Is C#'s null-conditional delegate invocation thread safe?
Checking for null before event dispatching... thread safe?
Easy way to determine the virtualization technology of a Linux machine? Prove: The pre-image of an ideal is an ideal. Let $\phi : R \to S$ be a homomorphism. If $N$ is an ideal of $S$, then $\phi ^{-1} (N)$ is an ideal of $R$. biblatex and new line for DOI, URL and Eprint Can mobs go through doors? I was wondering if mobs can go through doors or not. It's the difference between me just putting up a door to my house, or putting up dirt instead. 'sudo' not working after changing the permissions of '/usr' directory Recently I had to change the folder permissions of /usr folder. Now the whole sudo is not working. If I try to update my system using this command:  sudo apt-get update   I get the following message:  sudo: /usr/lib/sudo/sudoers.so must be only be writable by owner sudo: fatal error, unable to load plugins   So I tried to change the folder permissions by opening Nautilus as root by using this command  gksudo nautilus   But nothing happened. Nautilus didn't open. This is an emergency as the whole of sudo is not working. when I tried searching the solution from Google many suggested re-installation. I don't prefer re-installation. so is there any other solution? Router to router setup works fine with straight cable up to 2-3 meter it is not working for 45 meter why? Proof of Wolstenholme's theorem Creation and annihilation operators in the hydrogen atom Estimate on the derivative at fixed point Let $D$ be a bounded domain and let $f$ be analytic function from $D$ into $D$. Show that if $z_{0}$ $\in D$ is a fixed point for $f$ , then $|f' (z_{0} )| \leq 1$.  WHAT I WAS THINKING:   To have a conformal mapping from $D$ to the unit disc &amp; after composing it with $f$ ; I was thinking to use Pick's Lemma. So, can anyone tell me any conformal map from any bounded domain to the unit disc?? OR Is there any other way out to solve this problem?? Can I throw away contents of my Steam Inventory? I have a number of items in my Steam inventory that are never going to get used; for example, some AirMech 2 "gifts" that are marked as NO LONGER VALID.  Is there some way to get rid of them so I don't have them cluttering up my inventory?  Obviously I could do a one-sided trade with a friend, but then they'd have to deal with it instead, and I don't have any alt accounts. Can I mix MySQL APIs in PHP? Accessing System Databases/Tables using LINQ to SQL? How to get the client IP address in PHP Why are POSIX Find Parameters Different from Other Program styles? I don't see the gear button next to the Sign in button at the login screen. I have Nvidia drivers installed (for Nvidia 730 GT card). Ubuntu boots into Xorg as seen from the terminal with echo $XDG_SESSION_TYPE which outputs x11.  How do I switch to Wayland? Metapost label based on variable value I want to label a scale based on a variable value and I wrote a metapost code like this.  $ := 1; for i = -.3cm step .6cm until 3.6cm:     label.bot(str$,(i,-3.7cm));     $ := incr($); endfor;   It just labels "$" instead of its value. How to output the variables value? How do I resize my current ubuntu partition? I've got Ubuntu 10.10 installed to use my whole hard-disk, but would like to use the 130GB of free space I've got to create a new partition and do some development on Unity and Natty.  Can I use gparted to do this?  If so how? "It's cold today" -- what term do linguists use to call "it" when it's used as the subject of a sentence, but has no real antecedent? Prove that the square root of a positive integer is either an integer or irrational Is my proof that the square root of a positive integer is either an integer or an irrational number correct?  The proof goes like this:  Suppose an arbitrary number n, where n is non-negative. If $\sqrt{n}$ is an integer, then $\sqrt{n}$ must be rational. Since $\sqrt{n}$ is an integer, we can conclude that n is a square number, that is  for some integer a.  Therefore, if n is a square number, then $\sqrt{n}$ is rational.  Suppose now that n is not a square number, we want to show that the square root of any non-square number is irrational.  We prove by contradiction. That is, we suppose that the square root of any non-square number is rational. So $\sqrt{n} = \frac{a}{b}$, where $a,b \in Z^+, b \neq 0$. We also suppose that $a \neq 0$, otherwise $\frac ab = 0$ , and n will be a square number, which is rational.   Hence $n = \frac {a^2}{b^2}$, so $nb^2 = a^2$.    Suppose $b=1$. Then $\sqrt n = a$ , which shows that n is a square number. So $b \neq 1$. Since $\sqrt n &gt; 1$, then $a&gt;b&gt;1$.  By the unique factorization of integers theorem, every positive integer greater than $1$ can be expressed as the product of its primes.  Therefore, we can write $a$ as a product of primes and for every prime number that exists in $a$, there will be an even number of primes in $a^2$. Similarly, we can express $b$ as a product of primes and for every prime number that exists in $b$, there will be an even number of primes in $b^2$.  However, we can also express $n$ as a product of primes. Since $n$ is not a square number, then there exist at least one prime number that has an odd number of primes.  Therefore, there exists at least one prime in the product of $nb^2$  that has an odd number of primes. Since $nb^2=a^2$ , then this contradicts the fact that there is an even number of primes in $a^2$ since a number can neither be even and odd.  Therefore, this contradicts the fact that $\sqrt n$ is rational. Therefore, $\sqrt n$ must be irrational.  Is this sufficient? Or is there any parts I did not explain well? I remember reading a sci-fi story where astronauts visit a planet, which is covered in stationary aliens with mile long necks that filter feed from the atmosphere.  It was published between 1960 and 2000.    It was in English and involved a pair of astronauts that went from planet to planet on a tour or something, though I don't know details of any other planets they visited.