C# is forcing me to explicitly cast when it shouldn't, why?
Why enums require an explicit cast to int type?
How to run a server on port 80 as a normal user on Linux? I have Googled about a solution for quite some time, but couldn't find an answer.   I am on Ubuntu Linux and want to run a server on port 80, but due to security mechanism of Ubuntu, I get the following error:      java.net.BindException: Permission denied:80   I think it should be simple enough to either disable this security mechanism so that port 80 is available to all users or to assign required privileges to the current user to access port 80. How to parse a month name (string) to an integer for comparison in C#? I need to be able to compare some month names I have in an array.  It would be nice if there were some direct way like:  Month.toInt("January") &gt; Month.toInt("May")   My Google searching seems to suggest the only way is to write your own method, but this seems like a common enough problem that I would think it would have been already implemented in .Net, anyone done this before? XeLaTeX update [20201004] and item ball in Beamer How does a lurker gain reputation to receive privilege for commenting? Is there a word for somebody who cares for his/her child too much? I have just finished watching the 3rd season of .  I am wonder whether there is a word for somebody who cares for his/her son/daughter/child too much, like Dr. Cal Lightman? Explain proof by contradiction. TV show that starts with debris encompassing Earth question about the limit $\lim_{h\to0}\frac{\arcsin(x+h)-\arcsin(x)}{h}$ If, one fine day, someone found a contradiction in ZFC (or even ZF), what implications would such an event have for mathematicians? Is there currently any backup axiomatic system on par with ZFC that would enable mathematicians to continue pursuing mathematics, should the unthinkable occur? uniq command not working properly? So I'm checking the md5 hash of my files with this as my output:  657cf4512a77bf47c39a0482be8e41e0  ./dupes2.txt 657cf4512a77bf47c39a0482be8e41e0  ./dupes.txt 8d60a927ce0f411ec94ac26a4785f749  ./derpina.txt 15f63928b8a1d5337137c38b5d66eed3  ./foo.txt 8d60a927ce0f411ec94ac26a4785f749  ./derp.txt   However, after running find . -type f -exec md5sum '{}' ';' | uniq -w 33 to find the unique hashes I get this:  657cf4512a77bf47c39a0482be8e41e0  ./dupes2.txt 8d60a927ce0f411ec94ac26a4785f749  ./derpina.txt 15f63928b8a1d5337137c38b5d66eed3  ./foo.txt 8d60a927ce0f411ec94ac26a4785f749  ./derp.txt   From my understanding, only one of either derpina.txt or derp.txt should be showing up since their hashes are the same. Am I missing something? Can anyone enlighten me as to why it outputs like this? How to extract and join files xxx.zip, xxx.z01 and xxx.z02 I have three files 1.zip, 1.z01 1.z02. I would like to extract and join them into a file, by right clicking 1.zip and selecting "Extract Here". But then there comes an error:     7-Zip 9.04 beta  Copyright (c)   1999-2009 Igor Pavlov  2009-05-30   p7zip Version 9.04   (locale=en_US.utf8,Utf16=on,HugeFiles=on,2   CPUs)      Error: /home/tim/Desktop/1.zip:   Can not open file as archive      Errors: 1   I was wondering what the problem it is and how I can solve it?  Thanks and regards!    ADDED: Is my usage of "unzip -FF" wrong? It seems not work:  $ unzip -FF 1.zip  Archive:  1.zip warning [1.zip]:  zipfile claims to be last disk of a multi-part archive;   attempting to process anyway, assuming all parts have been concatenated   together in order.  Expect "errors" and warnings...true multi-part support   doesn't exist yet (coming soon). file #1:  bad zipfile offset (local header sig):  0   Thanks again!  2nd ADDED: Still ain't right.  $ cat 1.z01 1.z02 1.zip &gt; combined.zip &amp;&amp; unzip -FF combined.zip Archive:  combined.zip warning [combined.zip]:  zipfile claims to be last disk of a multi-part archive;   attempting to process anyway, assuming all parts have been concatenated   together in order.  Expect "errors" and warnings...true multi-part support   doesn't exist yet (coming soon). warning [combined.zip]:  209829313 extra bytes at beginning or within zipfile   (attempting to process anyway) file #1:  bad zipfile offset (local header sig):  209829313   (attempting to re-compensate)  extracting: 1.wmv   bad CRC ee181eef  (should be f3c61875)   It generated 1.wmv, which can play well only until  midway in Gnome Mplayer.  3rd ADDED: Am I using 7z wrongly:  $ 7z e 1.z01  7-Zip 9.04 beta  Copyright (c) 1999-2009 Igor Pavlov  2009-05-30 p7zip Version 9.04 (locale=en_US.utf8,Utf16=on,HugeFiles=on,2 CPUs)  Processing archive: 1.z01  Error: Can not open file as archive  $ 7z e 1.zip  7-Zip 9.04 beta  Copyright (c) 1999-2009 Igor Pavlov  2009-05-30 p7zip Version 9.04 (locale=en_US.utf8,Utf16=on,HugeFiles=on,2 CPUs)  Processing archive: 1.zip  Error: Can not open file as archive Solve the following system of equations  $ \begin{cases}  x_1^{'}(t)=x_1(t)+3x_2(t) \\  x_2^{'}(t)=3x_1(t)-2x_2(t)-x_3(t) \\  x_3^{'}=-x_2(t)+x_3(t)\end{cases} $.  First, I create the column vectors $X$ and $X^{'}$. Then the matrix $$A= \begin{bmatrix} 1 &amp; 3 &amp; 0  \\ 3 &amp; -2 &amp; -1 \\ 0 &amp; -1 &amp; 1  \\ \end{bmatrix} $$  Now, I find the eigenvalues, $-4,3,1$ and their corresponding eigenvectors $(-3,5,1)^T (-3,-2,1)^T (1,0,3)^T$.   I'm just not sure how to take it from here and solve the system of differential equations. I want a diagonal matrix $D$ so that I can read the solutions easy, but I'm not sure how to do it.  EDIT   Building on @Francisco 's answer, I'd have that:  $$X=c_1  (-3,5,1)^T  e^{-4t}   + c_2 (-3,-2,1)^T e^{3t} + c_3 (1,0,3)^T e^{t} $$.  But I believe this could be written in a simpler form. In PHP, what does the ${$ } syntax do? How can I display a short chapter name in the header and a long chapter name in the ToC? I am writing a book and one of my chapters has a very long title. As it can't be displayed correctly on the top of even pages, I provide a "short title name" via the   \chapter[short title]{long title}   command. But then, "short title" appears in the table of contents instead of the original/true/long one.  How can I have the "short title" in the header of even pages, and the "long title" in the ToC and in the "Title header"?  For reference, here is the code I use:  \documentclass[11pt,a4paper]{book} \begin{document} \tableofcontents  \chapter[Short title]{Very vey very very very very very very long title     i can't display in the header} Lorem ipsum dolor sit amet, consectetur adipiscing elit. \end{document} Why is "cupboard" pronounced with a silent "p"? Asking the position of a person in a sequence Fastest way to convert string to integer in PHP Using PHP, what's the fastest way to convert a string like this: "123" to an integer?  Why is that particular method the fastest? What happens if it gets unexpected input, such as "hello" or an array? Multi document printing from document library? Is there any one have solution for print multiple documents from Document library? User has option to select multiple documents or single document or inside folder.  I found commercial product "Unbounded Printing Services for SharePoint". But is so expensive. Please share me if you have any thoughts. Sign of determinant when using $det A^\top A$ We have been given matrix: $$A =          \begin{pmatrix}         a&amp; b&amp; c &amp;d \\         b &amp;−a&amp; d&amp; −c\\         c&amp; −d &amp;−a&amp; b \\         d &amp;c&amp; −b&amp; −a\\         \end{pmatrix}     $$ ...and have been asked to calculate $\det(A)$ using $AA^T$.  We see that:  $$AA^T=\begin{pmatrix}         a^2+ b^2+ c^2+ d^2&amp; 0&amp; 0&amp;0\\         0 &amp;a^2+ b^2+ c^2+ d^2&amp; 0&amp; 0\\         0&amp; 0 &amp;a^2+ b^2+ c^2+ d^2&amp; b \\        0 &amp;0&amp; 0&amp; a^2+ b^2+ c^2+ d^2\\         \end{pmatrix}     $$   So, $\det(AA^T)= (a^2+ b^2+ c^2+ d^2)^4$  Now which should I choose: $(a^2+ b^2+ c^2+ d^2)^2 $ or $ -(a^2+ b^2+ c^2+ d^2)^2$ ?  Please explain me which one and why. Proving that a number with digits 1...9 in some order, ending in 5, is not a perfect square. If "x" is a 9 digit number , which contains digits from 1 to 9 which ends in 5 . Prove that it can't be a perfect square ( digits are not to be repeated).  Please suggest the solution of this question.