String format Error "input string was not in a correct format."
How to escape braces (curly brackets) in a format string in .NET
Limiting sessions to one IP at a time On a site with premium user account subscriptions, I'd like to be able to limit logins to one computer at a time. The most straightforward way to accomplish this would be to limit by IP, but I haven't had any luck finding a plugin to accomplish this. Does anybody know of one I can use to get this functionality?  Related: Is gcc 4.8 or earlier buggy about regular expressions? On googling I got     Random numbers are numbers that occur in a sequence such that two conditions are met: (1) the values are uniformly distributed over a defined interval or set, and (2) it is impossible to predict future values based on past or present ones. Random numbers are important in statistical analysis and probability theory.   What if I want to pick a random integer from the number line ? The number line consists of infinitely many integers. My favorable event is to get a number (random) and the number of events possible is the complete number line i.e. infinity. I won't be considering any limits here as our number line doesn't tend to infinity but does contain infinitely many elements.  Now the probability of getting a random number  P(E) = 1 / ∞ = 0   So I won't be able to get a random number, ever. This implies, never would I ever be able to get a random number.  That's absurd as many of our science topics do depends on things being random. Why do I get blurry pictures when I take a lens attachment off? Finding an Inverse Function and Composition of Functions? Subfields of finite fields We know that if a finite field $F$ has characteristic $p$ (prime), then $F$ has cardinality $p^r$ where $r = [F:\mathbb{F}_p]$.   I'm now trying to say something about the possible cardinalities of subfields of $F$. I can see that there is a subfield of cardinality $p^s$ for each $s$ that divides $r$, given by the fixed field of the group generated by $\phi^s$, where $\phi$ is the Frobenius automorphism.  Now suppose $K$ is a subfield of $F$. Then (since both are additive groups), Lagrange gives us that $|K|$ divides $|F|$, so $|K| = p^t$ for some $1 \leq t \leq r $ (alternatively, $K$ contains $\mathbb{F}_p$ and so is a vector space over $\mathbb{F}_p$ and is thus isomorphic to $\mathbb{F}_p^t$, where $t = [K:\mathbb{F}_p]$). By considering the multiplicative group of units of $K$ and $F$ respectively, we get that $ p^t - 1$ divides $p^r -1$. I want to make the leap to $t|r$, but I'm failing to see why this needs to be true. Any help would be appreciated. Is the following layout possible with the subfigure package? MySQL: SELECT from another server I'm afraid that I already know the answer to my question, but I'll ask it anyway:  When there are two MySQL DB servers, can I access data that is stored on the other server?   In other words: Can I somehow do this:  INSERT INTO table (x, y, z)    SELECT x, y, x+y       FROM [otherserver].[database].[table]   Is the answer really as short as "No"? Why subtraction of a negative number from a positive number is addition? Eg: $a - (-b) = a + b$ When looking through the number scale I am unable to relate this, please kindly clarify ? The Center of $\operatorname{GL}(n,k)$ The given question:  Let $k$ be a ﬁeld and $n \in \mathbb{N}$. Show that the centre of $\operatorname{GL}(n, k)$ is $\lbrace\lambda I\mid λ ∈ k^∗\rbrace$.  I have spent a while trying to prove this and have succeeded if $ k \subseteq \mathbb{R}$. So I imagine there is a nicer way to go about this. I have seen people saying take matrices where every element except one is zero in proving similar results but such matrices are not in $\operatorname{GL}(n,k)$ as they are not invertible.  What I have done.  I have said let $B \in$ Center then $B$ commutes with everything in $\operatorname{GL}(n,k)$. So take a permutation matrix, $P$ which swaps rows $i,j$ $P \neq I$. From this you can deduce that $B^T = B$ and that $B_{ii} = B_{jj}$. We can use the fact that $B$ is symmetric to find an orthogonal diagonalisation of $B$ by the spectral theorem.  This gives $QBQ^T = D \rightarrow B=D$. And as $B_{ii} = B_{jj}$ we can say $B = k * I$.   But the spectral theorem requires $B$ to be a real matrix.   How do I prove this for a general $k$?. I have    iPhone 5 (with iOS 8.3) MacBook Pro Early 2013 (with OS X Yosemite 10.10.3) iPad Air (with iOS 8.3)   My iPhone is paired with the MacBook, so e.g when my phone rings, I can see it in my computer. But when I want to AirDrop photos from my iPhone the MacBook, then I can't see the MacBook in my iPhone AirDrop targets.  I can see the iPad, I can see my wife's iPhone, but not the MacBook.  On MacBook, I can see the iPad, my wife's phone and even my iPhone and I can drop to there, but not the other way around.  Before I started troubleshooting, the iPhone and MacBook were mutually invisible (I only saw iPad from both). After a while of trying things and sending files back and forth, my MacBook started to see the iPhone, but I did not manage to make it work from the other way. Still, the iPhone seems to be much "less" visible than the iPad - which I see almost instantly and always, the iPhone only seems to appear after a while.  Just to make it clear:   both iPhone and MBP have BT turned on both have AirDrop visibility set to Everyone   I would be glad for any suggestions to fix this - what settings to check, logs, whatever. How do you make a torus 'telescope' open? So I have a torus, and I need it to "telescope" open -- kind of like Pacman's death animation, but with a torus instead of a sphere.  I'm very new to 3D modeling/animating so I really have no clue how to tackle this problem.  Any advice to help me get pointed in the right direction would be most appreciated :) How to Fix:Though favicons are added, it doesn't show in Google Search The problems that appear in difficult math competitions such as the IMO or the Putnam exam are usually very difficult and require some ingenuity to solve. They also usually don't look like they can be solved by simply knowing more advanced theory and the such.  How do people typically come up with these problems? Do they arise naturally from advanced mathematics (the somewhat infamous 'grasshopper problem' from the 2009 IMO comes to mind - to my not exactly knowledgeable mind this problem looks like it popped out of basically nowhere)? What is the perspective that mathematicians take when seemingly "inventing" these problems with no theoretical motivation to them whatsoever? How can I get an underbrace and an overbrace to partially overlap in an equation? Sometimes I want to describe portions of an equation using \underbrace and \overbrace in such a way that the portions are not completely distinct, for example:          /-------\ x = A + B + C + D + E     \-------/   \---/      What is the best way to do this? Example of a group where $o(a)$ and $o(b)$ are finite but $o(ab)$ is infinite Let G be a group and $a,b \in G$. It is given that $o(a)$ and $o(b)$ are finite. Can you give an example of a group where $o(ab)$ is infinite? Reference — What does this symbol mean in PHP? Proof using Cauchy Integral Theorem How do I need to configure SMTP Auth? I have dedicated FreeBSD box, The email sending without  is working perfectly but very slow. The receiver receives email after almost 15 mins. I checked the maillog it says delays and timeouts that means the server keeps on trying to send email again and again until it successfully sends one. This is consuming lots of CPU.  I am seeking help for configuring smpt module to use my dedicated box's  smtp server, I need help in following places where I have ???? sign:  #SMTP server settings SMTP server -&gt; smpt.mydomain.com  # Tested by pinging, its working SMTP backup server -&gt; mail.mydomain.com  # Also verified by Ping SMTP port -&gt; 25 USE SSL  #SMTP Authentication Username -&gt; ??????? Password -&gt; ???????   I tried using localhost, also username and password of the dedicated box or leaving both lines blank which gives me:     Unable to send e-mail. Contact the site administrator if the problem   persists.   also checking the debug log doesn't show any log, it says check admin/reports/dblog but when I got to the link it just shows me the contents from admin/reports no any dblog I read about the new fancy progress bar which was added to apt 1.0 in  and how to enable it . However on running sudo apt-get update, I did not see the fancy progress bars. On reading the second link again, the solution explicitly requires you to use sudo apt update.  Hence my question is the difference between apt and apt-get. I thought they were one and the same.