\limits_{0}^{x} 1 dt = d(f,g) x  $$ Then $|\psi (f) - \psi(g)| \leq d(f,g) x $,  and taking supremum on the left hand side allows me to conclude that $d(\psi (f),\psi (g)) \leq d(f,g) x$  Is there any problem with this procedure? Does anybody has an insight on how to procede with the $\psi^{2}$ part?   Many Thanks!


I have lately got the privilege of protecting questions on Islam SE. Maybe I don't really know how this works, but i wonder, why on some questions i would get this opportunity as it is clearly displayed, while on other questions it isn't displayed at all:  For example  I can't even see the option to protect, even if the post is older than 24h.   While if i would like to protect  post. I could easily do so as the option is displayed.   I'd be glad if you could tell me whether this is a bug or not and why sometimes the "Button" appears and sometimes not.

I've found a trend that I'd like to discuss. There are users that have reached 15K+ reputation that are participating in a question and then protecting it so that low reputation users can't answer. This wouldn't be a problem if the question was attracting poor, low quality or spam answers from these low reputation users. However, in these cases, that's not what's happening.  The  privilege has the following guideline:     Questions should be protected when they are garnering lots of views and newbies are adding "me too!", "thanks!" and possibly even spam non-answers.   Some recent examples to consider:     This Q&A are by the same user. There are no deleted answers on the question. (14 views)   This Q&A are by the same user. There is a second answer that is not the highest of quality, but the user has 200+ reputation and wouldn't be affected by the protection anyway. (23 views)  This question was protected by the author of the only answer. There are no competing answers. (36 views)  This question was protected by the author of the only answer. There are no competing answers. (37 views)  This question was protected by the author of the only answer. There are no competing answers. (50 views)  This question was protected by the author of the only answer. There are no competing answers. (64 views)  This question was protected by the author of the question (not the answer). There is only one answer. (140 views)   Proposal  I'd like to propose a couple small changes to how a user can protect a question:  The question should not be able to be protected if all of the following are true   Has less a low number of views. We use 100 views to determine when . I think 100 would work here too. None of the posts have garnered low quality or spam flags None of the answers have a score less than 0 The user protecting the question is a participant   If any of those criteria are no longer true, then the participants should be able to protect it normally. I worked with  to spot check the flags on the above posts. There are not any low quality or spam flags.  This means that the final question in my example would be able to be protected by both participants.  If you haven't answered the question, you would be able to protect a question as normal. The idea is that the author of the question or answers can't protect if if the other three criteria are true  Motivation  The idea is to prevent locking out low reputation users from these questions. They are not gathering the types of answers mentioned in the  that require a post to be protected. Instead, it looks like a conflict of interest. The user protecting the post is one of the users that can benefit the most from preventing competing answers.


I am from Indian. My parents are staying in India but I am working in China. I need to apply for UK Visa from China. Do I need to submit any documents as a support for "evidence of family members remaining in your home country whilst you travel"? If so what kind of document I suppose to ?

Applying for Standard Visitor Visa from Russia while being Russian.  In case I have children and wife, I apply the marriage certificate and children birth certificates with the rest of documents.   Is that the evidence of family members remaining in your home country whilst you travel?  If it is not please let me know what kind of document it might be? I have not managed to find an answer to that question. Thank you in advance!


Let $K$ and $K'$ be two inertial frame and $K'$ moving with an infinitesimal velocity $\epsilon$ relative to $K$, i.e. $\vec{v'} = \vec{v} + \epsilon$. Let $L$ be the Lagrangian of $K$, $L'$ be the Lagrangian of $K'$ then we have $L' = L(v'^2) = L(v^2 + 2 \vec{v}\cdot\epsilon+ \epsilon^2)$ We can use Taylor series expansion and neglect second order terms: $$L(v'^2) = L(v^2) + 2\frac{\partial L}{\partial v^2}\vec{v}\cdot\epsilon$$ Landau then proceeds to argue that when $2\frac{\partial L}{\partial v^2}\vec{v}\cdot\epsilon$ is a linear function of $\vec{v}$, $L(v'^2) - L(v^2)$ is a total time derivative. Therefore he deduced that $L = \frac{m}{2}v^2$ My question is: Why $2\frac{\partial L}{\partial v^2}\vec{v}\cdot\epsilon$ is a linear function of $\vec{v}$? Why it is a total derivative of time of function $f(x,t)$?

We have two inertial coordinate systems, $K'$ and $K$. $K$ is moving with infinitesimal velocity ${\epsilon}$ relative to $K'$. Using Galilean relativity we can transform this into $v'=v+{\epsilon}$. Here Lagrangian is only a function of speed $L=L(|v|^2)$. $L'$ may differ from $L$ only by a total derivative of function of coordinates and time, i.e. $L'=L+\frac{d}{dt}f(q,t)$  $"$We have $L'=L(|v|^2+2{v}·{\epsilon}+{\epsilon}^2)$. [1. ] Expanding this expression in powers of ${\epsilon}$ and neglecting terms above the first order, we obtain $L(|v'|^2)=L(|v|^2)+\frac{\partial{L}}{\partial{|v|^2}}2v·{\epsilon}$ [2.] The second term on the right of this equation is a total time derivative only if it is a linear function of the velocity $v$ $"$  I understand nothing in the quoted part. How do we get [2.]? Why does $\frac{\partial{L}}{\partial{|v|^2}}2v·{\epsilon}$ being a linear function of velocity imply that it's a total time derivative?


Suppose $f: \mathbb{R} \rightarrow \mathbb{R} $ is differentiable and $g: \mathbb{R} \rightarrow \mathbb{R} $ is infinitely differentiable, i.e. $ g \in C^{\infty}(\mathbb{R})$, where we know $f'(x) = g(f(x)) $ on $\mathbb{R}$. Show that $ f \in C^{\infty}(\mathbb{R})$.   Thus I have to show that $f$ is infinitely differentiable, that is, derivatives of all orders exist. I can assume by induction that all derivatives of order less than, say $n$, exist, and have to show that the $nth$ derivative exists for $f$.   I came up with this: $f^{n}(x) = (g \circ  f)^{n-1}(x)$. I somehow have to show that the $(n-1)th$ derivative for this composite function exists. I tried using the chain rule, but it just seems to become more ugly as I continue taking more derivatives. Obviously, I have to use the fact that $g$ is infinitely differentiable as well as the inductive assumption, although I'm not sure how to complete this line of reasoning. Maybe induction isn't even the right way to proceed.   Ideas?

If there is a function $r(x)$ that is infinitely differentiable, prove that $k(x)$ is also infinitely differentiable if $k'(x) = r(k(x))$ for all $x$.   I am trying to somehow prove using induction. However, after taking many derivatives($1$st, $2$nd, $3$rd order), I have not been able to get to the necessary result.


I want to change my device name but I have not found any option in the settings to do it. I am using Ubuntu for first time. Is there any way to change the device name that appears when I lock the desktop?

I want to change the OS hostname but I do not want to restart.  I have edited /etc/hostname but it requires a restart to get implemented. How to avoid this?


As I understand it, AES in CBC mode does an XOR of the 128-bit initialization vector and the first 128 bits of plain text. Is this correct?  I am using the PHP 5.3 openssl extension in aes-256-cbc mode. I'm experimenting with a round trip, that is, encrypting and then decrypting to make sure I get back the original string.  I find that if I encrypt with one initialization vector and decrypt with another initialization vector, using the same secret key, the first 128 decrypted bits are corrupt (as expected) and the rest is the correct plain text.  I find that if I encrypt with one initialization vector, and decrypt with a barely-different initialization vector, the decryption shows barely-different plain text. That is, when I change the third byte of the initialization vector, the decrypted plain text shows the third byte corrupt and everything else correct.  This does seem to show how the initialization vector affects AES cipher-block chaining. Is the initialization vector simply XOR'd with with the first 128 bits of plain text, and that's it?

Let's say that there is a binary file encrypted with AES in CBC mode (i.e. using a key and initialization vector). If key is known, but IV is not, is it easy to fully decrypt the file?  How hard is it?


As I speak, I noticed that I use the word "have" a lot! Also, I was wondering whether this over-usage is right. For instance,       I have copied the file to your computer     where I could've used       I copied the file to your computer   instead! Am I right or do they differ subtly?

Non-native speakers often get confused about what the various tenses  and aspects mean in English. With input from some of the folk here I've put together a diagram that I hope will provide some clarity on the matter.  I offer it as the first answer to this question. Consider it a living document. Input is welcome, and good suggestions will be  incorporated into the diagram.    Nota bene: What this is not is a discussion of whether there are  more than two tenses in English. , to which this question is not intended to supply arguments one way or the other.  Here, the aim is to provide an overview of what constructions English-speaking people use for conveying information about actions referring to past, present, and future, and to provide it first and foremost to precisely the people who are likely to use "tense" as a catch-all term in their search, rather than to linguists who know better.  Breaking News There is now an excellent ELU blog article titled  . It is highly recommended reading.


Is there a filesystem and encryption combination that works across the 3 operating systems? Assume OSX High Sierra, Windows 10, 16.04. Bonus round if you have successfully done this before (please state in reply).  Read / Write access is required.  It would be best if I could encrypt the flash drive so that it is not easily accessed if it were lost.  That being said it makes sense to select a combination of filesystem and encryption that works across the three platforms in my environment (if possible)

How can I encrypt an external device (i.e. USB keys, external hard-drives, memory sticks, etc.) so that it would subsequently be readable/writable on any computer that I plug it in?   For instance, it should then be possible to easily plug the encrypted device on a Linux, Windows, or OS X machine, and be able to access the contents without any fuss.   How can I do that?


For each of the eight subsets of $\{ \mathrm{reflexive, symmetric, transitive} \}$, find a relation on $\{1,2,3\}$ that has the properties in that subset, but not the properties that are not in the subset.  Here is what I believe works for each. Let us denote the $8$ subsets by $1 = \emptyset, 2= \{r\}, 3 = \{s\}, 4 = \{t\}, 5 = \{r,s\}, 6 = \{r,t\}, 7 = \{s,t\}, 8 = \{r,s,t\}$.  Let $\rho_i$ be a relation with the properties of set $i$.  Then $\rho_1 = \emptyset$, $\rho_2 = \{(1,1)\}$, $\rho_3 = \{(1,2),(2,1)\}$, $\rho_4 = \{(1,2),(2,3),(1,3)\}$, $\rho_5 = \{(1,1), (1,2), (2,1)\} = \rho_1 \cup \rho_2$, $\rho_6 = \{(1,1), (1,2), (2,3), (1,3)\} = \rho_1 \cup \rho_4$, $\rho_7 = \{(1,2),(2,1), (2,3),(1,3)\} = \rho_3 \cup \rho_4$, and $\rho_8 = \{(1,1), (1,2), (2,1), (2,3), (1,3), (3,2), (3,1)\}$, the last one, somewhat surprisingly to me, is not $\rho_2 \cup \rho_3 \cup \rho_4$. My question is, do these $\rho_i$ work? For example in the only reflexive set I do not have $(2,2)$, but I believe this is fine, is it?

Definition: Given a set $X$, a relation $R$ on $X$ is any subset of $X\times X$.  A relation $R$ on $X$ is said to be  reflexive if $(x,x) \in R$ for all $x \in X$, irreflexive if $(x,x) \not\in R$ for all $x \in X$, transitive if $(x,y) \in R$ and $(y,z) \in R$ implies that $(x,z)\in R$, intransitive (or antitransitive) if $(x,y) \in R$ and $(y,z) \in R$ implies that $(x,z)\not\in R$, symmetric if $(x,y) \in R$ implies that $(y,x) \in R$, antisymmetric if $(x,y) \in R$ and $(y,x) \in R$ implies that $x=y$.   Given any combination of the properties listed above, is there a nontrivial (i.e. nonempty) relation which satisfies that combination of properties?


I created a simple shader to subdivide a circle in a specified number of bands. Each bands has a number between 0 and 1 and the factor (color) for each band is (previous bands + 1/NumberOfBands); I would like to know if there is a way to obtain a random number for each band ( between 0 and 1 possibly ) i.e. a number not related to the previous (or the following) band. It would be perfect a node that output a random number with a seed option, and I can plug the color in the seed, any one know if there is an addon for that?

Is it possible to generate a random number for use in the node editor?  I am aware of the Object Info node, but I want a random number which I have more control over. Ideally I could change it with some sort of "seed" vale and be able to define limits for maximum and minimum values.  How is this possible?


Can you raise and lower the indices of basis vectors and dual basis vectors? I know I can write  $$V^\mu = g^{\mu\nu} V_\nu $$  but can I also write  $$\vec{V} = V^\alpha \hat{e}_\alpha = V_\beta \hat{e}^\beta?$$

I have been trying to convince myself that it is consistent to replace basis vectors $\hat{e}_\mu$ with partial derivatives $\partial_\mu$. After some thought, I came to the conclusion that the basis vectors $\hat{e}_\mu$ were ultimately just symbols which represent what we think of as arrows, so it is not a problem to use a different symbol. The only requirement is that one can manipulate the $\partial_\mu$ in the same way as the $\hat{e}_\mu$.  However, raising/lowering indices seems to create an inconsistency. In switching our representation of the basis vectors, we make the substitutions:  $$\hat{e}_\mu \rightarrow \partial_\mu$$  $$\hat{e}^\mu \rightarrow dx^\mu$$  However, while we previously could write $\hat{e}^\mu=g^{\mu \nu}\hat{e}_\nu$, we fail to be able to write the same relationship in the new representation:  $$dx^\mu \neq \partial^\mu =g^{\mu \nu} \partial_{\nu}$$  My questions are:   Have I done something invalid here? If not, is it just an unwritten rule that one should never try to raise an index of a basis vector?  What is the motivation to write basis vectors as partial derivatives or differentials (for the tangent or cotangent space) as opposed to just writing some other symbol? Do we actually need the properties of a derivative or differential in our basis vectors? I am aware that the $\partial_\mu$ resemble the expression $\frac{\partial\vec{r}}{dx^\mu}$ which is a natural choice for the basis vectors $\hat{e}_\mu$, but the differentials seem to come out of nowhere.


While I have seen papers and posts about mean and variance of n i.i.d normal random variables, I am looking for the first order statistics of $n$ (specifically $11$) normal, non-identical (different mean and variances) but independent random variables. Specifically :   What distribution would that follow? Can it be approximated to known distributions? What would be the mean and variance?   I am currently using Monte-carlo simulation for finding this, but would like a more direct value to save computational time.

I just started taking survival analysis class and I'm stumped on this question.  Let $T_1,...,T_n$ random independent continuous variables, with hazard function of $h_1(t),...,h_n(t)$. $T=min(T_1,...,T_n)$. And we need to show that the hazard function of T is $\sum_j{h_j(t)} $  Any help or direction are welcome :)


I am extremely extremely new to Java and programming in general. I wrote this for a basic program to add 2 numbers input by the user and add them up and display them in the output box, however I'm getting Non-static method 'setText(java.lang.CharSequence)' cannot be referenced from a static context, but I don't know what the static thing is  private void onClick(View v) {     EditText input1 = (EditText) findViewById(R.id.input1);     double calc1 =  Double.parseDouble(String.valueOf(input1));     EditText input2 = (EditText) findViewById(R.id.input2);     double calc2 = Double.parseDouble(String.valueOf(input2));     double total = calc1 + calc2;     String result = Double.toString(total);     EditText output1 = (EditText) findViewById(R.id.output);     EditText.setText(result); }   The line giving the error:  EditText.setText(result);   Sorry if I'm being extremely incompetent but I searched and I couldn't really understand how to fix it. Thanks.

The very common beginner mistake is when you try to use a class property "statically" without making an instance of that class. It leaves you with the mentioned error message:     You can either make the non static method static or make an instance of that class to use its properties.   Why? I am not asking for solutions. I would be grateful to know what is the reason behind it. The very core reason!  private java.util.List<String> someMethod(){     /* Some Code */     return someList;             }  public static void main(String[] strArgs){                // The following statement causes the error. You know why..     java.util.List<String> someList = someMethod();          }


How can I downgrade from Safari 12 back to Safari 11?  I’m a blind Mac user, using Voiceover. Safari 12 has various accessibility bugs and thus I need to go back to the latest version of Safari 11.  Here’s what I’ve tried so far: restoring a previous version of my computer from a Time Machine backup. But that didn’t work, because as soon as the system restarted Safari updated automatically, before I could do basically anything.  Any other suggestions?  I’m running macOS High Sierra 10.13.6

Installing Safari beta version is apparently only possible by replacing the builtin Safari version. In this case version 11.1.2 with 12. If beta version does not work, gives too much trouble or is just unpleasant:  How do we downgrade to an official stable version, currently Safari 11.1.2 for 10.12.6 Sierra?  I tried to restore everything relating to 11.1.2 from a backup (going by what is found that is in the installer package), I located and re-applied the 11.1.2_Sierra.pkg. Yet Safari now refuses to work entirely.   Crashed Thread:        3  Dispatch queue: com.apple.SafariCore.RemotePlistController.internalQueue  Exception Type:        EXC_CRASH (SIGABRT) Exception Codes:       0x0000000000000000, 0x0000000000000000 Exception Note:        EXC_CORPSE_NOTIFY  Application Specific Information: *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '*** -[_NSPlaceholderData initWithContentsOfURL:options:error:]: nil URL argument' terminating with uncaught exception of type NSException abort() called   Is there any way around re-installing the entire system?


I remember an animated series from the mid nineties of a girl who is in a wheelchair and gets visited by a lady in a small bubble who could be considered as her guardian angel, this bubble gets bigger and this girl walks into it and goes to have many adventures. I do not remember that part well, but her foe is a really terrifying demon, even for an adult, made of fire who tries to catch the bubble but can't do anything to it and turns some people into vermines and other lower life forms when they fail. I'm assuming it is from the late 70s to early 90s but I'm not sure.

I remember watching a cartoon as a child, in the 80s or early 90s and I can't remember the title (or a lot of details about it). The setting reminds me of "Nadia: The Secret of Blue Water": we could be at the end of the 19th century, but my memory does not support me much in this.   Anyways, the protagonist of the cartoon is a girl on a wheelchair who is supported by a female genie wearing "One Thousand and One Nights" clothes. The genie protects the little girl from the assaults of the minions of an evil entity; in the first episode of the series the minions disguise themselves as workers of a circus just arrived in the town where the little girl lives. For some reasons I do not remember, the evil entity wants to terminate the little girl.  At the end of each episode the minions are inevitably defeated by the magic of the genie and the evil entity transforms them in little worms, for eternal punishment. The worms retain the original heads (so they continue to talk to the evil entity) and live in a rocky-bowl next to their master.


I tried to remove Chess.app from my Mac:    Moving to Trash: “Chess.app” can’t be modified or deleted because it’s required by macOS. Trying to change permissions for everyone in Get Info: The operation can’t be completed because you don’t have the necessary permission. chmod 755 Chess.app/ in Terminal works, but then doing sudo rm -rf Chess.app/: long list including rm: Chess.app/: Operation not permitted   So how can I remove it?

How can I delete Apple OS X application such as Photos, iPhoto, and others that I never use?


How can i set Caps Lock to set Russian language and Shift + Caps Lock to set english? Not just Ctrl + Shift to switch them.

Can I set up keyboard shortcuts for each input language? For example I want to set "Ctrl + 8" for English, "Ctrl + 9" for Ukrainian and "Ctrl + 0" for Russian languages. I don't want to switch and search language I need. It takes time. I want directly to turn language I want. I had this ability  in Windows. Maybe I need to install some plugin?


I am trying to get JSON data via http using Retrofit for Android I am getting the following error:  Attempt to invoke interface method 'retrofit2.Call com.webdealer.otexconnect.goldback.network.LoginServices.sendSMS(java.lang.String)' on a null object reference  Here are my classes.  Client  package com.webdealer.otexconnect.goldback.network;  import retrofit2.Retrofit; import retrofit2.converter.gson.GsonConverterFactory;  /**  * Created by fazal on 3/13/18.  */  public class RetrofitClient {      private static Retrofit retrofit = null;      public static Retrofit getClient(String baseUrl) {         if (retrofit==null) {             retrofit = new Retrofit.Builder()                     .baseUrl(baseUrl)                     .addConverterFactory(GsonConverterFactory.create())                     .build();         }         return retrofit;     }      public void getServies(){         // return RetrofitClient.getClient(BASE_URL).create(LoginServices.class);     } }   Interface:  package com.webdealer.otexconnect.goldback.network;  import com.webdealer.otexconnect.goldback.utils.Login; import com.webdealer.otexconnect.goldback.utils.Sms;  import org.json.JSONObject;  import retrofit2.Call; import retrofit2.http.Field; import retrofit2.http.FormUrlEncoded; import retrofit2.http.GET; import retrofit2.http.POST; import retrofit2.http.Part; import retrofit2.http.Path;  /**  * Created by fazal on 3/13/18.  */  public interface LoginServices {      @FormUrlEncoded     @POST("login")     Call<Login> login(@Field("email") String email, @Field("password") String password);     @FormUrlEncoded     @POST("register-customer")     Call<JSONObject> signUp(@Field("name") String name,@Field("email") String email,@Field("phone_number") String phoneNumber, @Field("password") String password);@FormUrlEncoded     @POST("register-driver")     Call<JSONObject> signUpDriver(@Field("name") String name,@Field("email") String email,@Field("phone_number") String phoneNumber, @Field("password") String password);     @FormUrlEncoded     @POST("send-sms-code")     Call<Sms> sendSMS (@Field("phone_number") String phone_number);    }   Model:  package com.webdealer.otexconnect.goldback.utils;  import com.google.gson.annotations.Expose; import com.google.gson.annotations.SerializedName;  public class Sms {      @SerializedName("status")     @Expose     private Boolean status;     @SerializedName("fourDigitCode")     @Expose     private String fourDigitCode;      public Boolean getStatus() {         return status;     }      public void setStatus(Boolean status) {         this.status = status;     }      public String getFourDigitCode() {         return fourDigitCode;     }      public void setFourDigitCode(String fourDigitCode) {         this.fourDigitCode = fourDigitCode;     }  }   try {                      mLoginServices.sendSMS("+923418007173").enqueue(new Callback<Sms>(){                          @Override                         public void onResponse(Call<Sms> call, Response<Sms> response) {                             Toast.makeText(getApplicationContext(), mPhone, Toast.LENGTH_SHORT).show();                         }                          @Override                         public void onFailure(Call<Sms> call, Throwable t) {                             Toast.makeText(getApplicationContext(),"Failed",Toast.LENGTH_LONG).show();                         }                     });                  }catch (Exception ex){                     Log.e("Error sms",ex.getMessage());                 }

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?