I have this in mind since quite a time. In book it was given that once kepler asked newton what is relation of distance with gravitational force. Newton said inverse square on which kepler concluded motion of planets will be elliptical, it is more easy to generalize it to circle as older attempts. I understand that circle is special eclipse with $a=b$. So my question is how inverse square determines/ derives to elliptical path motion?

How can analytically be  the ?   I found some extremely synthetic equations which from the  (in particular $\mathbf{F} = m \mathbf{a}$) tried to obtain the Kepler's laws, but even if it seemed to be a nice procedure they were too much incomplete.


Can I install Ubuntu on a pentium dual core with 1 gb of ram. If it is not possible please suggest a version. And please tell me, which is the best version of Ubuntu with min errors.

For a given hardware configuration, how do I find out if Ubuntu will run on it? What considerations should I take into account when choosing an Ubuntu version and  such as:    with a lighter desktop than the usual Gnome and Unity  with the even lighter LXDE desktop   Obviously Ubuntu does not run on some processor architectures. So how do I go about choosing the right version and derivate. How can I find out the minmal system requirements?


I want to the following using prepared statements:  <?php   $sql = "SELECT name FROM B WHERE profile_id IN (SELECT profile_id FROM A WHERE login_id='$email')     UNION SELECT name FROM C WHERE profile_id IN (SELECT profile_id FROM A WHERE login_id='$email')";  $result = mysqli_query($conn,$sql);  ?>   Here, A is the main table, B and C are sub tables. In the main table, the primary key is profile_id. The tables B and C have a foreign key referencing A's profile_id.

If user input is inserted without modification into an SQL query, then the application becomes vulnerable to , like in the following example:  $unsafe_variable = $_POST['user_input'];   mysql_query("INSERT INTO `table` (`column`) VALUES ('$unsafe_variable')");   That's because the user can input something like value'); DROP TABLE table;--, and the query becomes:  INSERT INTO `table` (`column`) VALUES('value'); DROP TABLE table;--')   What can be done to prevent this from happening?


Prove algebraically or otherwise:  $$\sum \limits_{r=0}^n {2r \choose r} {2n-2r \choose n-r} = 4^n $$  where ${n \choose r}$ denotes the usual binomial coefficient. I think there is a combinatorial proof using path counting arguments, but I haven't been able to find it.

It's not difficult to show that  $$(1-z^2)^{-1/2}=\sum_{n=0}^\infty \binom{2n}{n}2^{-2n}z^{2n}$$  On the other hand, we have $(1-z^2)^{-1}=\sum z^{2n}$.  Squaring the first power series and comparing terms gives us  $$\sum_{k=0}^n \binom{2k}{k}\binom{2(n-k)}{n-k}2^{-2n}=1$$  that is,  $$\sum_{k=0}^n \binom{2k}{k}\binom{2(n-k)}{n-k}=2^{2n}$$  My question: is there a more direct, combinatorial proof of this identity?  I've been racking my brains trying to come up with one but I'm not having much success.


I want to install ubuntu 16.04. After booting from usb stick and selecting "Install Ubuntu" the screen stays black.   Any ideas, what to do?    Edit:  I can boot from  one of the ubuntu advanced options in grub:  4.4 ubuntu (upstart)  And could figure out the error message:  gr: faild to load fecs_inst

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'm working on a model for VRChat, and the uploader in Unity requests as few meshes and materials as possible. This wouldn't be a problem normally but I went with an outside tool to pull multiple models into blender and the materials for them are separated into their respective texture maps. I would appreciate assistance in combining these maps in blender for export into Unity and beyond. I have the diffuse, normal, emissive, and specular maps and I've been trying to use the node view with materials to get them to mix correctly into the final material.

I created some texture maps in MindTex to use in Blender, but I can't figure out how to properly hook them up. The maps I have:  - Diffuse  - Normal  - Height   - Specular   - Gloss   - Self Illumination  - Reflection  - Ambient Occlusion   MindTex (left) vs. Blender (right)  As you can clearly see it's a pretty big difference.  is my material node setup in Blender. In the examples I don't actually use self illumination or reflection images, but including this in the answer would be helpful.


# touch $$ # gzip $$ # gzip --test $$.gz # echo $? 0 # OUT=$(gzip --test $$.gz) # echo $OUT  # if [ -z $OUT ] ; then echo $$ ; fi 26521 # if [ -n $OUT ] ; then echo $$ ; fi 26521 #      from bash(1)      -z string           True if the length of string is zero.    string    -n string           True if the length of string is non-zero.    I'm confused, how is it zero and non-zero at the same time? How does one do check against if key has a value (using bash)?

My question is on return values produced by this code:  if [ -n ]; then echo "true"; else echo "false"; fi   This prints true.  Its complementary test using [ -z ] also prints true:  if [ -z ]; then echo "true"; else  echo "false"; fi   In the above code, why does the [ -n ] test assume the string value that is not passed at all, as not null?  The code below prints false. This is expected since the passed string value is null and of zero length.  if [ -n "" ]; then echo "true"; else  echo "false"; fi


I just stumbled upon this user:   His nickname is probably some emoji which my browser is unable to detect. How to mention such users?

Once in a while you wish to address a comment to someone in particular, so you type @ and then you pause ... because the username is  or .  Short of using copy-paste to get their username right (which is even more cumbersome on a mobile device), is there another way around this? In a few cases I've noticed that the system will attempt to "promote" a comment to a reply, but I'm not sure if that's very reliable.  Should user names at least start with a normal letter?  Update  The auto-complete feature seems to normalize Zalgo'ish usernames, so typing the letter L will trigger the suggestions as expected; this is of course knowing that the name actually spells "lawrence", which may not always be obvious.  Knowing the way of the symbol ninja would surely help to type Ω using only the elbows as well, but not everyone is blessed with the same keyboard layout.  Suggestion  It would be nice to have a reply feature like this:        When ↳ reply is clicked, it will automatically populate the comment box with @username pre-filled in.   Additional notes  To clarify, I'm not suggesting to put the "reply" on all comments, as proposed , because in most cases the owner of a post is already notified of a new comment; this would strictly be to effect cross-commenter communication.


By default biber simply numbers the entries in my bibliography. How can I change the style to reflect the citation reference names I choose instead?  Example:  @book{myownreferencename,   author    = {Abcd Somebody},    title     = {some title},   publisher = {some publisher},   year      = 1990,   volume    = 2 }     % no additional configuration of biblography \bibliography{references} % ... \MakeBibliography      Hey this is a source \cite{myownreference} that should show up.     Result: Hey this is a source[myownreference] that should show up.

I tried to use natbib and other attempts but for vain. I would like to produce a cite key which is identical to the BibTeX entry. For example, if I have a BibTeX entry Bt98, then I want \cite{Bt98} to produce something like [Bt98].  Any ideas how can I accomplish this?


There are only 4 edges to vertex Z, so why does it have a degree of 5?

Why do you count a loop as a double in graph degree?  Rather than just as a single?  From :     a vertex with a loop "sees" itself as an adjacent vertex from both ends of the edge thus adding two, not one, to the degree.   Or perhaps this is just a feature of undirected graphs.  However, I wonder, what's the usefulness of counting a loop as a double in undirected graph?


Let $\gamma\in C^2(I)$ be a parametric continous curve with $\parallel \  \dot\gamma(t)\parallel^2 \ = \ 1$.  Show that $\langle \ddot\gamma,\dot\gamma \rangle \equiv 0$.    From that i know that we have to show that the vector of the velocity is perpendicular to the vector of acceleration. From $\gamma \in C^2(I)$ i know that $\gamma$ is exactly two times differentiable so $\ddot\gamma$ has to be constant. Then $\dot\gamma$ should have a constant slope which makes it a straight line. This straight line represents the velocity which should be 1. I think its not very far from here. Any tips?

How can I prove the following?  $\gamma (t)$ is unit speed, $\dot \gamma(t) \not= 0 \Rightarrow \ddot \gamma(t)$ is perpendicular to $\gamma(t)$   I don't really see where a problem would arise when $\dot \gamma(t)=0$ would cause such a problem


I'm traveling to Toronto and my flight there has a 3 hour layover in Zurich. My return flight has a 7 hour layover, also in Zurich.    Do I need a transit visa?

I found many related questions on this site but I am still not sure about the rules. How can I decide if I need a visa to transit? Schengen members as of May 2021 are as follows:  Austria Belgium Czech Republic Denmark (excluding Greenland and the Faroe Islands - but an open border with the Schengen Area is maintained) Estonia Finland France (excluding overseas departments and collectivities) Germany Greece Hungary Iceland Italy Latvia Liechtenstein Lithuania Luxembourg Malta Netherlands (excluding Aruba, Curaçao, Sint Maarten and the Caribbean Netherlands) Norway (excluding Svalbard) Poland Portugal Slovakia Slovenia Spain (except Ceuta and Melilla) Sweden Switzerland


I know similar questions have been asked before, but I found none of the answers to be very explanatory or upvoted enough for an effective solution.   Keeping it short, quite evidently I am running out of space on my /home (sda6), and have about 197 gigs unused (which I plan to completely dedicate to my ubuntu home assuming it's legal)  I am aware that only logically adjacent partitions can be exchanged memory between, but apparently there is a way using a live USB that allows us to do otherwise.  Ps: sda1 is my windows data drive.

Previously, I have installed Windows 7 on my 320 GB laptop with three partitions 173, 84 and 63 GB each. The 63 GB partition was where the Windows was installed. The rest were for file containers.  Now I changed my OS to Ubuntu 12.04 LTS. I installed Ubuntu by replacing the entire Windows 7 on the 63 GB partition. The rest of the partitions remain as an NTFS Windows partition and I can still access them both (the 173 and 84 GB partitions).  Now I want to change the two partitions of Windows into an Ubuntu format partitions plus most importantly, I want to extend the 63 GB partition to more than a 100 GB because at the moment I am running out of disk space. Whenever I try to install any application, especially using wine, it always complains for a shortage of disk space.  How do I do the extending activity before I entirely format my laptop again and lose all the important files on my partitions?


How can I stop the random popup of a program indicating a file location of C:\Users\<username>\Appdata\roaming\ijvctjd:

What should I do if my Windows computer seems to be infected with a virus or malware?  What are the symptoms of an infection? What should I do after noticing an infection? What can I do to get rid of it? how to prevent from infection by malware?   This question comes up frequently, and the suggested solutions are usually the same. This community wiki is an attempt to serve as the definitive, most comprehensive answer possible. Feel free to add your contributions via edits.


I am a competition math student, and I was doing a practice test when I found this problem: "The fourth degree polynomial equation $x^4 - 7x^3 + 4x^2 + 7x - 4 = 0$ has four real roots a, b, c and d. What is the value of the sum $1/a + 1/b + 1/c + 1/d$? Express your answer as a common fraction." First of all, I don't think this polynomial can be factored, so I don't know how to find the reciprocals of the roots. I need help and any help would be appreciated! Note: When they say "Express your answer as a common fraction.", it means that the answer IS a fraction. I tried asking this question already, but I worded it differently, and they didn't get a fraction so I knew it was wrong. This is the link to my other question:

I am a student learning competition math, and in a past test I found, you need to find the sum of the reciprocals of the roots of the polynomial $x^4-7x^3+4x^2+7x-4 = 0$. I have watched some videos on how to solve higher order polynomials, and I found that I like grouping. However, when I try to solve this with grouping, I end up with $x^4+4(x^2-1)-7x(x^2-1)$ which can be simplified to $x^4 + (x^2-1)(4-7x)$. I am not sure what to do now, and am having trouble figuring it out from here. Any help would be appreciated!


For a custom post type I have enabled support for comments via php and CPT.       'supports' => array(         'title',         'editor',         'revisions',         'comments',      )   But each post still has its "Allow comments" box in the discussion-field unchecked. I am now looking for a way to automatically check this box, as I have quite a big number posts of this custom post type and do not think, this can only be done manually.     However, I have other custom post types where I still want comments to be disabled. So I am looking for a way, to check all "enable comments" for one specific post type.

I am trying to add defualt comments to my custom post type.  I have added to my supports argument:  'supports' => array('title','editor','thumbnail','tags','post-formats','comments','revision'),   I've also tried a few functions like this:  function default_comments_on( $data ) { if( $data['post_type'] == 'your_custom_post_name' ) {     $data['comment_status'] = 1; }  return $data; } add_filter( 'wp_insert_post_data', 'default_comments_on' );   But nothing seems to work.Here is my custom post type:  function spark_register() { $labels = array(     'name' => _x('Spark Garden', 'post type general name'),     'singular_name' => _x('Spark', 'post type singular name'),     'add_new' => _x('Add New Spark', 'sparknz'),     'add_new_item' => __('Add New Spark'),     'edit_item' => __('Edit Spark'),     'new_item' => __('New Spark'),     'view_item' => __('View Spark'),     'search_items' => __('Search Sparks'),     'not_found' =>  __('Nothing found'),     'not_found_in_trash' => __('Nothing found in Trash'),     'parent_item_colon' => ''   );  $args = array(     'labels' => $labels,     'public' => true,     'has_archive' => true,     'publicly_queryable' => true,     'show_ui' => true,     'query_var' => true,     'rewrite' => true,     'capability_type' => 'post',     'hierarchical' => false,     'menu_position' => 8,     'supports' => array('title','editor','thumbnail','tags','post-  formats','comments','revision'),           'taxonomies' => array('post_tag'), );  register_post_type( 'spark' , $args ); }   add_action('init', 'spark_register');   I've tried adding to 'twentysixteen' and activating but that still doesn't show the comments by default. I have it checked to allow comments in my settings, it does appear in my regular 'post' post type, but in my cutom post type the discussion checkbox is not checked and comments are not showing by default?  Thanks in advance..


I want to export a selection of objects as .obj. I want each object as its own .obj. Is there a way to export individual .obj for each object selected? There does not seem to be an python reference for exporting, nor is a command echoed when I export a single .obj

It is possible to export all object from a scene into different obj files (each object in a obj file separately)?


I would like to know if there is a way for me to determine if the existing process can be launch or open instead of creating a new instance if I click the exe application of my winforms. Currently I only have a checker if there is an existing process with the same name but I'm not familiar with opening an existing exe application. Note: I do not have main form that shows in taskbar Note: my system is running in background and only show a system tray icon

Using C# and WPF under .NET (rather than  or console), what is the correct way to create an application that can only be run as a single instance?  I know it has something to do with some mythical thing called a mutex, rarely can I find someone that bothers to stop and explain what one of these are.  The code needs to also inform the already-running instance that the user tried to start a second one, and maybe also pass any command-line arguments if any existed.


I am getting the following error:  Conversion loses qualifiers   when trying to implement indexing operator without the code duplication (I will show the segment of code):  Point* BufferedList::indexTemp(size_t idx) {     if (idx >= size) return nullptr;      return &arr[idx]; }  const Point* BufferedList::operator [](size_t idx) const {     return indexTemp(idx); } Point* BufferedList::operator [](size_t idx) {     return indexTemp(idx); }   However, the following works (this doesn't use the helper function indexTemp and it is duplication of code):  const Point* BufferedList::operator [](size_t idx) const {     if (idx >= size) return nullptr;      return &arr[idx]; } Point* BufferedList::operator [](size_t idx) {     if (idx >= size) return nullptr;      return &arr[idx]; }   Do I really need two functions for indexing (one that returns Point* and the other that returns const Point*)?

Let's say I have the following class X where I want to return access to an internal member:  class Z {     // details };  class X {     std::vector<Z> vecZ;  public:     Z& Z(size_t index)     {         // massive amounts of code for validating index          Z& ret = vecZ[index];          // even more code for determining that the Z instance         // at index is *exactly* the right sort of Z (a process         // which involves calculating leap years in which         // religious holidays fall on Tuesdays for         // the next thousand years or so)          return ret;     }     const Z& Z(size_t index) const     {         // identical to non-const X::Z(), except printed in         // a lighter shade of gray since         // we're running low on toner by this point     } };   The two member functions X::Z() and X::Z() const have identical code inside the braces.  This is duplicate code and can cause maintenance problems for long functions with complex logic.    Is there a way to avoid this code duplication?


with python 2 reaching EOL how to make python3 the default python ? ...  system wide not just a specific script  should we do a symbolic link ?   sudo ln -s /usr/bin/python3 /usr/local/bin/python   or possibly   sudo update-alternatives --set python /usr/bin/python3    Does it matter which ?  I am on a box without python 2 so I have no python 2 specific libraries to deal with.  of course just making below edit to your ~/.bashrc will only impact one user   alias python=python3   I am looking for a system wide change for all processes and userids not just  me

The python program command executes Python 2. Python 3 can be executed using the python3 command.  How can Python 3 be executed using the python command?


I know about the history command and Ctrl+R and !, but there is still something I find frustrating when working with bash. Let's say I start typing "ssh username@serv" and then I've just remembered that, about a week ago, I had already typed "ssh username@server-012.somecompany.somedomain.etc.co.uk". If I was typing this in Windows, I would immediately simply press F8, and it would complete my command. In bash, is there something equivalent, or do I have to press Ctrl+u, Ctrl+r and re-type what I had just typed? Continuing from the above, let's say I typed "ssh username@serv" F8 and it recalled "ssh username@server-012.anothercompany.somedomain.etc.co.uk" instead of the one I was looking for. I would then simply press F8 again and it would cycle through the history, starting from what I had already typed, and it would find the command I was looking for. Is this also possible in bash?

I have searched, but not found anything on this. I am looking for a functionality in bash, using a terminal.  Way back when, I had a user on a debian system, and a friend set me up with a convenient history search feature (I believe I used tcsh then), where I would type the beginning of a previous command, hit up-arrow, and it would do a search, based on the partial string.  E.g. if my history is:  ./script.pl ./script.pl arg1 cat output cat output | grep yada   And I type ., and press up-arrow, it would show me: ./script.pl arg1. Press it again and it would show ./script.pl, etc.  Very much like it would perform a grep on .bash_history. Is there a way to get this functionality?


Given $\alpha _{n}= 1+\frac{1}{1!}+...+\frac{1}{n!} ,\forall n\in \mathbb{N}$.  How would I prove the following statement?  $\alpha _{n}< 3$

Have had a lot of trouble with this question. Cannot seem to use the components from the induction hypothesis properly.   Prove by induction:  P(n): $1+\frac{1}{1!}+\frac{1}{2!}+...+\frac{1}{n!} \leq 3 - \frac{1}{n}$ for all $n \in \Bbb N$  So i'm able to prove the base case:  P(1): $1+\frac{1}{(1)!} \leq  3 - \frac{1}{1} = 2 \leq 2$ is true.  Induction: Assume that   P(k): $1+\frac{1}{1!}+\frac{1}{2!}+...+\frac{1}{k!} \leq 3 - \frac{1}{k}$  is true for some $k \in \mathbb N$  Have to show that  P(k+1): $1+\frac{1}{1!}+\frac{1}{2!}+...+\frac{1}{k!}+\frac{1}{(k+1)!} \leq 3 - \frac{1}{(k+1)}$  So I get up to this step but don't know how to proceed after it: $1+\frac{1}{1!}+\frac{1}{2!}+...+\frac{1}{k!}+\frac{1}{(k+1)!} \leq 3 - \frac{1}{k}+\frac{1}{(k+1)!}$  (using the $3-\frac{1}{k}$ from the induction hypothesis (P(k)) but I have no idea how to start working with with these factorials to arrive at a conclusion that matches the RHS of P(k+1). I'm thinking there's something to do with that $(k+1)! = (k+1)k!$ but can't connect it all.   Any help would be appreciated - I checked everywhere on here and closest question I found that was similar was  question but it involves equality vs inequality. Thanks hopefully can help. I think this is question is unique enough that it shouldn't be a duplicate.


As far as I know, the probability of a quantum object being in a certain position depends on the wave function value for each position. That raises a question: Is this probability strictly greater than 0 for all points? If I place an electron in a box, it can be anywhere on the box, or anywhere on the universe?  For example, there is always a small possibility of finding a value however far from the mean, while that is not the case for a triangular distribution.  Also, slightly related: Is this property maintained when studying classical objects? Is there any possibility, even if unimaginably small, that all of the particles of a cat will simply move somewhere else at the same time, "teleporting" it?

Quantum physics dictates that there are probabilities that determine the outcome of an event, ie: the probability of a quark passing through a wall is X, due to the size of the quark in comparison to the wall), but couldn't macroscale events be predicted this same way, assuming all variables were accounted for (lets hypothetically say we have a computer powerful enough to take into account all factors).  My understanding is that as the scale of an object increases, the probability of it doing anything other than what classical physics dictates is almost 0%, but could conceivably do something not predicted (Example: a human being cannot pass through a wall, but given an infinite timescale, eventually s/he would because of probability not being 0). Is this accurate? If so, then shouldn't we, (except as a tool for conceptualization, like how we round 9.8 m/s^2 or pi to 3.14), use only quantum mechanics to explain events in any scale (for more accuracy)?


import java.util.*;  public class Main {     public static boolean isVowel(String c) {         return (c == "a" || c == "A"                 || c == "e" || c == "E"                 || c == "i" || c == "I"                 || c == "o" || c == "O"                 || c == "u" || c == "U");     }      public static void main(String[] args) {         Scanner sc = new Scanner(System.in);         String input1[] = new String[4];          for (int i = 0; i < input1.length; i++) {             input1[i] = sc.nextLine();         }          int i = 0;         int j = input1.length;          while (i < 1) {             if (isVowel(input1[i]) && isVowel(input1[j - 1])) {                 System.out.print("isVowel string");             }             i++;         }     } }  For example, input String array is oreo. so as its first character o (which is vowel) and its last character o (which is also vowel, it could be another vowel character also like e), so at last it should print isvowel string. But the problem in my program is if condition I am not able to enter in it. I am not able to understand why?

I've been using the == operator in my program to compare all my strings so far. However, I ran into a bug, changed one of them into .equals() instead, and it fixed the bug.  Is == bad? When should it and should it not be used? What's the difference?


If I have a child and elect not to get him a social security number, will he still be required to pay social security taxes? I am a U.S. citizen, as is he.  I'm trying to weigh the cost of not claiming him as a dependent on my tax return against this potential upside—but I want to be sure the upside is real first.  Edit:  First, this is not a duplicate of . That question is about opting oneself out of SS; this one is about opting a dependent out.  Second, if you find yourself more occupied about my motivations than my question, please read the following disclaimers:   I understand that this will be inconvenient for him (and us). I am not trying to evade taxes or do anything illegal. Even if I were a nutjob tax evader, speculation and comment about such only serves to divert attention from the question at hand.   Third, a correction. A SSN is not required to get a job. Not even a non-religious/non-exempt job (See David's answer below.)  Lastly, a request. Please justify your answer. I'd like to see it in the applicable tax code, or at least on a reputable source like irs.gov. Thanks!

How do I opt-out of the Social Security system and give up my Social Security Number? If I do this, am I then exempt from paying Social Security income tax?


I am new to DB. We have galera cluster on Test environment where mariaDB is replicated in 3 servers. Now customer want us to work with only 1 server. they cant provide 3.  Can we do replication on single physical server? Is it good idea? How can we achieve that?  Thanks in advance.

I'm contemplating setting up a Master-Slave replication for my database. The slave server will be used for redundancy and possibly a reports server. However, one of the biggest issues I'm running into is that we're already maxed out on power at our datacenter. So adding another physical server is not an option.  Our existing database server is fairly under-utilized as far as cpu (load averages never really get above 1 on a quad-core). So the leading idea is to toss in some new drives and double the memory (from 8GB to 16) and run a second mysql instance on the same physical machine. Each instance would have separate disks for the database.   Is there anything wrong with this idea?  Edit (more info): I've (luckily) never had anything bad enough happen to take down the server, but am trying to plan ahead. We of course have nightly backups that we could recover from. But I figured having the redundant data on separate disks would provide a quicker solution if the master server's drives failed (obviously not if the entire machine goes out).   As for the reporting aspect, any tables we would report off of are MyIsam. So doing expensive reads on the same tables that are being written to can bog down the server. My assumption was having a slave server to report off of wouldn't affect the main server as long as we threw enough RAM at it (since cpu load hasn't been an issue yet).


It's a resistor like coloured 2 pin component from a power supply board of a home theatre, with a glass-transparent aspect. I'm a hobbyist and not into complex circuit till now...  looking for an answer online but couldn't find anything.  Thanks

I know the symbol at the bottom is ground, but I can't get what the other symbol is. It can be a single symbol of two arrows pointing at each other, or two symbols each one a arrow.  I also have the PCB for this schematic. It is nothing but two traces (1mm long) with a little gap in between. Is it a RF filter of some sort?


When a body travels at a very high speed, as far as its concerned, everything is in motion except itself, so according to the body everything around it looses length in the direction of motion and gains mass. Keeping these two facts in mind, should it not be possible to travel so close to the speed of light, that due to increase in mass and decrease in volume that for the moving body many things around it should turn into a black hole?   What's wrong here? The only thing that I'm unsure about in this obviously false assertion is the relativity of mass and how its supposed to increase with velocity.  All answers will be appreciated, thanks a lot.

I'm a big fan of the podcast Astronomy Cast and a while back I was listening to a Q&A episode they did. A listener sent in a question that I found fascinating and have been wondering about ever since.   From the show :      Arunus Gidgowdusk from Lithuania asks: "If you took a one kilogram mass and accelerated it close to the speed of light would it form into a black hole? Would it stay a black hole if you then decreased the speed?"   Dr. Gay, an astrophysicist and one of the hosts, explained that she'd asked a number of her colleagues and that none of them could provide a satisfactory answer. I asked her more recently on Facebook if anyone had come forward with one and she said they had not.  So I thought maybe this would be a good place to ask.


I am following the instructions on  for Grails installation, but am not able to write to /etc/profile like it instructs.    sudo echo ‘JAVA_HOME=/usr/lib/jvm/java-6-sun’ >> /etc/profile   Instead, I get this error:   bash: /etc/profile: Permission denied   Why am I not able to write to the file even using sudo?

I want to do:  echo "something" >> /etc/config_file  But, since only the root user has write permission to this file, I can't do that. But the following also doesn't work.  sudo echo "something" >> /etc/config_file   Is there a way to append to a file in that situation without having to first open it with a sudo'd editor and then appending the new content by hand?


This is not a duplicate of the following questions     because:  My question specifically states that I am interested in end-user experience, not ease or efficiency of development, which is what the other question largely refers to.  As has been noted, development/deployment affects end-user experience, but it is not all there is to it, and neither of the referenced questions address issues that directly impact an end-user's ability to use the application (e.g. trouble accessing data on other partitions, sluggishness, etc.) Maybe "compelling" wasn't the right word to use; my intent was to ask about real-world, experiential consequences, i.e., things that happen or  don't happen, as opposed to theoretical/architectural statements that, while presumably accurate, don't appear to be backed up with any real-world examples to support the statement. I should have stated more directly that my intention was to get answers that consider the balance of "advantages" to snaps against the real-world downsides experienced by end-users. The "duplicate" question is largely theoretical, and doesn't discuss end-user experience at all. The "duplicate" question makes no mention of anything remotely similar to the example I used here, i.e., that there is an end-user downside to snaps (in this case, lack of access to data on other partitions and snap app performance) that isn't discussed in any available documentation that I can find.   While I understand that snap has a big advantage in making apps more widely available, is there any compelling reason to choose snap over apt, if the app is available for my distro/version via an apt package? I am curious because I've been doing some reading about snaps, and all the excitement about the method seems to be about things that are advantageous for app developers, but I've seen virtually nothing on how this makes life easier for end users (aside from the obvious; that they may be able to install apps that aren't otherwise available on their distro/version). I installed snapd and installed a couple of snaps and was really frustrated and disappointed.  The snap apps are slow and it's difficult, if not impossible, to access files on other partitions from within the snap. While I've seen plenty of info that says snaps are "faster," "easier," "safer," etc., I haven't been able to find anything that explains why or how this is actually the case. Being very new to Linux, I am wondering if maybe I'm just missing something obvious? To be clear, I understand why the technology might be useful overall, but I can't find anything that explains whether/why it is a better option even when the app in question is available for install via a more traditional method, and all dependencies are met.

Assumptions  Honestly, I do not know much about  snap-packages - but this is irrelevant for this question - see below. I assume the system is significantly different from the existing one.  Does the change make sense?  Is there an actual need, strong enough? That is - is there a new use case, that is important enough to develop a new format - and the associated infrastructure?  Was it not feasible to change the current method to cover the new use cases too?  Or am I missing the point?  It is possible that what I see is mostly marketing - new names and presentation for minimal technical change, to get a chance that any body assumes it as "new and better" and it may be actually used. Also, it may be that the new packages are so closely based on the existing format that it is mostly a change in presentation to the user. That could be a good solution, of course. In this case, this question would have not much relevance.  Then, I would hope that is still useful enough to answer to side aspects.  Just let me know if the question is not helpful or confusing to new users, I'm happy to delete it.  So, why do they exist?    Background  My first reaction was "That makes no sense!"  This is similar to a situation where, on the physics site, someone asked, unhappy, why nobody discusses his brilliant new ideas in answers. It looked strongly like crackpot ideas; So far from actual physical knowledge that it was hard to find a point to even start. I wrote an answer that did not touch his ideas with a single word, but explained why one would just not discuss assuming crackpot ideas - not the first case of that. The answer was actually hitting the point, I think.    If my assumptions were right, this case is similar.    But then, maybe not - let's see.


Here is my command :  \newpage  \cvitem{$\rhd$}{\small{\textbf{Xxxxxxx}}} \cvline{-}{\small{Yyyyyyyyyyyyy}} \cvline{-}{\small{Zzzzzzzzzzzzz}}  \cvitem{$\rhd$}{\small{\textbf{Aaaaaaaaaaaaaaaaaa}}} \cvline{-}{\small{Bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb}} \cvline{-}{\small{Cccccccccccccccccccccccccccccc}} \cvline{-}{\small{Dddddddddddddddddddddddddddddddddddd}} \cvline{-}{\small{Eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee}}   Minimum code to compile :  \documentclass[10pt,a4paper]{moderncv} \moderncvtheme[blue]{classic}                 \usepackage[utf8]{inputenc} \usepackage{color} \usepackage{amsmath} \usepackage{amssymb}  \usepackage{soul}% http://ctan.org/pkg/soul \usepackage[normalem]{ulem} \usepackage[T1]{fontenc} \renewcommand*{\sectionfont}{\Large\textbf\textsc}  \usepackage[inner=1.2cm,outer=1.2cm,top=0.9cm,bottom=0.9cm]{geometry}  \firstname{\Large{AAAA}} \familyname{\Large{\textsc{CCCC}}} \title{\small \textcolor{light-gray}{\textbf{CCCCC} }} \address{\textbf{CCCCC}}{\textbf{CCCCC}}   \mobile{\textbf{CCCC}}              \email{CCCC@aol.fr}  \extrainfo{\textbf{CCC}}     \renewcommand*{\emaillink}[2][]{ \ifthenelse{\equal{#1}{}} {\textbf{\href{mailto:#2}{#2}}} {\textbf{\href{mailto:#2}{#1}}}}  \renewcommand*{\emailsymbol}{\marvosymbol{66}}   \begin{document}  \cvitem{$\rhd$}{\small{\textbf{Xxxxxxx}}} \cvline{-}{\small{Yyyyyyyyyyyyy}} \cvline{-}{\small{Zzzzzzzzzzzzz}}  \cvitem{$\rhd$}{\small{\textbf{Aaaaaaaaaaaaaaaaaa}}} \cvline{-}       {\small{Bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb}} \cvline{-}{\small{Cccccccccccccccccccccccccccccc}} \cvline{-}{\small{Dddddddddddddddddddddddddddddddddddd}} \cvline{-}{\small{Eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee}}   \end{document}   Result :    But I would like second cvline to be next to firt one e.g. something like that :    How could I do this simply ?  Notes : after some tries, it seems the following (see "Mini page" codes) is what I would need but how could I automatically adjust text on the right ? I would like text not to overlap amrgins and to adapt to mini page.  \documentclass[10pt,a4paper]{moderncv} \moderncvtheme[blue]{classic}                 \usepackage[utf8]{inputenc} \usepackage{color} \usepackage{amsmath} \usepackage{amssymb}  \usepackage{soul}% http://ctan.org/pkg/soul \usepackage[normalem]{ulem} \usepackage[T1]{fontenc} \renewcommand*{\sectionfont}{\Large\textbf\textsc}  \usepackage[inner=1.2cm,outer=1.2cm,top=0.9cm,bottom=0.9cm]{geometry}  \firstname{\Large{AAAA}} \familyname{\Large{\textsc{CCCC}}} \title{\small \textcolor{light-gray}{\textbf{CCCCC} }} \address{\textbf{CCCCC}}{\textbf{CCCCC}}   \mobile{\textbf{CCCC}}              \email{CCCC@aol.fr}  \extrainfo{\textbf{CCC}}     \renewcommand*{\emaillink}[2][]{ \ifthenelse{\equal{#1}{}} {\textbf{\href{mailto:#2}{#2}}} {\textbf{\href{mailto:#2}{#1}}}}  \renewcommand*{\emailsymbol}{\marvosymbol{66}}   \begin{document}  \cvitem{$\rhd$}{\small{\textbf{Xxxxxxx}}} \cvline{-}{\small{Yyyyyyyyyyyyy}} \cvline{-}{\small{Zzzzzzzzzzzzz}}  \cvitem{$\rhd$}{\small{\textbf{Aaaaaaaaaaaaaaaaaa}}} \cvline{-}      {\small{Bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb}} \cvline{-}{\small{Cccccccccccccccccccccccccccccc}} \cvline{-}{\small{Dddddddddddddddddddddddddddddddddddd}} \cvline{-}{\small{Eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee}}  \vspace{4cm}  %Mini page ?  \begin{minipage}[t]{0.1\textwidth} \cvitem{$\rhd$}{\small{\textbf{Xxxxxxx}}} \cvline{-}{\small{Yyyyyyyyyyyyy}} \cvline{-}{\small{Zzzzzzzzzzzzz}} \end{minipage} \hfill \begin{minipage}[t]{0.6\textwidth} \cvitem{$\rhd$}{\small{\textbf{Aaaaaaaaaaaaaaaaaa}}} \cvline{-}{\small{Bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb}} \cvline{-}{\small{Cccccccccccccccccccccccccccccc}} \cvline{-}{\small{Dddddddddddddddddddddddddddddddddddd}} \cvline{-}{\small{Eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee}} \end{minipage}    \end{document}

See code :  \documentclass[10pt,a4paper]{moderncv} \moderncvtheme[blue]{classic}                 \usepackage[utf8]{inputenc} \usepackage{color} \usepackage{amsmath} \usepackage{amssymb}  \usepackage{soul}% http://ctan.org/pkg/soul \usepackage[normalem]{ulem} \usepackage[T1]{fontenc} \renewcommand*{\sectionfont}{\Large\textbf\textsc}  \usepackage[inner=1.2cm,outer=1.2cm,top=0.9cm,bottom=0.9cm]{geometry}  \firstname{\Large{AAAA}} \familyname{\Large{\textsc{CCCC}}} \title{\small \textcolor{light-gray}{\textbf{CCCCC} }} \address{\textbf{CCCCC}}{\textbf{CCCCC}}   \mobile{\textbf{CCCC}}              \email{CCCC@aol.fr}  \extrainfo{\textbf{CCC}}     \renewcommand*{\emaillink}[2][]{ \ifthenelse{\equal{#1}{}} {\textbf{\href{mailto:#2}{#2}}} {\textbf{\href{mailto:#2}{#1}}}}  \renewcommand*{\emailsymbol}{\marvosymbol{66}}   \begin{document}  \cvitem{$\rhd$}{\small{\textbf{Xxxxxxx}}} \cvline{-}{\small{Yyyyyyyyyyyyy}} \cvline{-}{\small{Zzzzzzzzzzzzz}}  \cvitem{$\rhd$}{\small{\textbf{Aaaaaaaaaaaaaaaaaa}}} \cvline{-}      {\small{Bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb}} \cvline{-}{\small{Cccccccccccccccccccccccccccccc}} \cvline{-}{\small{Dddddddddddddddddddddddddddddddddddd}} \cvline{-}{\small{Eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee}}  \vspace{4cm}  %Mini page ?  \begin{minipage}[t]{0.1\textwidth} \cvitem{$\rhd$}{\small{\textbf{Xxxxxxx}}} \cvline{-}{\small{Yyyyyyyyyyyyy}} \cvline{-}{\small{Zzzzzzzzzzzzz}} \end{minipage} \hfill \begin{minipage}[t]{0.6\textwidth} \cvitem{$\rhd$}{\small{\textbf{Aaaaaaaaaaaaaaaaaa}}} \cvline{-}{\small{Bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb}} \cvline{-}{\small{Cccccccccccccccccccccccccccccc}} \cvline{-}{\small{Dddddddddddddddddddddddddddddddddddd}} \cvline{-}{\small{Eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee}} \end{minipage}    \end{document}   What I want to do is what you see after %Mini Page ? but as you can see some text goes outside page margin. How could I adapt it automatically (go to the next line) ?


I have a matrix $A$ of size $n \times n$ defined as  $$a_{ij} = \frac{1}{i+j-1}$$ Where $a_{ij}$ is the $i^{th}$ row, $j^{th}$ column element of the matrix. So, $1 \leq i \leq n$ and $1 \leq j \leq n$. Is this matrix, Positive Definite or Positive Semi-Definite? How to prove it?

This is so called Hilbert matrix which is known as a poorly conditioned matrix.  $$ A =        \left(\begin{matrix}         1 & \frac{1}{2} & \frac{1}{3} & ... & \frac{1}{n} \\         \frac{1}{2} & \frac{1}{3} & \frac{1}{4} & ... & \frac{1}{n + 1} \\         \vdots & \vdots & \vdots & \ddots & \vdots \\         \frac{1}{n} & \frac{1}{n + 1} & \frac{1}{n + 2} & ... & \frac{1}{2n - 1} \\         \end{matrix}\right) $$  The task is to prove that matrix A is positively definite.   A possible way to go is to look at the scalar product:   $$(f, g) = \int_0^1f(x)g(x)dx$$   within the space of polynomials of degree not higher than $(n - 1)$. There our form $(\cdot, \cdot)$ is bilinear, symmetric and positively defined thus our polynomials' space with that form is, in fact, an euclidean space of degree $n$.   Now let's take the following basis: ${1, x, x^2, ..., x^\left(n - 1\right)}$. It's easy to see that matrix A is a Gram matrix for this basis.   But is there another approach?


there are some users in my project, that are using auto refresher or similar extension and each of them are making like 40 requests per second. It is not good for my server. Is it possible from apache side, to temporary block those users? Just temporary? For example for 5 mins?

What techniques and/or modules are available to implement robust rate limiting (requests|bytes/ip/unit time) in apache?


I want to know how to use target selectors such as @p and @r to display a person's gamertag in chat. I've tried to use /tellraw @a {"rawtext":[{"text":"@p"}]} but it just puts @p in chat, not any player names. Could anyone tell me how to get the selector to work?

I'm working on a map in Bedrock Edition, and I need to have a command block at the end that says "Congratulations [player], you win!" Because it's a map, I cant just have it say a specific player's name, and I've seen maps that do exactly what I want mine to do. How do I format the command to allow this?


I know statistical test usually requires "enough" samples to estimate the uncertainties. But what if I have only one sample in each group, and also I "know" which distribution this one sample comes from, is it reasonable to construct a statistical test? To be more specific, Let's say we have a case-control study and we want to know if there is some significant improvement in "case" than in "control". The data we have is only one sample, $x$ for the case group, and $y$ for the control group, so $x$ and $y$ are both scalars. The other information we also have is that $X\sim exp(\lambda)$ and $Y\sim exp(\mu)$, where $\lambda$ and $\mu$ are both some known parameters. My question is to test my hypothesis that "case is greater than control", can I just construct a test statistic $T = X-Y$? And I will be able to find the distribution of $T$ by this . Does this make sense? My confusion mainly comes from how to write up $H_0$ and $H_1$. It seems like I am testing $H_0:x=y$ but obviously this looks very weird... Because usually we will test $H_0: \lambda = \mu$...

I am wondering what we can say, if anything, about the population mean, $\mu$ when all I have is one measurement, $y_1$ (sample size of 1).  Obviously, we'd love to have more measurements, but we can't get them.  It seems to me that since the sample mean, $\bar{y}$, is trivially equal to $y_1$, then $E[\bar{y}]=E[y_1]=\mu$.  However, with a sample size of 1, the sample variance is undefined, and thus our confidence in using $\bar{y}$ as an estimator of $\mu$ is also undefined, correct?  Would there be any way to constrain our estimate of $\mu$ at all?


I'm looking for suggestions for a name for a new baby shop (with kids clothes and accessories).  What Stack Exchange site can I ask about this on?

Stuff like polls, recommendations based on subjective constraints, puzzles, webcomics etc. do not belong on the serious main SE sites, where professionals should be considered at work and having just a few spare minutes ("code's compiling") to answer questions, so they should not be distracted by such things. However, I'd also like to have a home for these things still using the SE engine. For the reason I mentioned before, this needs to be a separate place though. Let's call it four.[sitename-here].com.


I want to convert 3 to 3.00. I have done explicit type conversion. but guess it doesn't seem to work!whats wrong? my code: #include<iostream>   using namespace std;  int main(){     int PI = 3;     double a = static_cast<double>(PI);     cout<<a;   }  output: 3

In my  I was printing a double using cout that got rounded when I wasn't expecting it.  How can I make cout print a double using full precision?


Hi I'm having some problems with c++, when I try to compile my 3 files, main.cpp, My_Stack.cpp and My_Stack.hpp I get the "Undefined symbols for architecture x86_64" error, as if I hadn't included the header file and if I add "#include "My_Stack.cpp"" instead of "#include "My_Steak.hpp"" in main.cpp it works fine.    //main.cpp #include <iostream> #include "My_Stack.hpp"  int main(int argc, const char * argv[]) {     My_Stack<int> s = My_Stack<int>();     s.push(1);     s.push(2);     cout << s.pop() << endl;     cout << s.pop() << endl;   }     //My_Stack.hpp #ifndef My_Stack_hpp #define My_Stack_hpp  #include <stdio.h>  #include <string>  using namespace std;  template<class T> class My_Stack {  public:     My_Stack();     void push(T v);     T pop();  private:     class My_Stack_Node {     public:         T data;         My_Stack_Node* next;         My_Stack_Node(T n) {             data = n;             next = NULL;         }     };     My_Stack_Node* head;  };   #endif /* My_Stack_hpp */     //My_Stack.cpp #include "My_Stack.hpp"  #include <string> #include <sstream>  using namespace std;  template<class T> My_Stack<T>::My_Stack() {     this->head = NULL; }  template<class T> void My_Stack<T>::push(T v) {     if (this->head == NULL) {         this->head = new My_Stack_Node(v);     }     else {         My_Stack_Node* aux = new My_Stack_Node(v);         aux->next = this->head;         this->head = aux;     } }  template<class T> T My_Stack<T>::pop() {     My_Stack_Node* aux = this->head;     this->head = this->head->next;     return aux->data; }

Quote from :     The only portable way of using templates at the moment is to implement them in header files by using inline functions.   Why is this?  (Clarification: header files are not the only portable solution. But they are the most convenient portable solution.)


If I understand correctly, the bug allows to read in another process's virtual address space. To exploit this, the adversary needs to have a legal account on your personal computer, which is not usually the case on a personal home computer, or there must be some malware on the computer, which is bad anyway. Right?

Any questions relating to status updates, or asking if anything is going to be patched for these vulnerabilities should be closed as duplicates of this question.    are in the news right now and sound pretty severe. I don't see any security updates from Ubuntu that cover these vulnerabilities.  What is Ubuntu doing about these vulnerabilities, and what should Ubuntu users be doing?  These are CVE-2017-5753, CVE-2017-5715 and CVE-2017-5754.


I want to create a template from CentOS 6 so that each VM feels like a unique install. I found sys-unconfig which works but I don't like that it asks for a new password on boot. What alternatives are there?

I cloned a disk (SSD) and put the cloned disk into another machine. Now both systems have the same value in /etc/machine-id. Is it any problem to simply edit /etc/machine-id to change the value? Can I do this while the system is running (or do I need to boot from a Live USB)?  Is  a better alternative?  The naive use of systemd-machine-id-setup doesn't work. I tried these steps:  nano /etc/machine-id (to remove the existing value) systemd-machine-id-setup > Initializing machine ID from D-Bus machine ID. cat /etc/machine-id   The new value is the same as the old value.


F is a homomorphism from the field of real numbers into itself. I have proved that F is injective. I have also proved that F(q)=q for all 'q' belonging to the set of rational numbers. Is F the identity map? I am not able to come to a conclusion about images of irrational numbers under F. Can I get a few hints?

Is an automorphism of the field of real numbers $\mathbb{R}$ the identity map? If yes, how can we prove it?  Remark An automorphism of $\mathbb{R}$ may not be continuous.


I'm new to Ubuntu server and I'm having a little trouble with a project.  I need to add a user account, add a comment, create and set the home directory, add primary group to user, and set the password for the user.    I have no problem doing these individually, but for my project I need to use a single command to create the user with the properties listed.    Can anybody help me with this?

Here's the script code I'm using now:  getent group $MYGROUP if [ $? -ne 0 ] ; then     sudo su -c "groupadd $MYGROUP" fi sudo su -c "useradd mynewuser -p mypassword -m -g $PRIMARYGRP -G $MYGROUP"   This approach works well on openSuse. But there are several problems with the user account it creates on Ubuntu so I am seeking help here.   the terminal prompt isn't set (echo $PS1 returns nothing) the arrow keys and tab key do not work correctly in the terminal the password doesn't seem to work (although I'm still unclear exactly what this issue is) the /etc/sudoers rights set for this new user are not honored   If instead I manually create the user with adduser (instead of useradd) I don't have these problems on Ubuntu. But I can't use adduser on openSuse (afaik). Therefore, I need a non-Debian-exclusive script or method of adding user accounts via my bash script that works on Ubuntu (and doesn't stop working on other distros).  Finally, I would like to understand  between adduser and useradd. For example, I want to know which skeleton directory is used by adduser as that might be the reason useradd isn't working as expected (because I just accepted the default).   Thanks


{   "total": 5,   "start": 0,   "limit": 10,   "results": [     {       "firstName": "stage",       "lastName": "user",       "country": "IN",       "state": "TN",       "city": "Saint Thomas Mount",       "containersSelected": [         "CIEP",         "CIES",         "sthub"       ],       "created": "2020-07-23T03:29:49.394Z",       "updated": "2020-07-23T03:29:59.663Z"     },     {       "firstName": "Automation",       "lastName": "Testing",       "country": "IN",       "state": "TN",       "city": "Saint Thomas Mount",       "containersSelected": [         "CIEP",         "CIES"       ],       "created": "2020-07-22T03:25:00.510Z"     },     {        "firstName": "Test",       "lastName": "elite",       "country": "IN",       "state": "TN",       "city": "Chennai",       "containersSelected": [         "iothub",         "CIEP",         "CIES"       ],       "created": "2020-03-05T06:41:07.884Z"     },     {       "firstName": "Amani",       "lastName": "I",       "country": "IN",       "state": "TG",       "city": "Hyderabad",       "containersSelected": [         "iothub",         "CIEP",         "CIES"       ],       "created": "2019-10-15T06:23:14.857Z"     },     {       "firstName": "Venkatesh",       "lastName": "Chandu",       "country": "IN",       "state": "TG",       "city": "Hyderabad",       "containersSelected": [         "iothub",         "CIEP",         "CIES"       ],       "created": "2019-09-11T09:56:02.540Z"     }   ] }  here I used code was Containerallowedlist = [] data = response.json() for containers in data['results']:     for list in containers['containersSelected']:         Containerallowedlist.append(list)  Output I Got was : ['CIEP', 'CIES', 'sthub', 'CIEP', 'CIES', 'iothub', 'CIEP', 'CIES', 'iothub', 'CIEP', 'CIES', 'iothub', 'CIEP', 'CIES'] But I need output was :  [[‘CIEP', 'CIES', 'sthub’],[’CIEP', 'CIES', 'iothub’],[ 'CIEP', 'CIES', 'iothub’], [‘CIEP', 'CIES', 'iothub’], [‘CIEP', 'CIES’]]

What's the difference between the list methods append() and extend()?


I am looking to count the number of cells based on three dependent select statements. each select corresponds to a column.  For example,   Select nth-Day (Column A)   Select Issue Type (Column B)     Select Severity Level (Column C)  ColumnA, ColumnB, ColumnC  Day1, Backend, Sev1  Day1, Backend, Sev1 Day1, Frontend, Sev1  Day1, Frontend, Sev2  Day1, Frontend, Sev1  Day2, Backend, Sev3  Day2, Frontend, Sev3   So for Day1, Backend, Sev1 = 2 Day1, Frontend, Sev1 = 2 Day2, Backend, Sev1 = 1    I can start with countifs but I'm stuck after that.  Any gurus?

I have some Excel columns that look like this.  1774    David                    Not Upheld 1770    James                    Upheld 1771    David                    Upheld 1772    Sam                      Upheld 1768    Arthur                   Not Upheld 1769    James                    Upheld   How can I write an Excel formula for: count the number of rows with 'James' in column B, and 'Upheld' in column C?  I know how to do each of these conditions individually (using COUNTIF), but I'm not sure of the best way to combine them.  Thanks!


The line $l$ has gradient $-2$ and passes through $A=(3,5)$. $B$ is a point on $l$ such that $\overline{AB}=6\sqrt5$. Find coordinates of each possible points of $B$.   There are two $x$-coordinates and two $y$-coordinates and so far I have got to $(3-x)^2+(5-y)^2=180$. What do I do from here?

You are given a point P(x,y) on a line of equation ax + by + c = 0. I want another point, Q(a,b) on the same line which is at a distance d from P. There will be two points. I want to find both and know which side of P each one lies.  There are related questions in this forum, but I didn't get what I am exactly looking for. Sorry for beating around the bush, and thanks for any help... Please give me an elaborate explanation. Thanks once again!!!


I have a binary that repacks android kernel (not mkbootimg). I'm making a script to automate the process.   I don't want any output on the screen. So I have redirected the output to a file named foo.log.  The command is - kernel_make "$zImage" "$ramdisk" "$cmdline" "$image" &> data/local/working/foo.log  My current working folder is data/local/working/. What I've to do is, if the last line of the output (foo.log) is this -  "targed file $1 is patched sucesfully, enjoy new kernel"  Then return 0 and exit. Else return 1 and exit.  I'm trying to do with grep, but as I'm new to command line, do don't have any idea of doing it the right way.

As a part of this script, I need to be able to check if the first argument given matches the first word of file. If it does, exit with an error message; if it doesn't, append the arguments to the file. I understand how to write the if statement, but not how to use grep within a script. I understand that grep will look something like this  grep ^$1 schemas.txt   I feel like this should be much easier than I am making it.  I'm getting an error "too many arguments" on the if statement. I got rid of the space between grep -q and then got an error binary operator expected.   if [ grep -q ^$1 schemas.txt ] then         echo "Schema already exists. Please try again"         exit 1 else         echo "$@" >> schemas.txt fi


I have two EXACT same uvc unity cameras that have no serial number so I have abandoned mounting them via some unique attribute. Instead I specifically looking to mount it via the usb port I have plugged them into   Camera 1 PLUGS ONLY INTO USB Port 1 Camera 2 PLUGS ONLY INTO USB Port 2   I wrote the udev rule like so  SUBSYSTEM=="usb", KERNELS=="2-2.2", ATTR{idVendor}=="0000", ATTR{idProduct}=="0001", NAME="video0", MODE="0666", SYMLINK+="video0" SUBSYSTEM=="usb", KERNELS=="2-2.3", ATTR{idVendor}=="0000", ATTR{idProduct}=="0001", NAME="video1", MODE="0666", SYMLINK+="video1"   However, this still does not work they get crossed it only depends on which device gets recognized first

So I have a udev rules file to remap the serial ports as they are not in the order required. The rules work fine on SLES11 SP1 but do not seem to do anything on 14.04.  SUBSYSTEM=="platform", DRIVERS=="serial8250", KERNELS=="serial8250", KERNEL=="ttyS2", NAME="ttyS10" SUBSYSTEM=="platform", DRIVERS=="serial8250", KERNELS=="serial8250", KERNEL=="ttyS3", NAME="ttyS11" SUBSYSTEM=="tty", DRIVERS=="serial", ATTRS{id}=="PNP0501", KERNEL=="ttyS4", NAME="ttyS12" SUBSYSTEM=="tty", DRIVERS=="serial", ATTRS{id}=="PNP0501", KERNEL=="ttyS5", NAME="ttyS13" SUBSYSTEM=="tty", DRIVERS=="serial", ATTRS{subsystem_vendor}=="0x1415", KERNEL=="ttyS6", NAME="ttyS2" SUBSYSTEM=="tty", DRIVERS=="serial", ATTRS{subsystem_vendor}=="0x1415", KERNEL=="ttyS7", NAME="ttyS3" SUBSYSTEM=="tty", DRIVERS=="serial", ATTRS{subsystem_vendor}=="0x1415", KERNEL=="ttyS8", NAME="ttyS4" SUBSYSTEM=="tty", DRIVERS=="serial", ATTRS{subsystem_vendor}=="0x1415", KERNEL=="ttyS9", NAME="ttyS5" SUBSYSTEM=="tty", DRIVERS=="serial", ATTRS{subsystem_vendor}=="0x1415", KERNEL=="ttyS10", NAME="ttyS6" SUBSYSTEM=="tty", DRIVERS=="serial", ATTRS{subsystem_vendor}=="0x1415", KERNEL=="ttyS11", NAME="ttyS7" SUBSYSTEM=="tty", DRIVERS=="serial", ATTRS{subsystem_vendor}=="0x1415", KERNEL=="ttyS12", NAME="ttyS8" SUBSYSTEM=="tty", DRIVERS=="serial", ATTRS{subsystem_vendor}=="0x1415", KERNEL=="ttyS13", NAME="ttyS9"   I have tried using setserial a swell but with no progress.


What are the basic differences between $\mathbb C$ and $\mathbb R^2$?  The points in these two sets are written as ordered pairs, I mean the structure looks similar to me. So what is the reason to denote these two sets differently?

I haven't taken any complex analysis course yet, but now I have this question that relates to it.  Let's have a look at a very simple example. Suppose $x,y$ and $z$ are the Cartesian coordinates and we have a function $z=f(x,y)=\cos(x)+\sin(y)$. However, now I change the $\mathbb{R}^2$ plane $x,y$ to complex plane and make a new function, $z=\cos(t)+i\sin(t)$.  So, can anyone tell me some famous and fundamental differences between complex plane and $\mathbb{R}^2$ by this example, like some features $\mathbb{R}^2 $ has but complex plane doesn't or the other way around. (Actually I am trying to understand why electrical engineers always want to put signal into the complex numbers rather than $\mathbb{R}^2$, if a signal is affected by 2 components)   Thanks for help me out!


I'm assuming that if you don't select an answer for a question that you have asked within a certain time-span, that you will get no bounty. I asked the question shown in the image a couple months back.    Correct me if I'm wrong. And if I'm wrong, I'd like to see it implemented, because I think it's a good idea :)

What does it mean to accept an answer? How do I accept an answer, and what are the rules? Can I accept my own self-answer? What are the reputation benefits? Which answer should I accept? Should I upvote the accepted answer? What should I do if none of the answers are suitable?      For more information, see "" in the .


I have had letters delivered to me that had abuses.   I had letters delivered to me that had abuses.

If I seem tired, can I say: "I haven't slept last night"? If not, why have I been told that we use present perfect for actions that have present effects?


This is the following problem: Let $X$ be a set.  Prove that there is not a surjection from $X \rightarrow 2^X$ (Hint: Assume to the contrary that $f\colon X \rightarrow 2^X$ is a surjection and consider the set $M=\{ x \in X | x  \notin f(x) \}$). Use these facts to conclude that there is an infinite number of sizes of infinity cardinals.   My answer to the question is: This function is only bijective if its inverse is bijective and since $f(x)=\log(x)$ isn't bijective then the original function $f(x)=2^x$ isn't bijective. Since this function isn't bijective, then it isn't surjective.   As for the cardinality, I'm not sure what that really means. Thank you in advance for any hints or help!

Show that: Let A be a set and let $P(A)$ be the set of all subsets of $A$. Then there is no surjection $f: A→P(A)$.  Here is what I thought:  if $A=\{a,b\}$ then it has only two elements where $P(A)=\{∅,\{a\},\{b\},\{a,b\}\}$ has 4 elements. Therefore $f:A→P(A)$ cannot be surjective. But I have some problems:  1) How is it possible that any $f$ function to take $\{a\}$ from set $A$ to $\{a,b\}$? Maybe because I am thinking mainly about functions with real values like $f(x)=2x$, I find it a little bit strange that a function to take an element of a set to another set which has more elements. Is it possible?  edit: Now I thought that if $f(x)$ is $\sqrt{x}$, then $f(4)=±2$ which means it took an element from a set to a set which has 2 elements. But still I find it kind of strange to denote $f(\{a\})=\{a,b,c,...\}$  2) How can I construct a explicit proof for this question?  Regards


I have to show that $a_n=\sqrt[n]4$ is convergent and then find the limiting value. How can I formally show that an is convergent? To find the limiting value is it enough to use this line of reasoning:  $\sqrt[n]4=4^{\frac{1}{n}}$ and then $\frac{1}{n}\rightarrow0$ for $n\rightarrow\infty$ so $\sqrt[n]4=4^{\frac{1}{n}}\rightarrow1$ for $n\rightarrow\infty$ ?

I am looking for a proof possibly using the sandwich theorem and/or Bernoulli's inequality for proving the following statement:  If $x>0$ then $x^{1/n}$ tends to $1$.


I have read an article from Tektronix on how oscilloscope probes affect your meausurment here i read that:  The consequence of a large capacitance value is severe because the front edge of the signal contains the high frequency content.(page 6)

Say, I want to limit the rise time of my digital signal edges to avoid dealing with transmission line effects.   How do I determine the maximum frequency of harmonics in my signal knowing that my rise time is, say 5ns?  How do I determine corner frequency of my low pass filter knowing that hold time on the receiver chip is, say 10ns?  In wikipedia I've found the formula   $$ BW=\frac{0.34}{t_{rise}} $$   does it apply in this case?     Edit  I failed to make myself clear, so I'll try to explain my line of thought.  Say, I have a 30HMz signal and my trace length is well below 1/10 of the wavelength. So I don't have to deal with transmission line effects in respect to that. But my edges are steep - 5ns. This adds some high frequency components into my signal that potentially will suffer from transmission line effects.   My idea is that I slow down edge transitions up to a point where I don't have to deal with transmission line phenomena. The question is twofold:    how do I calculate fastest rise/fall time that with the given trace length would enable me to traeat my circuit as "lumped"? how do I slow down the rise/fall time?   Rise/fall time is time for voltage to change from 10% to 90% of max value. I know how to calculate the approximate speed of signal on FR4 board.


I'm trying to get my head around zero-knowledge proofs and my definition of a zero-knowledge proof is as follows.     A zero-knowledge proof is a method by which one party can prove to   another party that they know a secret value, without sharing any   data about this secret value.   To implement a zero-knowledge proof some data will need to be exchanged.  With that in mind, is the following assertion correct?    To implement a zero-knowledge proof, data can be exchanged but this data should not contain any information about the secret value?   If the above holds true, could a digital signature be classified as a zero-knowledge proof?

Suppose Alice publishes a book with a public key in it, and later wants to prove that she wrote the book. She could sign challenge messages with her private key, and others could verify those signed messages. Alice is proving she knows a private key without revealing anything about it.  Is this an example of an interactive zero knowledge proof?   Or does this example somehow not apply to zero knowledge proofs?


What is the cause of the surface tension of the liquids? How to know the direction of tension force on the free surface of the liquid?   I know that the surface tension is the force acting normally per unit length of the imaginary line drawn on the free surface of a liquid, but don't have any idea about the cause of the surface tension. More over I don't know the direction of the tension force i.e. where it towards liquid or away from the liquid.  Any idea or help will be greatly appreciated!

A text says:     The surface tension of a liquid results from an imbalance of intermolecular attractive forces, the cohesive forces between molecules:   A molecule in the bulk liquid experiences cohesive forces with other molecules in all directions.   A molecule at the surface of a liquid experiences only net inward cohesive forces   If this is the case: Why is surface tension considered or experienced as a force parallel to the interface while it is so obvious that it must be perpendicular to it?


I'm trying to get the Hairnet add-on installed on Blender 2.8.1, but it won't show I've had the same issue since I found out about add-ons.  It shows that it's in my Blender folder, but it doesn't show up in my preferences not even if I I scrolled down.

I have downloaded an add-on hosted on github today using this green 'Download/Clone' button. However, when I try to install it via User Preferences > Addons > Install it does not appear in the Add-on list, no matter what I do.    Q: How to download and install add-ons hosted on github properly? What am I doing wrong?


I recently had Linux Mint running as my sole OS, which I later decided to remove to reinstall Windows 8. Windows Update is current, however after the installation I found some missing functionality (USB ports, FN Keys, etc.).  I want my computer to run as if I just bought it. Is there a one-stop shop to ensure that all of my Windows drivers are up-to-date?

I have a Dell laptop issued by my employer, and I always find it a real pain to search for, download and maintain their drivers. It baffles me that there does not seem to be a nice way (product, website, ...) to just download the stuff you need, without hassle. The same goes for the other Windows based laptops in my direct environment.  Are there any (preferably free) automated solutions available? Or do you have a nice workflow - other than searching the manufacturers website - to help smoothing this process?


The original question I was asking was . The obvious answer is that the torque ${\bf r}\times{\bf F}$ is zero as ${\bf f}$ is zero at the centre of mass, but that defeats the purpose of my question. Why is the centre of mass so special that we take torque about it and force applied on it does not provide rotation? I am looking for the intuition behind this.

I found after searching that this question has been asked before. But all the answers were not convincing. Suppose I have a body which is free, not constrained always rotate about its center of mass (COM). Why is that so? A convincing answer that I found was that in most cases the moment of inertial about the center of mass is the least and that's why the body rotates about the center of mass. But I ask it again with hope of the question not getting closed and getting a better succinct answer. I was thinking that motion about the COM is the most stable one and the rotation about other points degenerates. I don't think it's right. Is it? —————————————————————————————————— Note-: 1). This question has been wrongly closed. The other questions linked don’t answer my question at all. It asks me to ask a new question if my question is still not resolved. I did make it clear that I am not satisfied with the answers in the linked questions. 2). The answer to this question is that a free body never rotates about its center of mass ( the instantaneous axis of rotation never passes through the center of mass). In fact we choose a point about which we want to decompose the motion into rotation and translation and we could very well have chosen any point other than the center of mass and analysed rotation about it. Moreover the instantaneous axis of rotation for a free body never passes through the center of mass. I would urge the moderators to give me the right to add my answer to this question. This is the correct answer, the one which satisfied me the most and it is nowhere in the linked answers. So kindly give me the right to open this question and let me add my answer to it.


Suppose we have two finite, ordered sequences $x = (x_1,\dots,x_m)$ and $y = (y_1,\dots,y_n)$. How many ways can we create a new sequence of length $m+n$ from $x$ and $y$ so that the order of elements is preserved?  How many ways can we do this using $k$ ordered sequences instead of two?

Suppose that, for a real application, I have ended up with a sorted list A = {$a_1, a_2, ..., a_{|A|}$} of elements of a certain kind (say, Type-A), and another sorted list B = {$b_1, b_2, ..., b_{|B|}$} of elements of a different kind (Type-B), such that Type-A elements are only comparable with Type-A elements, and likewise for Type-B.  At this point I seek to count the following: in how many ways can I merge both lists together, in such a way that the relative ordering of Type-A and Type-B elements, respectively, is preserved? (i.e. that if $P_M(x)$ represents the position of an element of A or B in the merged list, then $P_M(a_i)<P_M(a_j)$ and $P_M(b_i)<P_M(b_j)$ for all $i<j$)  I've tried to figure this out constructively by starting with an empty merged list and inserting elements of A or B one at a time, counting in how many ways each insertion can be done, but since this depends on the placement of previous elements of the same type, I've had little luck so far. I also tried explicitly counting all possibilities for different (small) lengths of A and B, but I've been unable to extract any potential general principle in this way.


Oke, so my mind is blown by Einsteins view on gravity, at least as far as I understand the basics and principles he based his views on.  One of the first things that struck me was that most of his equations were based on the fact the photons had no mass, something which is virtually impossible to verify experimentally and something I can not convince myself of.  Now he states that mass bends spacetime: in other words, objects can only move in straight paths, but because the "surface" of a path can be bent, the trajectory relative to the mass is bent. So as a reaction, I did the following thought experiment that would separate Newton from Einstein:  Image in space (completely empty) and two objects with mass M and m (whatever the values) placed from a distance R apart completely at rest. According to my interpretation of Einstein, nothing will happen because even if spacetime is bent they have no motion themselves. Our earth orbits the sun because it initially had a motion and is now travelling in an infinite straight line, which becomes an ellipse due to curvature. Newtons would of course state that in my thought experiment, the two masses would collide.  After I realized this, I started to question Einstein's view. Is my basic interpretation correct about Einstein, and how would you (expert/physics professor/lover) respond to this thought? I personally find it pretty confronting...  BTW I'm just a high school student, potentially going to Cambridge to study physical sciences...

It is fine to say that for an object flying past a massive object, the spacetime is curved by the massive object, and so the object flying past follows the curved path of the geodesic, so it "appears" to be experiencing gravitational acceleration. Do we also say along with it, that the object flying past in reality exeriences NO attraction force towards the massive object? Is it just following the spacetime geodesic curve while experiencing NO attractive force?  Now come to the other issue: Supposing two objects are at rest relative to each other, ie they are not following any spacetime geodesic. Then why will they experience gravitational attraction towards each other? E.g. why will an apple fall to earth? Why won't it sit there in its original position high above the earth? How does the curvature of spacetime cause it to experience an attraction force towards the earth, and why would we need to exert a force in reverse direction to prevent it from falling? How does the curvature of spacetime cause this?  When the apple was detatched from the branch of the tree, it was stationary, so it did not have to follow any geodesic curve. So we cannot just say that it fell to earth because its geodesic curve passed through the earth. Why did the spacetime curvature cause it to start moving in the first place?


I have been dealing with a multitude of equations in my thesis and it gets really repetitive defining each parameter verbally after each equation; personally I find it inefficient and at times messy. So I came up with the idea of displaying the units as follows (Word example):    With the parameters listed below with a 1/2 or 2/3 page indent (I will have to test out what works) and a single line spacing between each definition to save page space (currently using 1.5). If possible I would like to have the parameters aligned as well as the definitions but this isn't crucial, I believe it will look neater, as seen in the above example.  I tried including the list within the equation environment but it does not work well as the equation number is displaced.  I offer my MWE for my document class:  \documentclass{scrreprt} \usepackage{amsmath} \usepackage{siunitx}  \renewcommand{\labelitemi}{}  \begin{document}  I would like to have the units neatly defined for Equation~\eqref{eq:newton_gravity}\dots  \begin{equation} \label{eq:newton_gravity} F=G\frac{{m_1}{m_2}}{{r^2}} \end{equation} \begin{itemize} \item $F$: Force. \item $G$: Gravitational constant. \item $m_x$: Mass of object x. \item $r$: Distance between centres of two objects. \end{itemize}  \end{document}

Equation's symbols or parameters description are shown below, however, the = and the details of parameter are not aligned.     I want to make a parameters list for a equation like this, in which symbols, = and detail information are aligned in terms of vertical position.  where    ...    P   =   notional permeability factor    N   =   number of waves    Sd  =   damage level    ...   A tabular environment will produce more spacing before and after text. So how to make the = vertical aligned?   The codes is provided:  \begin{equation} \frac{H_s}{\Delta D_{n50} } = 1.0~ P^{0.13}~ \left(\frac{S_d}{N} \right)^{0.2} \xi_m^P~ \sqrt{\cot \alpha} \end{equation} where:   $H_s$ = significant wave height, equal to the average of the highest 1/3 of the waves  $\Delta$  = relative buoyant density, equal to $\rho_r / \rho_w - 1$, where     $\rho_w$ is the water density  $D_{n50}$ = nominal  diameter defined in Equation (2)  $P$ = notional permeability factor  $S_d$ = damage level   $N$ = number of waves  $\xi_m$ = breaker parameter based on mean wave period $T_m$  $\alpha$ = slope angle


When voting to close (or in such cases migrate) a question:  Community specific What site does this question belong on?  expatriates.meta.stackexchange.com   Here one site is given, but I would like to suggest  law.stackexchange.com  but there isn't any way to add this suggestion to the migration list.

I think there should be more Stack Exchange networks available when flagging a question to be migrated. For example I'd think that the following question:    ...would be a better fit on unix.stackexchange.com instead of Stack Overflow. But without the option to mark it so, I feel hesitant to mark it.


If x^(1/3) is an irrational number, then x is also irrational.  I tried using contrapositive, but it's not the right way.

If $x^{ 1/3 } \in\mathbb{R}$ is an irrational number, then $x$ is also irrational.   I want to use the contrapositive to disprove this. The contrapositive would be:  If $x$ is rational, then $x^{ 1/3 }$ is rational.   I am thinking that if I use $10$ as a counter example, then it will disprove the claim. I am not sure that this is the right approach. I am also unsure of whether or not I'll have to prove that the cubed root of $10$ is irrational to make the proof complete.


I have downloaded the latest ubuntu onto a external hard drive, but i cant seem to figure out how to open it and run that os from the HD. I'm in school so i need to keep my primary OS as windows for conveniency purposes. Is there a way i can have a duel os, pulling one from a External Hard Drive?  John

OS: Windows 7. I want install Linux Ubuntu on portable external Hard Drive USB 2.0. I read a variety of methods, but I'm confused by contradictory advices and methods. Actually, installing Ubuntu on anything other than clean PC looks much more complicated than installing Windows. I need to have Ubuntu package + Python 2.7 + C compiler installed. My external hard drive is partially filled, I need to keep all files.   Will Ubuntu run entirely on external hardrive, without writing anythinhg to Windows system folders and to registry?  Can anyone post detailed guide how install Ubuntu on external hard drive?


Currently in the history for one's votes it says (deleted) if a vote has either been accepted, denied, or deleted.  So, for instance if I cast a close vote on a question and the question is closed, it will look like this:    Which is slightly confusing and may be misleading to some as they may think that the close vote was rejected in some way. It also does this for reopen votes, delete, and undelete votes, basically it does it for any vote if it's been accepted, rejected (in some way), or deleted (like if I cast a close vote, and then retract it).  So I think that the text displayed about the status of the vote should be more descriptive, so that it says (deleted), (denied), and (accepted).

Background Within the voting tab of my profile any questions that I have voted to close which have then actually been closed are shown as deleted.  The related   has been marked  I shall argue that it should be changed; Proposal I propose that any close vote that ultimately leads to a closure be marked "successful" rather than "deleted" as whatever the internal processes the end user will see it that way Reasoning From a users perspective their vote has not been deleted; it is still shown under the closed question as "closed by Voter1, Voter2 ...." as such the vote is "successful" not "deleted", from a users point of view a deleted vote would be one that aged away or was invalidated in some other way.


For example: Group workers have 2 users: john and jony . What command do I have to use to list the members of group workers ?   These commands don't serve my purpose:  compgen -u , compgen -g, cut -d ":" -f 1 /etc/passwd

How can I see all the members of a group in Linux?


I have windows 7 installed on my hard drive (installed it in BIOS mode not UEFI), and I want to buy another hard drive and install ubuntu 16.04 on it through UEFI. When I have both hard drives connected to my motherboard will I be able to choose which OS to boot into when I turn on my computer or would I have to always go into my BIOS and change the boot order?  I was thinking of just installing ubuntu 16.04 on my current hard drive alongside dual boot but I don't know if I can install it in BIOS mode instead of UEFI?

I'm absolutely new to Linux. I would like to know how to install Ubuntu alongside the pre-installed Windows 8+ OS.  Should I do it with Wubi, or through the Live USB/DVD?  What steps do I need to take to correctly install Ubuntu?


I am non-tech entrepreneur. I have developed a Rails app. I want to deploy it live. Can anyone guide me on how to use Ubuntu server + Juju on my laptop to deploy the app live on internet.

I want to mess with , how do I get started?


What do we mean, concretely, by the unit $\rm N\:m$ (newton $\cdot$ meter)?  For example, $1 \:\rm m/s$ mean that each second, we make one meter. Also, $1\:\rm  m/s^2$ mean that each second the speed is $1 \:\rm m/s$ faster. Also, $1\:\rm  N/m$ mean that each meter, the force is $1\:\rm N$ more powerful.   Now, I don't understand when instead of division we have a multiplication. For example, what mean $1 \:\rm N\:m$? How can I interpret this? (instead the fact that $\:\rm N\:m=J$). What is the phenomena behind?

I can easily understand what divisive units imply, but not what multiplicative units imply.    What I mean is, when I read "$12 \:\mathrm{eggs/carton}$", I mentally convert it to, "There are 12 eggs for each carton". I get that.  But, when I see units like joules * seconds, $\:\mathrm{J \cdot s}$, it really bothers me for some reason. I'm not sure what it's saying. It's not saying the amount of joules expended over a certain time-frame, otherwise it would be joules/second, wouldn't it? So, when I read units like joules * second, I try to relate it to my egg-carton analogy, and it comes out like this:  "There are 12 eggs for each reciprocal of a carton."    It just doesn't make any sense for me. Is this a problem for anybody else? Should this be on math SE? Is this even a valid question or am I just asking why $2 + 2 = 4$?


By mistake my bashrc has changed (using source command to a .com file). Now sudo su command is not working. To use root I have to give bash command. How can I return previous stage? Or use sudo su?

I've been trying to install Android developer tools on Eclipse, and I followed the information in .  Like the video said, I added the following two lines of code to the .bashrc file:  export PATH=$(PATH):-/android-sdk/tools/ export PATH=$(PATH):-/android-sdk/platform-tools/   It seems, however, that this did not help me to install the Android files on Eclipse, as I am still having problems with that. What's more, it seems that I can't make any command in the terminal without getting a major error now! Every time I try to make a command in the terminal I get the following message:     The command could not be located because '/usr/bin' is not included in   the PATH environment variable.   Is there away to restore .bashrc to its default?


I have tried to remove makemkv multiple times and it is still here. I tried snap remove makemkv. And is said I don't have permission. I tried from the app store and it comes right back. Please tell me what I am doing wrong and how to fix this.  Also. When I try to launch makemkv nothing happens. But I can't reinstall it because it is already installed and can't be removed.

I want to know how to check how many snaps programs/apps I'm running. Also, how to replace them (if possible) without losing GUI?


I'm a beginner when it comes to Android app development. I have finished some Udemy courses, but then I realized that the Android Docs might actually be better for building a good foundation on app development. I'm trying to build my own app now but I encountered this problem.    My Class  public class deleteInfluencer extends AppCompatActivity {  private Repository instanceOfRepository;  public static final int EDIT_BLOGGER_ACTIVITY_REQUEST_CODE = 1;  deleteInfluencer(Application application) {     super();     instanceOfRepository = new Repository(application); }  void callDeleteInfluencer(int id) {     instanceOfRepository.deleteInfluencerCalledFromRepository(id); }  void callUpdateInfluencer() {     Intent intent = new Intent(this, EditBloggerActivity.class);     startActivityForResult(intent, EDIT_BLOGGER_ACTIVITY_REQUEST_CODE); }  @Override protected void onActivityResult(int requestCode, int resultCode, @Nullable Intent data) {     super.onActivityResult(requestCode, resultCode, data);      if (requestCode == EDIT_BLOGGER_ACTIVITY_REQUEST_CODE && resultCode == RESULT_OK) {         int id = 0;         Tables tables = new Tables(                 id, // Blogger ID                 data.getStringExtra(EditBloggerActivity.EXTRA_EDIT_REPLY_NAME),                 data.getStringExtra(EditBloggerActivity.EXTRA_EDIT_REPLY_ADDRESS),                 data.getStringExtra(EditBloggerActivity.EXTRA_EDIT_REPLY_MOBILE_NUMBER),                 data.getStringExtra(EditBloggerActivity.EXTRA_EDIT_REPLY_FB),                 data.getStringExtra(EditBloggerActivity.EXTRA_EDIT_REPLY_IG),                 data.getStringExtra(EditBloggerActivity.EXTRA_EDIT_REPLY_EMAIL));         instanceOfRepository.updateInfluencerCalledFromRepository(tables);     } else {         Toast.makeText(this, "Save Failed! fill out all fields", Toast.LENGTH_LONG).show();      } } }   LOGCAT  2020-02-28 16:10:06.071 4258-4258/com.kurtsample.digitalcirclesprototype E/AndroidRuntime: FATAL EXCEPTION: main Process: com.kurtsample.digitalcirclesprototype, PID: 4258 java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String android.content.Context.getPackageName()' on a null object reference     at android.content.ContextWrapper.getPackageName(ContextWrapper.java:142)     at android.content.ComponentName.<init>(ComponentName.java:130)     at android.content.Intent.<init>(Intent.java:6082)     at com.kurtsample.digitalcirclesprototype.deleteInfluencer.callUpdateInfluencer(deleteInfluencer.java:31)     at com.kurtsample.digitalcirclesprototype.Adapter$2.onClick(Adapter.java:95)     at android.view.View.performClick(View.java:6597)     at android.view.View.performClickInternal(View.java:6574)     at android.view.View.access$3100(View.java:778)     at android.view.View$PerformClick.run(View.java:25885)     at android.os.Handler.handleCallback(Handler.java:873)     at android.os.Handler.dispatchMessage(Handler.java:99)     at android.os.Looper.loop(Looper.java:193)     at android.app.ActivityThread.main(ActivityThread.java:6669)     at java.lang.reflect.Method.invoke(Native Method)     at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)

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?


How does one show that if $2^n-1$ is a prime, then $n$ is a prime?

Let $M_p=2^p-1$, $p$ prime. Show that $M_p$ is either prime or pseudoprime : $2^{n-1} \equiv1\pmod n$  My attempt: 1)Show p not prime  $\Rightarrow$ $M_p$ not prime Assume that $p$ is not prime, then write $p=n \cdot q ,\quad n,q>1$, so $$M_p=2^p-1=2^{n \cdot q}-1=\underbrace{(2^n-1)}_{>1}\underbrace{(2^{n(q-1)}+...+2^p+1)}_{>1}$$ $\Rightarrow M_p$ is prime, if $p$ is prime. 2)Show $2^{M_p-1} \equiv 1 \pmod {M_p}$ With fermat theorem: $$2^{p-1} \equiv 1 \pmod p$$ $$2^{p-1}=1+ k\cdot p ,\quad k\in \Bbb Z$$ $$\Rightarrow 2^{M_p-1} = 2^{2kp}= (2^p)^{2k}=(M_p+1)^{2k}\equiv 1 \pmod {M_p}$$ I think this is not correct, so I would appreciate any hint, thank you.


I know the title might not be that helpful. I do apologize. My problem is that I am trying to create a simple MPG application to help me get used to GUI. I have a gallons and a miles JTextField. I also have an ueditable JTextField to display the MPG. I am trying to convert the user input to an integer then divide the gallons and miles input for the result of the miles per gallon, which will be displayed.  I have the parseInt as displayed below:      int mil = Integer.parseInt(miles1.getText());     int gal = Integer.parseInt(gallons1.getText());   then the MPG:  int mpg = gal / mil;   It shows no error in the code, but once I go to run the application I get these errors:  java.lang.NullPointerException at app.<init>(app.java:40) at app$1.run(app.java:26) at java.awt.event.InvocationEvent.dispatch(Unknown Source) at java.awt.EventQueue.dispatchEventImpl(Unknown Source) at java.awt.EventQueue.access$200(Unknown Source) at java.awt.EventQueue$3.run(Unknown Source) at java.awt.EventQueue$3.run(Unknown Source) at java.security.AccessController.doPrivileged(Native Method) at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source) at java.awt.EventQueue.dispatchEvent(Unknown Source) at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source) at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source) at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source) at java.awt.EventDispatchThread.pumpEvents(Unknown Source) at java.awt.EventDispatchThread.pumpEvents(Unknown Source) at java.awt.EventDispatchThread.run(Unknown Source)   Does anybody know what is causing this?


MWE:  \documentclass[english]{article}  \usepackage[T1]{fontenc} % This seems to be the problem \begin{document} Test \end{document}   If the fontec line is not commented out I get:    If I comment it out I get:    I need to use that package for the reasons given in .  How can I get the quality of the second version while using the fontec package?  I am using TeXworks, but a colleague with LyX has the same results.

I have the following LaTeX-document:  \documentclass[a4paper,11pt]{article} \usepackage[utf8x]{inputenc} \usepackage[ngerman]{babel}  \usepackage[T1]{fontenc}  \title{Sample} \author{Litb}  \begin{document}  \maketitle  \begin{abstract}  This is a report about my praxis phase at the company foo bar which I enjoyed. \end{abstract}  \end{document}   Much to my frustration, compiling with pdflatex creates a document with Bitmap-fonts. The reader's font-list shows that it uses Type3 fonts.  If I remove the fontenc loading, then I get a PDF using vector-fonts, but I heard that I should keep it, for having native German Umlauts working.  Does anyone know how I can get vector fonts with T1, and why it keeps using bitmap fonts if I have that package-load line in it?


I'm an amateur currently owning a Canon 700D and a few lenses (18-55, 10-18, 55-250 and 50 1.8 STM). As I try to do some stock photography, image quality is my main concern, and 700D has a very average sensor scoring only 61 at dxomark. I've heard dxomark ratings shouldn't be taken literally, but Canon APS-C sensors are known for poor performance for both DR and noise, and this is the case for 700D. I'm on a limited budget, so going full frame is out of my reach. Nikon APS-C cameras have about 10% larger sensor and don't have low pass filter, giving sharper images with less noise. I prefer Canon aesthetics more, but my main concern is image quality - do you think I should switch to Nikon or maybe the gains in IQ wouldn't be worth the cost (again, I'm selling stock photography, so with better image quality I could possibly sell more and cover the cost of the switch)?

I'm considering purchasing a DSLR for the first time here in the next couple of months. The two "big boys" seem to be Canon and Nikon. I've looked at both companies, and I can't really see significant differences between the two.  Are there any?  (Please don't go into a flame war here -- I'm looking for factual differences between the cameras, not "I enjoy X because of Y...")    Also see   for another important part of the story.


Plot Details/Summary  As best as I can remember, the focus of the story is a planet that is in deep/interstellar space, balanced between the gravitational pulls of two distant star systems.  One of these is Sol, and I do not remember the other (Centauri or Proxima, perhaps?).  The planet's is comprised of a number of frozen chemical compounds that are able to exist in this form because of the planet's unique location.  I believe supercooled helium might be one, but I can't say for sure.  The relevant point here is that the compounds form the "building blocks" for the planet's development of intelligence.  The first few paragraphs of the story chart the progress of the planet's development.  Naturally occurring electrical impulses begin a process where, over the course of several millennia, simple chemicals develop into rudimentary circuits, which in turn become more sophisticated circuits, which eventually develops into a complex, planet-wide matrix.  In effect, the planet has become a massive organic supercomputer.  Once the planet reaches the supercomputer stage, it develops an imperative to reach out and make contact.  The planet creates icy, comet-like "probes" which it launches into space in the direction of Sol system.  I'm not sure how these are launched. I do recall the story details how the first few probes were destroyed when they were close enough to a system that a change of even a few degrees Kelvin was enough for them to dissipate from the heat.   Each generation of probe survives just a bit longer than the one before.  This is the sum total of my knowledge of the story.  It was either a long-ish short story, or a novella.  In either case, I never got around to finishing the tale. Hopefully what I did read is enough to identify the story.  Publication Details/Timeframe   I read this story in the mid-1980s.  I think it was in a sci-fi magazine rather than an anthology or collection.  I am not certain of that, however.

Arthur C. Clarke wrote a short story about a huge matrioshka brain that sends out probes to the far reaches of the Universe to find out more information.  I cannot recall the title, and was hoping someone else knew.


Every search engine out there displays the number of results at the top of the result listing. Yet SO and after several requests and no feedback, still does not display the number. It feels like hiding the number is intentional!  Only showing number of pages and result/per page at the bottom is not very helpful. Plus if the results are more than 500, you have no idea what the number is.

I'd love to see the approximate number of matches found for a search from the "Search" box. This would be useful to determine the effect of your search terms - did they, or did they not, narrow down the search?  This would be especially useful now that we have the new search operators.


I need to have job interview soon, one of the questions may be L1. vs L2 regularization. Yann LeCun explained best to my knowledge the difference between L1 and L2 regularization.  : Weights are shrunk every iteration towards zero by a constant equals to ηα. It will eliminate weights (he said inputs in error) that are not very useful.   : In the absence of any gradients from C the weights exponentially decay to zero. It tried to tell the system: Minimize my cost function but to it with the weight vector that is as short as possible.  As you can see both the regularization types will minimize/eliminate the weights that are not very useful to zero. So what's the difference really? If I just say L1 will do it based on L1 norm, and L2 based on L2 norm this is probable true but that will not be the scientific mindset.

Note: I know that L1 has feature selection property. I am trying to understand  which one to choose when feature selection is completely irrelevant.   How to decide which regularization (L1 or L2) to use? What are the pros & cons of each of L1 / L2 regularization? Is it recommended to 1st do feature selection using L1 & then apply L2 on these selected variables?


My father broke his power supply for his recliner. He's on oxygen 24/7 and I was trying to find out if it was safe to use a slightly different power supply cause I can't find an exact match. The one he broke says it's input is 100-240V and output is 25V,2.5A. I can only find ones that have the output slightly different saying 24-29V, 2A or 29V, 2A. Not sure if that .5 is a big deal or weather it wouldn't work or most of all if it was unsafe being he's in his 70's and on oxygen 24/7. I wanna make him as comfortable as possible and get him back in his beloved recliner lol but didn't want to waste my time if it wouldn't work or risk his safety before buying one and leaving it plug in with him. Any help is much appreciated and thank you in advance for taking the time to read this and answer. Theres a pic of the old broken one attached also.

Power supplies are available in a wide range of voltage and current ratings.  If I have a device that has specific voltage and current ratings, how do those relate to the power ratings I need to specify?  What if I don't know the device's specs, but am replacing a previous power supply with particular ratings?  Is it OK to go lower voltage, or should it always be higher?  What about current?  I don't want a 10 A supply to damage my 1 A device.