A have found the following snippet:  int a[100]; ... int value = 42[a];   Which appears to do exactly what a[42] does.   Is it a bogus with undefined behavior or a perfectly legal C++ code?
As Joel points out in , in  (aka: K &amp; R), there is mention of this property of arrays in C: a[5] == 5[a]  Joel says that it's because of pointer arithmetic but I still don't understand. Why does a[5] == 5[a]?
Why does do-release-upgrade on Xubuntu running version 18.04 not offer to update to 20.04.1? I've understood that when I run the Software Updater GUI program, or when I execute do-release-upgrade on the command line, it should offer to upgrade to the latest LTS release if (1) I've got the release update behaviour set to lts, and (2) if the first point release of a newer LTS release is available. I have a couple of machines running the Xubuntu LTS release 18.04. The contents of file etc/update-manager/release-upgrades on all these machines is [DEFAULT] Prompt=lts  That satisfies condition (1). Also, it seems that Xubuntu 20.04.1 is already released, satisfying condition (2). However, neither do-release-upgrade nor the Software Updater offers to upgrade to that release. Why? Edited to add: It seems this question is a duplicate of . I know that for every $n\in\mathbb{N}$, $n\ge 1$, there exists $p(x)\in\mathbb{F}_p[x]$ s.t. $\deg p(x)=n$ and $p(x)$ is irreducible over $\mathbb{F}_p$.     I am interested in counting how many such $p(x)$ there exist (that is, given $n\in\mathbb{N}$, $n\ge 1$, how many irreducible polynomials of degree $n$ exist over $\mathbb{F}_p$).   I don't have a counting strategy and I don't expect a closed formula, but maybe we can find something like "there exist $X$ irreducible polynomials of degree $n$ where $X$ is the number of...".  What are your thoughts ? $x^TAx=0$ for all $x$ when $A$ is a skew symmetric matrix Let $A$ be an $n\times n$ skew symmetric matrix. Show that $x^TAx =0 \ \forall x \in \mathbb R^n$.  How to prove this? Show a connected metric space (X,d) is $\epsilon$-chainable for $\epsilon &gt;0$       $\epsilon$-chainable Definition      (X,d) is $\epsilon$-chainable if given any two points $a,b \in X$, $\exists$ a finite set $\{ x_0,x_1,...,x_n\}$ of points in X with $x_0 = a$, $x_n=b$ and $d(x_{n-1},x_n)&lt;\epsilon$, $\forall$ n      Clopen Definition      A set is clopen $iff$ its boundary is empty      Connected Metric Definition      Let $(X,d)$ be a metric space. It is connected $iff$ it has no proper subsets which are clopen      The following are equivalent:         $X$ is connected   The only subsets of $X$ that are clopen are $\emptyset$ and $X$   $\nexists$ non-empty, disjoint, open sets $A$ and $B$ such that $X=A\bigcup B$   $\nexists$ non-empty, disjoint, closed sets $A$ and $B$ such that $X=A\bigcup B$         Theorem      Let $(X,d)$ be a metric space, and let $S$ be any subset of $X$. Then $S$ is closed $iff$ $S^c$ is open.     So how would someone show this exactly?  Let the set $A$ be the set of all elements in $(X,d)$ that can be reached from $a\in X$ in a finite number of steps with length less than $\epsilon$.  Does this mean that there should be no isolated points? (an isolated point would be at distance infinity from any other point, or distance zero from itself, since $\epsilon &gt;0$ we don't want this?)  Knowing that $X$ is connected $\implies$the only subsets of X that are clopen are $\emptyset$ and $X$  Also, by construction of A;   A is the set of all $\epsilon$-chains between any element and $a$  And, so $A^c = \emptyset$ if $A$ was $\epsilon$-chainable (because A has to have a path from all elements to a, for this to be true, the complement wouldn't have anything in it)  Leaving only $A$ as the remaining clopen set  Does this make any sense? Or should an alternative be used, and how to formalize it.  Thanks. How to pin either a Shortcut or a Batch file to the new Windows 7, 8 and 10 Taskbar and start menu? We are having trouble adding our batch scripts to the Windows 7, 8 or 10 taskbar or start menu.  Our batchfiles take some arguments and just execute other application based on those arguments . To keep it simple, we created some shortcuts for our customers which they just need to click in order to get everything running.  The question here is: How can we pin those shortcuts or batch files to the taskbar or start menu?  (The customers aren't supposed to access the file system and that's why they can only see the taskbar) Better way of saying "those who are curious"? I am writing a note in which I want to ask (curious) readers to refer another paper. The phrase curious minded comes to mind, but it seems somewhat awkward.  Is there a better way to convey similar import? How to "find and replace" in a PDF on OS X? Sending the output from 'dd' to awk/sed/grep I'm trying to approximate the computer's write speed using dd:  dd if=/dev/urandom of=/dev/null bs=1K count=10000   which gives the following output  10000+0 records in 10000+0 records out 10240000 bytes (10 MB) copied, 0.746313 s, 13.7 MB/s   How can I get the '13.7 MB/s' into a bash variable? I've tried piping the output from dd to progs like awk, sed and grep to no avail.  Ultimately, I'm calling this via os.system(...) from within a python script. If anyone knows of a more direct way to get a similar result inside python I'd be interested in that also. I'm trying to predict how long a file copy will take, based on the file size.  Any help would be greatly appreciated. Are there areas of mathematics (current or future) that cannot be formalized in set theory? I often read that ZFC can formalize "most" of everyday mathematics, but I could never find an example which it cannot. The closest I got is differential geometry (DF), where some article mentions that the translation from concepts of DF into set theory would render it almost "meaningless", in the sense that any intuitions one could have would be lost in the translation. So, if I didn't misunderstand, at least in theory, the translation could be done, so that a future AI machine could prove theorems of differential geometry using set theory. On the other hand, the Stanford Encyclopedia of  Philosophy states that "all of mathematics can be reduced to a formal system of set theory"().   So, the questions are:    Is there any current area of mathematics that could be not formalized within set theory? If so, which ones? and  Is there any reason to think that every math of the future will be always formalizable in set theory? any references to read on the subject are welcomed!   Clarification: by set theory I am not restricting to ZFC, but to any flavor of it, including all additional axioms you want (about large cardinals or any kind) What exactly are DLL files, and how do they work? How exactly do DLL files work? There seems to be an awful lot of them, but I don't know what they are or how they work.  So, what's the deal with them? Set limit to one author when using "et al." in biblatex In the header of my document I set the maximal author names to 2:  \usepackage[style=authoryear-icomp, maxbibnames=9, maxcitenames=2, backend=biber]{biblatex}   Now, when I cite texts with two authors, LaTeX gives me the following:     (Author_A/ Author_B 2012: 232)   When I have three authors or more, LaTeX makes it:     (Author_A/ Author_B et al. 2012: 232)   But I want LaTeX to just name the first author if there are more than two authors ... like this:     (Author_A et al. 2012: 232)   Any tips on how to achieve this ? When I apply to a PhD program, may I ask a postdoc who closely supervised me to write a recommendation letter? My supervisor also has written one for me, but I believe the postdoc can comment on my research skills in a more detailed and precise way. Does a postdoc have enough authority to convince admission committee? TeX Live 2016: minted doesn't work with LuaLaTeX (bug fixed in June 2016) I've installed TeX Live 2016 as it was released today (thanks to the developers for that).  Now I have the following MWE:  \documentclass{article} \usepackage{minted} \begin{document}  \begin{minted}{ruby} class Foo   def init     pi = Math::PI     @var = "Pi is approx. #{pi}"   end end \end{minted}  \end{document}   When running pdflatex --shell-escape test.tex it runs through the file and produces the PDF as expected. Doing the same with lualatex --shell-escape test.tex fails with  This is LuaTeX, Version 0.95.0 (TeX Live 2016)   system commands enabled. (./test.tex LaTeX2e &lt;2016/03/31&gt; Babel &lt;3.9r&gt; and hyphenation patterns for 1 language(s) loaded. (/opt/texlive/2016/texmf-dist/tex/latex/base/article.cls Document Class: article 2014/09/29 v1.4h Standard LaTeX document class (/opt/texlive/2016/texmf-dist/tex/latex/base/size10.clo)) (/opt/texlive/2016/texmf-dist/tex/latex/minted/minted.sty (/opt/texlive/2016/texmf-dist/tex/latex/graphics/keyval.sty) (/opt/texlive/2016/texmf-dist/tex/latex/oberdiek/kvoptions.sty (/opt/texlive/2016/texmf-dist/tex/generic/oberdiek/ltxcmds.sty) (/opt/texlive/2016/texmf-dist/tex/generic/oberdiek/kvsetkeys.sty (/opt/texlive/2016/texmf-dist/tex/generic/oberdiek/infwarerr.sty) (/opt/texlive/2016/texmf-dist/tex/generic/oberdiek/etexcmds.sty (/opt/texlive/2016/texmf-dist/tex/generic/oberdiek/ifluatex.sty)))) (/opt/texlive/2016/texmf-dist/tex/latex/fancyvrb/fancyvrb.sty Style option: `fancyvrb' v2.7a, with DG/SPQR fixes, and firstline=lastline fix  &lt;2008/02/07&gt; (tvz)) (/opt/texlive/2016/texmf-dist/tex/latex/float/float.sty) (/opt/texlive/2016/texmf-dist/tex/latex/base/ifthen.sty) (/opt/texlive/2016/texmf-dist/tex/latex/tools/calc.sty) (/opt/texlive/2016/texmf-dist/tex/latex/ifplatform/ifplatform.sty (/opt/texlive/2016/texmf-dist/tex/generic/oberdiek/pdftexcmds.sty (/opt/texlive/2016/texmf-dist/tex/generic/oberdiek/ifpdf.sty) (/opt/texlive/2016/texmf-dist/tex/generic/oberdiek/luatex-loader.sty (/opt/texlive/2016/texmf-dist/scripts/oberdiek/oberdiek.luatex.lua))) (/opt/texlive/2016/texmf-dist/tex/generic/oberdiek/catchfile.sty) uname -s &gt; "test.w18"  ! Package catchfile Error: File `test.w18' not found.  See the catchfile package documentation for explanation. Type  H &lt;return&gt;  for immediate help.  ...                                                l.93     \CatchFileDef\@tempa{\ip@file}{}  ?    As mentioned, it's a freshly installed TeX Live 2016; I've also checked the installed Pygmentize version, which is 2.1.3 (installed via pip). Testing with TeX Live 2015, the above code, however, works without problems with both pdflatex and lualatex.  Does anybody have a hint for me on how to fix this, or is it a bug in lualatex or minted (or whatever other package) that should be reported?  Update: With minted version 2.2 (2016-06-08), the package loads the shellesc package automatically, as mentioned by @David Carlisle and @egreg in the comments; therefore, the problem should not occur any more, if you have this version installed. How were the solar masses and distance of the GW150914 merger event calculated from the signal? I'm new here. Can anyone give me a hint in this problem? Please.  Let $X$ be a topological space. If $f,g$ are continuous function of $f,g\colon X\to S^n$  which $f(x)\neq -g(x)$ for all $x$ in $X$, show $f$ and $g$ are homotopics. I try to show a homotopy using $$q(x) = \frac{f(x)+g(x)}{\|f(x)+g(x)\|}$$ it's well defined, but i'm stuck. Transfer / security in Istanbul Ataturk airport after 29/06/2016 I would like to have my whole document showing a blank line (or space) between paragraphs and no indentation. I know how to do it line by line, but is there a way to change that with just one or a few lines at the beginning of the document? Like a package you can install, or a new command? I was reading a review of 5e game play  from our RSS Feed. And in it he makes an observation about a strange rule:     Weird rule thing. You recover half your level in hit dice during a   long rest, which means level 1 characters never recover hit dice.   That didn't make sense to me, and so I double checked the long rest rule and it reads:     At the end of a long rest, a character regains all lost it points.   The  character also regains spent Hit Dice, up to a number of dice   equal to half of the character’s total number of them. For example, if   a  character has eight Hit Dice, he or she can regain four spent Hit   Dice upon finishing a long rest.   And of course the rules state that in general you always round down.  Am I missing something? Is there a good explanation for why at first level you can't recover hitdice? If $a+\frac1b=b+\frac1c=c+\frac1a$ for distinct $a$, $b$, $c$, how to find the value of $abc$? If $a, b, c$ be distinct reals such that $$a + \frac1b = b + \frac1c = c + \frac1a$$ how do I find the value of $abc$?  The answer says $1$, but I am not sure how to derive it. There are $2n+1$ people. For each $n$ people there is somebody who is friend with each of them. Prove there is a "know-them-all" person. I have a following problem.     There are $2n+1$ people in the room. For any group of $n$ people there is a person (different from them) who is friends with each person in this group. Prove that there is a person, who knows all $2n$ other people.   One can easily see, that $\min_v \deg v = n$. From this we can get a lower bound for number of edges $N_e$: $$ N_e = \frac{1}{2}\sum_v \deg v \geq \frac{(2n+1) \cdot n}{2} $$ I do not know where to move from this point (and do not think, that this is the right direction) and pretty stuck right now. Can you give a hint?