What is the relation between web server like "apache2" and this user www-data?
What is the www-data user?
Is kinetic energy a relative quantity? Will it make inconsistent equations when applying it to the conservation of energy equations? If the velocity is a relative quantity, will it make inconsistent equations when applying it to the conservation of energy equations?  For example:  In the train moving at $V$ relative to ground, there is an object moving at $v$ relative to the frame in the same direction the frame moves. Observer on the ground calculates the object kinetic energy as $\frac{1}{2}m(v+V)^2$. However, another observer on the frame calculates the energy as $\frac{1}{2}mv^2$. When each of these equations is plugged into the conservation of energy, they will result in 2 distinct results (I think). Ephemeral error message: "cryptsetup: unknown fstype, bad password or options?" For all square matrices $A$ and $B$ of the same size, it is true that $(A+B)^2 = A^2 + 2AB + B^2$? The below statement is a true/false exercise.  Statement: For all square matrices A and B of the same size, it is true that $(A + B)2 = A^2 + 2AB + B^2$.  My thought process: Since it is not a proof, I figure I can show by example and come to a valid conclusion based on such example.  My work:  Come up with a square matrix A and B let both be a 2 by 2 matrix(rows and cols must be same).  Matrix $A$:  $A = \begin{array}{ccc} 3 &amp; 5 \\ 4 &amp; 6 \\ \end{array} $  Matrix $B$:  $B =  \begin{array}{ccc} 5 &amp; 8 \\ 9 &amp; 4 \\ \end{array} $  $A + B =  \begin{array}{ccc} 8 &amp; 13 \\ 13 &amp; 10 \\ \end{array}$  $(A + B)^2 =  \begin{array}{ccc} 233 &amp; 234 \\ 234 &amp; 264 \\ \end{array}$  $A^2 =  \begin{array}{ccc} 29 &amp; 45 \\ 36 &amp; 56 \\ \end{array}$  $(AB) =  \begin{array}{ccc} 60 &amp; 44 \\ 74 &amp; 56 \\ \end{array}$  $2(AB) =  \begin{array}{ccc} 120 &amp; 88 \\ 234 &amp; 112 \\ \end{array}$  $B^2 =  \begin{array}{ccc} 97 &amp; 72 \\ 81 &amp; 88 \\ \end{array}$  $A^2 + 2AB + B^2 =  \begin{array}{ccc} 246 &amp; 205 \\ 265 &amp; 256 \\ \end{array}$  Based my above work, the answer is false.   Is there another way to approach the problem? It seems like a lot of work needed to be done for a true/false question which raised my suspicion about whether there is a better way to look at the problem. Contraction mapping in the context of $f(x_n)=x_{n+1}$. How to copy the complete folder structure one location to another location include all files and sub folders I have folder structure like in Shared documents/East using CSOM and C# Main Folder   Folder1   Folder2      File1      File2  I have to copy all folders with files to Shared documents/West .... How to prove floor identities? From what I've seen it looks to me like Libav's avconv is thought to become a successor of ffmpeg — is that correct?   If that's true, why is that so? What exactly is libav doing better and why would I want to choose it over ffmpeg?   I came to this "issue" while installing Ubuntu 12.04.1 LTS in a VM and when I've installed ffmpeg it came up with a message telling mit ffmpeg is deprecated and I first thought "wtf?".  If I use avconvWill I be able to migrate my scripts depending on ffmpeg more or less easy to libav? I'm mostly using it for writing meta data and doing audio conversion (wav, flac, vorbis, mp3, aac). Show that $A$ is an invertible matrix if $\left(A+I\right)^3=0$ and find $A^{-1}$ What is LookDev/Material Preview mode for in 2.8? Should spam posts be edited? I noticed that the posts of the recent spam surge on Ask Ubuntu were all edited by community members into something like     EDITED - REMOVED SPAM ANSWER   or similar.  While I understand why the users have done this and I too don't want to leave the spam content visible for longer than absolutely necessary, it made flagging more time consuming for me.  I had to check the revision for every of those possible spam posts to check if it is actually spam, instead of being able to flag directly from the review site. Spam flags carry a serious penalty, I'm not willing to add my spam flag without checking that it's really spam.  I see the benefit of editing spam, but it also causes some more effort for the later flaggers. Should we as a general policy edit out the content of spam posts, or should we deal with them solely by flagging? How to center column values in a table? How can I center the column values in this code?  \documentclass[conference]{IEEEtran} \begin{document} \begin{table}   \centering   \begin{tabular}{|p{2.5cm}|p{2.5cm}|p{2.5cm}|}     \hline     Reconstruction strategy &amp; aa          &amp; bb( \%) \\ \hline     Classic                 &amp; 3342 voxels &amp; 68 \%   \\ \hline     VC                      &amp; 4296 voxels &amp; 87 \%   \\ \hline     V m=7                   &amp; 4745 voxels &amp; 96 \%   \\ \hline   \end{tabular}   \newline\newline    \caption{title}\label{tab1} \end{table} \end{document} Should outdated answers be voted down? On Stack Overflow, there are many questions that relate to currently-developing technologies such as HTML5 and CSS3. The accepted answers to these questions may become outdated, and better answers may appear. However, because the answers are accepted, it is likely that they will continue to be upvoted by users reading the answers, and the OP will likely not notice that a better, more up-to-date answer may exist.  Should these outdated answers be voted down? If not, what should we do with them? Best File System for Sharing Between OS X and Windows Prove that the product of two continuous functions is continuous I have really no idea about this:  Problem: Show that there exists a function $f:[0,1]\rightarrow\mathbb{R}$ such that:  $f$ is discontinuous in all $x\in \mathbb Q$. $f$ is increasing in $[0,1]$. $f$ is integrable.   EDIT: Sorry, it is not discontinuous in all $x\in \mathbb R \setminus \mathbb Q$, just in $\mathbb Q$. Counting points in polygons using QGIS Can every even integer be expressed as the difference of two primes? Can every even integer be expressed as the difference of two primes? If so, is there any elementary proof? How do you split a list into evenly sized chunks? Can we implement the low quality flags to delete posts after a certain number of flags? Can we implement the low quality flag to be like the abuse or spam flags. So if a certain number of low quality flags are made  on a post (whether it be a questions or an answer) the post is deleted?  This would also relieve *close vote and delete vote queues. *which is a much discussed topic.        related: Increase progress bar each second -- error