JavaScript - How to check if alphabetical characters or symbols have been entered in a number input field
HTML text input allow only numeric input
How do you validate that two math expressions are equal? Let's say you have a few expressions like the following:  $$\begin{array}((x+17)^2 \\ x^2 + 34x + 289 \end{array} \\ 288 + \frac{x^2}{2} + \frac{x^2}{2} + 34x + 1 \\ [...] $$  You get the idea: there's an infinite number of ways of representing the same expression. The challenge is that my application is validating answers to math problems, input by math students practicing a certain concept. I'd like to make sure that whenever they submit an answer, the checker is completely insensitive to the issues of ordering, simplification, formatting etc. which would be plain frustrating. I'd also ideally like to support more than one variable such as x, y, z in the same expression.  What's a reliable way to check that two expressions are the same? The most brute force way I can think of would be to just plug a number into each variable of each expression and see what the result is.  I have a hunch however that there might be ways of getting false positives this way. That, and I might be either missing something really obvious about this, or there might be a better way of doing the comparison.  Would love to hear what you think. Huge close votes review queue on Stack Overflow Today, the size of the close-votes queue is 55k questions and growing. Is that a problem?   Which action could be taken to clean it up? It seems to be growing faster than Stack&nbsp;Overflow members are capable to handle it. How to set up RAID 1? How to *quickly* get a list of files that have bad sectors/blocks/clusters/whatever? My wife's Windows 7 (64-bit) box has suddenly developed a SMART "disk is bad" status.  I'm attempting to copy everything off (no admonishments about lacking a backup regimen, please, I know already  :(  ) by creating a System Image across the network to a different machine, but it gets to a certain point and starts taking forever.  Doing a chkdsk reveals that certain files cause this by having many bad blocks (like dozens of thousands in a row, if the event log is any indication) and causing the system to do its standard try-to-recover-and-relocate-upon-access thing.  But this is taking so long, I'm afraid the disk will fail completely before I can get the damned thing copied.  However, several of the files so far have been ones that she has copies of elsewhere, so I am able to just delete them prior to retrying the backup to speed things up considerably.  So:  is there some tool or procedure that will try reading each file, and upon hitting a bad block, just tell me about it and skip to the next file?  So I can see which ones I can just dump and which I need to let it try to recover? What's the easiest way to run GUI apps on Windows Subsystem for Linux as of 2018? How to Make Multi User Objects Single User? NOTE: Signing up or Logging in via OpenID is no longer officially supported from any of our sites. Currently,  is still on the internet, but we are in the process of completely removing it.    Stack Overflow was an early and strong supporter of . We built our sign up/log in flow around it. We were idealistic and had , but these hopes weren't realized. Over the years people have wondered if  is . We've had to remove support as OpenID providers  or .  The time has come to part ways. The reality is OpenID support has created a ton of complexity in our codebase and the number of users actively using OpenID simply don't justify that cost. Users have spoken with their actions. You prefer Google, Facebook and Stack Exchange (aka email/PW) based account auth.     Of our 9,813,747 accounts around one-tenth of a percent are actively using OpenID (defined as having visited a site in the past 12 months). If you include all the inactive accounts it is still less than 2.9% of all accounts.  So, what is the plan?   DONE - Remove the ability to create a new account with most OpenID providers DONE - Add the ability to  DONE - Convert Launchpad (used on  ) to OAuth DONE - Remove creating new accounts with Steam from  (Steam removed support for OAuth. Only 454 accounts that use Steam have visited Arqade in the past 6 months.) DONE - We will email all users that rely on deprecated OpenID credentials and request that they  DONE - Run an announcement banner for any user who signs in with deprecated OpenID credentials urging them to  DONE - End support for logging in via OpenID  In progress - Remove  (note: only 0.2% of authorized sites in the last year have been non-Stack Exchange sites.)   Obviously, if your account relies on OpenID, then make life easy and go now and .   If you've forgotten what you credentials you use to login, then  to refresh your memory. If you don't see one of the three account types shown in the image below (Google, Facebook or Stack Exchange) then click the "add more logins" button to add one of them to your account.    Remember, SE OpenID accounts are really email/pw accounts handled through complicated OpenID code path. They will be migrated to a simpler email/pw only code path as a part of this change. No extra work required on your part. Book about a teenager programmed to be an assassin and trying to get back at the people responsible I read this book a few years back (around 5?). It was part of a multibook series but I have only read one book.  It's about this teenager who I think had been programmed to be an assassin but then somehow retained his conscience and was out to get the people responsible for that.  I briefly remember a girl, and this one part where he realizes he can ride a horse and then there's a chase too. I think the guy's name was Jim or Jimmy, but not quite. I'm also quite sure he was either part computer, or had a chip or something in his brain which had been programmed to give him a host of mental and physical abilities which he retained and used as well as the series progressed. error when adding user to wheel group in CentOS 7 I am trying to grant sudo privileges to a user of a CentOS 7 devbox by typing gpasswd -a user wheel, but the attempt is failing.  What am I doing wrong?    Here is what I typed:    I discovered that the user is not in the wheel group as follows:    [user@localhost git]$ sudo yum install git [sudo] password for user:  Sorry, try again. [sudo] password for user:  Sorry, try again. [sudo] password for user:  user is not in the sudoers file.  This incident will be reported.   I then logged in as root and typed the following to add the user to the wheel group:    [user@localhost git]$ su - Password:  Last login: Thu Aug 20 17:11:31 PDT 2015 on pts/0 [root@localhost ~]# gpasswd -a user wheel Adding user user to group wheel [root@localhost ~]# exit logout   I finally tried to use the sudo command again but it failed as follows:    [user@localhost git]$ sudo yum install git [sudo] password for user:  user is not in the sudoers file.  This incident will be reported. [user@localhost git]$ Minor grid in pgfplot not showing when specifying ticks manualy Firstly I would like a minor tick between every major tick in my pgfplots graph (and minor grids). I wrote minor tick num=1 but it doesn't work. I am not using a logarithmic axis so it is unrelated to other questions.  Secondly, why is the tick label "1" missing for each axis? How do I get that to display?     \documentclass{standalone} \usepackage{pgfplots} \begin{document} \begin{tikzpicture}  \begin{axis}[% footnotesize, width=4in, height=4in, axis equal, scale only axis, separate axis lines, grid=both, every outer x axis line/.append style={black}, every x tick label/.append style={font=\color{black}}, xmin=0, xmax=1, xlabel={X}, every outer y axis line/.append style={black}, every y tick label/.append style={font=\color{black}}, ymin=0, ymax=1, ylabel={Y}, minor tick num=1, xtick={0,0.1,...,1}, ytick={0,0.1,...,1}, axis background/.style={fill=white}, legend style={at={(0.97,0.03)},anchor=south east,legend cell align=left,align=left,fill=white} ]  \end{axis}  \end{tikzpicture}% \end{document} A function that is continuous at the origin but not differentiable there Show that $f(x,y)=\dfrac{xy^2}{x^2+y^2}$ (with $(x,y)\not=(0,0)$ and $f(0,0)=0$) is continuous but not differentiable at $(0,0)$.  I tried to show continuity with an $\epsilon -\delta$ argument but I don't know how to factorize the expression so that I can have something useful.   For differentiability I think I should show that the partials are not continuous at $(0,0)$. But finding the partials is also painful. What is a word for getting an award in a competition or being one of the top three? Did Voldemort refer to Snape as one of the missing Death Eaters at his rebirth? In Goblet of Fire, Voldemort says of the missing six Death Eaters:     “And here we have six missing Death Eaters… three dead in my service. One,   too cowardly to return… he will pay. One, who I believe has left me forever… he   will be killed, of course… and one, who remains my most faithful servant, and   who has already reentered my service."   The answer to  speculates about who the three dead Death Eaters were.  We know who the last one is. I’m guessing one of the remaining two is Karkaroff. Is the other Snape? If so, which one? Breakthrough at the Large Hadron Collider -Leptoquarks skepticism? Expected number of siblings, given the expectation of children in a family The number of children in a family is a random variable $X$, where    $ \mathbb E(X) = 1.8 $ and $Var(X) = 0.36 $. If we randomly choose a child, which is the expected number of its siblings?      (it is given that the answer $&gt; 0.8$)   Let the number of a child's siblings be a random variable $Y$, so we are looking for $ \mathbb E(Y) $.  If found that,  $$ \mathbb E(Y) = \sum_{k=0}^{\infty}k \; \mathbb P(Y=k) = \sum_{k=0}^{\infty}k \; \mathbb P(X=k+1) = $$ $$ = \sum_{k=0}^{\infty}k \; \mathbb P(X=k)-\sum_{k=0}^{\infty}\mathbb P(X=k) +\mathbb P(X=0)= \mathbb E(X)-1+\mathbb P(X=0) $$ which is indeed greater than $0.8$, if  $\; \mathbb P(X=0)&gt;0$.  But, I don't know how to make use of the variance to find the final answer.  Thank you in advance Where's my 5 minute grace period gone? I just posted an answer. (Revision history .)  I edited it a few seconds later (within my 5 minute grace period), and it comes up as a new revision!? Why?    ^^You can see 'answered 1 min ago', and then 'edited 1 min ago' - how is that possible?  Additionally, !?    I don't think this is a dupe of . See my comments on AI.E's answer. If you're a mod, it will really help. $f : S^1 \to\mathbb R$ is continuous then $f(x)=f(-x)$ for some $x\in S^1$ Question is to prove :  $f : S^1 \to \mathbb R$ is continuous then $f(x)=f(-x)$ for some $x\in S^1$  I guess it would be helpful to use intermediate value theorem  Assuming $f(x)\neq f(-x)$ then given any $p\in (f(-x),f(x))$ (assuming $f(-x)&lt;f(x)$) there exists $y\in (-x, x)$ such that $f(y)=p$  I am not very sure of how to use this..   It would be helpful if someone can give some hint which would help me to solve this..  Thank you. In Windows 7, How to query times, when the computer was locked? Is there a way to get a list of the following times:  whenever windows is locked (using windows+l combo)  whenever windows is unlocked in the past week?   I've tried poking in the event log, to no avail. Many thanks! A singularity, by the definition I know, is a point in space with infinite of a property such as density.  Density is Mass/Volume.  Since the volume of a singularity is 0, then the density will thus become infinite because Mass/0 = undefined  However, is it possible to have a singularity with a mass of 0? 0/0 is indeterminate, but would it be possible for a singularity to exist even if its mass and volume were zero? I want to write applications for Ubuntu. I do not know much about programming but I want develop for Ubuntu. I am trying to learn Python right now. What do you think is the best way to develop apps for Ubuntu? Can you point me to some resources that can help me get started with developing for Ubuntu?  Related .