How can you find all matrices A such that, for all matrices B, AB = BA?
Matrices that commute with all matrices
I just refreshed the [iphone]-tag-site, and I'm seeing all kinds of questions with what appears to be a grey overlay (see image). What is this for/why am I seeing this? I have never seen anything like this before.. What is a simple way of downloading a file from a URL path? I am new to Salesforce and hence a lot of silly questions. What is the rational behind defining future methods as Static? I mean has these methods been not static what harm would have they caused? Etiquette for username changes I'm thinking of changing my username to my real name. Given that I have a presence on some of the sites here (mostly codegolf), what etiquette should I follow when choosing my new name?  As my username is currently Quincunx, I'm thinking of changing it to MyRealName (formerly Quincunx). Or perhaps MyRealName (né Quincunx). Maybe just MyRealName (Quincunx). If I do use these usernames, will people be able to recognize who I am? Or is there already a standard way to change usernames (such as relying on the profile picture)? What does "Baking" mean? I want to make some cool items for Dota 2 and I'm reading a guide. There are some tips about baking  etc. but I can't use them, because I don't even know what baking means. Can somebody explain this, please?  Thanks for all answers. Book recommendation on plane Euclidean geometry I consider myself relatively good at math, though I don't know it at a high level (yet). One of my problems is that I'm not very comfortable with geometry, unlike algebra, or to restate, I'm much more comfortable with algebra than geometry. I think that's mainly because my geometry education was sparse through the years, lacking in consistency etc. So I'd like to revise (and learn more) all at once, catching the basic axioms, understanding why such is such, etc. Essentially, a moderately rigorous textbook in plain Euclidean geometry (nothing fancy). Please don't say "The Elements" - I have browsed it at the bookstore, it is quite good, but not really what I'm looking for right now. I need to cite the journal "Sankhyā: The Indian Journal of Statistics" in a paper.  I would like to draw the ā character properly (i.e., I'd prefer, if possible, to avoid using some hack, such as \bar or \overline).  Question: How do I properly write ā in LaTeX?  I didn't have luck with  (after several attempts in both in capital and lowercase) nor The Comprehensive LATEX Symbol List (searching the file for "ā" and "india" revealed no hits). Who invented "dooblidoo"? The word  is used by several different youtube channels as a different word for the youtube description bar. I've seen it used by the  and by .  Who was first to use and invent the term? Is any particular punctuation necessary when a quotation is the subject of a sentence? For example, in the sentence below, should there be a comma after export? Or anything else?     "Laughter is America's most important export" is a well-known quote of Walt Disney's. Correct position of "only" Which is grammatically correct?  I can only do so much in this time.  or  I can do only so much in this time. Grepping logs for IP addresses Vortex in liquid collects particles in center BJT Astable Multivibrator What is this style of text-as-image called? I'm seeing a trend (especially on pinterest) of these posters where the image and subject is the text. THe text itself isn't really that important either, it's mainly the typefaces used and composition.  Does this style have a name to it? Is it just called text-as-image? Why don't the White Walkers take the sea route to Westeros $B^n/S^{n-1}$ is homeomorphic to $S^n$ Let $B^n$ be a n-dimensional disc (ball) with boundary $S^{n-1}$. Prove that  $B^n/S^{n-1}$ is homeomorphic to $S^n$.  Could someone check my proving, please?  Let  put the ball $B^n=\{x\in\mathbb R^n\mid |x|\leq 1\}$ into $\mathbb R^{n+1}$ using $x\mapsto (x,0)$. Then we can use homeomorphism $(x,0)\mapsto (x,\sqrt{1-|x|})$. We fall into the half of the sphere $S^n$. The boundary of the ball after this will turn just into points of the form $(*,0)$. If you pull it to the point then we will get suspension $S^{n-1}$ or $S^n$. I'm trying to wrap my head around this for a while now. It's a simple question for what I think will be a not so simple answer. How would you go about modeling one leg of this stool? I would like to do it without using sculpt mode because I can't get things quite clean enough. I have a finite $p$-group $G$ and a normal subgroup $N$ which is not the trivial subgroup. I am asked to show that $|N \cap Z(G)| &gt; 1$.  There has been a similar question on MSE here:  Unfortunately, I cannot comment on other user's questions yet, so I made my own question.  I know that the trick is to use conjugation but I can't follow the logic. Can someone elaborate? Especially on the last part of the top answer. Can suspended users post comments? Get notifications? What happens to their reputation? How to troubleshoot unwanted white spaces smartly?