$p = x^2 + xy + 3y^2$ if and only if $p \equiv 1$, $3$, $4$, $5$, $9$ mod $11$?
$p = x^2 + xy + y^2$ if and only if $p \equiv 1 \text{ mod }3$?
How to retrieve the first n characters of a variable in Bash? I've occasionally noticed an OP who will edit their own question and destroy it completely, always leaving some kind of graffiti-like scrawl in its place.  This seems to happen on low-quality questions by low-rep users, and the moment of self destruction tends to occur after the question is closed or otherwise devolves into a communication breakdown between the poster and the community.  Here are some examples:        It seems like the poster usually does this in anger (1st and 3rd example), or else is just confused about how to delete their question (2nd example).  My question is about how this should be handled. In the past I wasn't sure, so I always flagged for mod attention. However I notice the solution on all of these was to just do a rollback (I did this myself on the 2nd example after nothing happened). Is this a good enough approach, or should a flag be used?  What about with particularly obnoxious users like in the third example, where the guy came back to do it again? Why is the potential energy equal to the negative integral of a force? Why is the potential energy equals to the negative integral of a force? I am really confused with this negative sign. For example, why there is a negative sign in the gravitational potential energy and what does it mean?   I read that the negative sign means you do the same force but in the opposite direction.  Doesn't that mean the object shouldn't move? Is there a single word for 'without a beginning'? Drupal SA-CORE-2014-005 - How to tell if my server / sites were compromised? I just updated all my sites using the patch method of resolving the Drupal SA-CORE-2014-005 exploit. I just read reports that just yesterday there is someone from a Russian IP infiltrating drupal sites.     My main concerns are now:   How do I tell if my sites have been comprised? What should I search for in my apache access logs to detect if my site was a victim or not? So far what are these hackers doing to comprised sites? If I squeeze something really hard, will it ever become two-dimensional? Frequently when I ask a question I have to include quite a bit of code for reference purposes.    It would be nice to be able to bold the important lines in the code.  For example this code:  &lt;DataTrigger Value="True"&gt;     &lt;DataTrigger.Binding&gt;         &lt;MultiBinding Converter="{StaticResource DisableWorkItemConverter}"&gt;             &lt;Binding ElementName="MainForm" Path="PickedWorkItemID"/&gt;             &lt;Binding Path="Id"/&gt;         &lt;/MultiBinding&gt;     &lt;/DataTrigger.Binding&gt;     &lt;Setter Property="IsEnabled" Value="False"/&gt;     &lt;Setter Property="IsSelected" Value="False"/&gt; &lt;/DataTrigger&gt;   Is not way long, but the important part (as seen in ) is the second to last line.  However, if I try to bold it, it ends up looking like this:  &lt;DataTrigger Value="True"&gt;     &lt;DataTrigger.Binding&gt;         &lt;MultiBinding Converter="{StaticResource DisableWorkItemConverter}"&gt;             &lt;Binding ElementName="MainForm" Path="PickedWorkItemID"/&gt;             &lt;Binding Path="Id"/&gt;         &lt;/MultiBinding&gt;     &lt;/DataTrigger.Binding&gt;     &lt;Setter Property="IsEnabled" Value="False"/&gt;     **&lt;Setter Property="IsSelected" Value="False"/&gt;** &lt;/DataTrigger&gt;   Not what I was going for.  It would be nice to be able to draw attention to specific lines of code when needed.    (One suggestion of a different color background sounds nice too (ie a yellow background to draw the eye to the relevant code.)  If there is some other way to bold in code please let me know. How would I have a  action that may have some effects on the current page but would also change the URL in the browser so if the user hits reload or bookmark, then the new URL is used?    It would also be nice if the back button would reload the original URL.  I am trying to record JavaScript state in the URL. Increasing Flood Plain I am doing a project where we are attempting to determine how many more buildings would flood if the FEMA flood plain increased by 1',2' and 5'. I have the elev of the flood plain rasters from lidar data, but I am having trouble figuring out how much more area will be inundated by the rises I described earlier. I'm using raster calc and I have a buffer for the flood plain that also has elevation data. Any thoughts? Why are there humans in the Star Wars Universe? How do I troubleshoot random rebooting? I am not an expert of random forest but I clearly understand that the key issue with random forest is the (random) tree generation. Can you explain me how the trees are generated? (i.e. What is the used distribution for tree generation?)  Thanks in advance ! Massless charged particles What's up with Vic? In the TV series Glitch, we see several people returning from the dead.  We see Vic, a police officer,      getting in a car accident and seemingly .   Afterwards, he behaves very strange. He searches for the doctor and the risen, whom he seems to know a lot about, sometimes even more than they know themselves.      He rekills Maria by breaking her neck.   And he threatens the other risen.  Most of the risen, especially those that have been dead longer, have some difficulty remembering their past lives. Vic on the other hand, seems to remember fine but to have a complete change of character instead.   Why is he after the other risen? How does he know so much about them? Why did he have such a change of character?     Yes, these are several questions, but they are closely related, so I opted to ask them in one question. ASP.NET MVC controller actions that return JSON or partial html How to salt PBKDF2, when generating both an AES key and a HMAC key for Encrypt then MAC? When using Encrypt-then-MAC with AES and HMAC by password, and given 128 bits of payload with the ciphertext to store a random salt, which would be more secure:   Using PBKDF2 with then entire 128 bit random salt and generating 512 bits of derived data to split into a 256 bit AES Key and a 256 bit HMAC key.   or   Using PBKDF2 separately for each key using two 64 bits salts to generate a 256 bit AES Key and a 256 bit HMAC key.    or    Better option? IDW Publishing are bringing out a , Assimilation2, set in ST:TNG (presumably the TNG in the timeline prior to the events of the 2009 movie) featuring the 11th Doctor.  Is the canon of these two universes at all compatible?  Both feature a lot of time travel, so we've seen the development of humanity through many centuries in the future.  Are there direct contradictions that make it impossible for these two universes to merge, without some sort of trick like having the Doctor come from an alternate universe?  Or, given the vast spread of time, is it possible that the events of the Star Trek stories have taken place between Doctor Who stories? In the following code, I need to use +/- where I have used it, and plus/minus where I have used it.  \documentclass{amsart} \usepackage{lipsum} \usepackage{calc} \parindent=0 pt \parskip=12 pt plus 0 pt minus 0 pt \def\headsep{25pt} \newcommand{\LAYOUT}[5]{%   \setlength{\hoffset}{0pt}   \setlength{\voffset}{0pt}   \setlength{\topmargin}{#1 - 1in - \voffset - \headheight - \headsep}    \setlength{\oddsidemargin}{#2 - 1in - \hoffset}   \setlength{\evensidemargin}{\oddsidemargin}   \setlength{\textheight}{#3}   \setlength{\textwidth}{#4}   \setlength{\marginparwidth}{#5} } \newcommand{\FULLPAGE}{\LAYOUT{1in}{1in}{9in}{6.5in}{0.75in}}  \FULLPAGE \begin{document} \lipsum[1-10] \end{document}   If I replace the plus and minus in line 5 with + and - I get an enigmatic  "missing begin{document}" error.    If I replace the minus's in lines 10-11 with "-" I get an equally enigmatic error:  "Missing number, treated as zero. Illegal unit of measure (pt inserted)."   Can somebody please explain when one can use +/- and when one can use plus/minus?  Presumably there's a good reason why latex's error messages are so completely mystifying, and why inconsistencies like these keep appearing? It's really hard to explain to my coauthors, who regularly complain that latex is the most infuriating program ever devised by mankind.  Thanks very much for any advice!   Leo User is able to edit a file using vim I have a file named file.txt with some contents located in /home/myuser directory and I change the permissions of that file to root.root and 644.   -rw-r--r-- 1 root root 1.4K Jan 15 07:09 file.txt   Then I switched to myuser and opened it with vim(without sudo) and tried to edit and save it :wq! and I got output like:  "file.txt" E949: File changed while writing Press ENTER or type command to continue   Then I hit enter and then again :wq! and the output was :  WARNING: The file has been changed since reading it!!! Do you really want to write to it (y/n)?   I hit y, the file got saved and exit from vim.  The question is why a file with ROOT privileges and read-only access for a user changed by the way?  PS: 1. myuser is a sudo user.   The user permissions after saving the file changed to myuser.myuser as well.  -rw-r--r-- 1 myuser myuser 1.4K Jun 27 18:57 file.txt How can I remove fonts that I never use, from libreoffice and linux in general? There are a lot of extra fonts that all seem to look alike and just annoy me in the drop down list. Is there anyway I can erase them from the system?