If $AB = I$. Prove that $BA = I.$(use only linear combination/matrix multiplication)
If $AB = I$ then $BA = I$
What is the difference between "Have you seen this?" and "Did you see this?" What's the difference between these two phrases? How do I set access permissions per field? What I need to do  I have some user account fields that should be administrator-only (read/write/edit).  For example, the IP address used to register.  So, I want to restrict access to administrators.  Current state of field permissions in Drupal 8 (as of Feb 19, 2016)   In Drupal 7, I used the  module.  However,  There is a , but this has been going on for seven years and is marked as "postponed." $(m, m+2)$ is twin prime, iff $4((m-1)! + 1) \equiv -m \pmod {m(m+2)}$ The Wiki page on  says     The pair $(m, m+2)$ is twin prime, iff $4((m-1)! + 1) \equiv -m \pmod {m(m+2)}$.   This is obviously connected to Wilson's Theorem. Can anybody provide a proof for that, along the lines of thought given here, ? How does accepting an answer work? I think my MacBook Pro battery was replaced by Apple's service team. How do I check that? Why can't I have a non-integral static const member in a class? I noticed C++ will not compile the following:  class No_Good {   static double const d = 1.0; };   However it will happily allow a variation where the double is changed to an int, unsigned, or any integral type:  class Happy_Times {   static unsigned const u = 1; };   My solution was to alter it to read:  class Now_Good {   static double d() { return 1.0; } };   and figure that the compiler will be smart enough to inline where necessary... but it left me curious.  Why would the C++ designer(s) allow me to static const an int or unsigned, but not a double?  Edit: I am using visual studio 7.1 (.net 2003) on Windows XP.  Edit2:  Question has been answered, but for completion, the error I was seeing:  error C2864: 'd' : only const static integral data members can be initialized inside a class or struct What does a "real" quantum computer need for cryptanalysis and/or cryptographic attack purposes? The cryptographic world has been buzzing the word "quantum" for a while now (even the NSA is currently preparing itself for a post-quantum crypto world) and quantum-related hardware engineering is evolving constantly. For example: the 5-qubit quantum computer created at MIT by using the technique of ion traps succeeded in prime-factorizing 15. Does that mean that since it succesfully managed that, that it is a all-purpose quantum computer which could be used for cryptanalysis and/or cryptographic attacks?   If that's not the case, what exactly would a "real" quantum computer need (think: rough description of expected technical abilities, aka specs) to enable its users to use it for cryptanalysis and/or cryptographic attack purposes? And - ignoring rumours about potentially existing but confidential governmental projects - does any such system already exist today? Proof of $\int_0^\infty \left(\frac{\sin x}{x}\right)^2 \mathrm dx=\frac{\pi}{2}.$ Sorting seach results in Android app The issue is how can user sort questions for a particular search query?   For example, I would want to search questions for  and want to see newest questions first.  Right now If we write any query in Search Questions... input box then users are not able to select sorting options as search questions input box takes whole width of the screen and sorting options box gets hidden.  I have also tried to select sorting options first and then search for  but still its showing questions without considering sorting option I have selected. It is a general and maybe basic question, but it will help me to avoid a mistake. To test the dependence of continuous variable on nominal with parametric test (t-test, ANOVA), "the data has to be normally distributed" (distribution have to be not much violated from normal).   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What my intuition directed me to do next is to find the probability that any of the three events are true, or the union of the three events, which involved adding up the probabilities.   To put this generally, the equation for the probability SHOULD be:  ${n \choose 2} \div 365$  However, clearly this is not the case since $n \choose 2$ gets over 365 when $n = 28$. And with 28 people, there is obviously still a chance that they all have different birthdays (Person 1 has Jan. 1, Person 2 has Jan. 2. ... Person 28 has Jan. 28). Could anyone tell me what's wrong with my intuition? I don't want to know what's the right solution, I just want to know what's wrong with my solution .. it makes sense to my intuition, even though it's incorrect. Adding 2 favorite tags at one time combines tag after page refresh I was adding some tags to my favorite tags today and noticed tags being joined together when the page is refreshed.  Steps to reproduce:   Click edit on Favorite Tags Create the first tag and click add [I added django in this case] Add a second tag and click add [I added python in this case] Refresh page  Favorite tags now show previous tags + djangopython   In the edit section, it appears as if the tags are added separately, but when I refresh the page they get joined  OS: MacOSX-10.8  Browser: Chrome, Firefox A secant inequality for convex functions A book with two twins and a difficult relation, a room where time flows slower than the outside and a mysterious monster coming from a sink There was this pocket book, back when I was a teenager (20 years ago), with these two twins discovering a room (it was something like a tool shed) where time flowed slower (1 hour inside was equal to 1 minute outside if i remember correctly).   Inside the shed there was a sink and a strange snake-ish metallic monster was slowly coming out from it.  One of the twins, the one with the "less marked" personality, decides to spend one year (during a whole night) inside the room in order to get out older than his twin.  I don't remember what happens with the monster, I think they destroyed the whole shed in the very end. I can't destroy blocks in Minecraft Creative Mode (singleplayer) I tried many ways to make a bigger screen in VirtualBox because I do not like the small size of the guest window? Is there is a way to make it big? Property of $\{0^n, 1^n, \ldots\}$ The differences between adjacent squares are the odd numbers:  0 1 4 9 16 25 …  1 3 5 7  9 …  The differences between adjacent odd numbers are 2 = 2!  I found that this truth is more general: If the differences between adjacent powers of $n$ are written out, and the differences of those differences etc, the $(n+1)$th sequence is {n!, n!, …}  Is this a well-known theorem? Does it have an easy proof? How to get hardware specs of Android device? Probability associated with experiencing all outcomes How to set font to Arial throughout the entire document? In preamble, I have:  \renewcommand{\familydefault}{\sfdefault} \usepackage{arial}   but it doesn't work.  How do I set my font to Arial throughout the entire document?  UPDATE:  In short, both answers work.  It really depends on the environment. If using LaTeX and don't mind using a cloned font then choose Herbert's answer. If using XeLaTex and want to use the authentic font then choose Leo's.