What is a good user group size for going out into the field to test your interface with users, one to one?
What's recommended for the number of users to use in qualitative and quantitative testing?
Regarding running Watir scripts for UI Automation from Jenkins CI - How to hook up Jenkins and watir-automations Is there a closed form expression for $\text{n}^{\text{th}}$ coefficient of Maclaurin series of  $$\ln(1-x+x^2)$$ Joda Time: How to get dates of weekdays on some date interval? From the limit definition of differentiability, determine where the function $f(x) = \sqrt{x}$ is differentiable? For the function $f(x) = \sqrt{x}, \hspace{2 mm} f: [0, \infty) \rightarrow \mathbb{R}$, I wish to find all points $a$ at which $f(x)$ is differentiable, from the definition $$ f'(x) = \lim_{x \rightarrow a}\frac{f(x)-f(a)}{x-a} $$  Clearly, for our function $f(x) = \sqrt{x}$, this gives us $$ f'(x) = \lim_{x\to a}\frac{\sqrt{x} - \sqrt{a}}{x-a} $$  So the question boils down to showing for what values of $a$ this limit exists. How would this be done? Intuitively, I know that the answer is that the limit exists for all $a \in [0, \infty)$ and is given by $\frac{1}{2 \sqrt{a}}$, but I'm unsure of how to prove this from the limit definition of differentiability. Find $P(n+1)$ for a polynomial P Fix a nonnegative integer $n$. Let $P(x)$ be a polynomial of degree $n$ (over the real numbers) such that for all $k\in\left\{0,1,\ldots,n\right\}$, we have $P(k)=\dfrac{k}{k+1}$. Find $P(n+1)$.   My attempt:  I consider a new polynomial $Q(x)=(x+1)P(x)-x$. This $Q$ is a polynomial of degree $\leq n+1$ satisfying: $Q(k)=0\quad\forall k\in\{0,1,2,\ldots,n\}$. Therefore $Q(x)=\lambda x(x-1)\cdots(x-n)$ for a certain real $\lambda$. Hence, $Q(-1)=\lambda \cdot (-1)^{n+1}(n+1)!$, but also $Q(-1)=(-1+1)P(-1)+1=1$. Comparing these, we get $\lambda \cdot (-1)^{n+1} (n+1)! = 1$, thus $\lambda=\dfrac{(-1)^{n+1}}{(n+1)!}$. Since $Q(n+1)=\lambda(n+1)!$ then $Q(n+1)=(-1)^{n+1} $, and from $P(x) = \dfrac{Q(x)+x}{x+1}$ we deduce: $$P(n+1)=\dfrac{(-1)^{n+1}+n+1}{n+2}.$$ I doubted that the answer were $P(n+1)=\frac{n+1}{n+2}$ but the term $(-1)^{n+1}$ wasn’t expected. Is there any mistake here? Does someone have an alternative proof? Among other sources,  states that triple DES using three seperate keys (k1, k2, k3) is vulnerable to meet-in-the-middle-attacks, while triple DES using only two keys (k1, k2, k1) is not.  Why is this attack possible if all the keys are different, but not if the first and the third are the same? Does anyone know of a .NET date/time parser similar to  (handles stuff like "tomorrow" or "3pm next thursday")?  Note: I do write Ruby (which is how I know about Chronic) but this project must use .NET. Let $R$ be a Artinian commutative ring with $1 \neq 0$. If $I$ is prime, then $I$ is maximal. Prove: Let $R$ a Artinian commutative ring with $1 \neq 0$. If $I$ is prime, then $I$ is maximal.     I got stuck on this. I understand that every ideal is generated by finitely many elements. Here is my approach:  It's enough to prove the following implication: $$ R/I \ \text{is a domain} \quad \Rightarrow \quad R/I \ \text{is a field}$$ Then only finding an inverse is left. So let $x \in R$ in arbitrary. We have to find an inverse element of $x+I \in R/I$, that as an element y so that $x \cdot y \in I+1$. We denote $I = (x_1, x_2, \cdots, x_n)$.  Now I have to find y so that $xy -1 = r_1 x_1+ r_2 x_2 + \cdots + r_n x_n$. This is where I no more knew what to do.   Your advise will be appreciated. How to find the URLs accessed by iOS Apps? What is wrong with code snippets/blocks on Stack Overflow? I wasn't following Meta Stack Overflow too closely so maybe something was changed that I'm not aware of. But each time I paste a snippet from a Python console I'm getting wrong formatting. What I'm doing wrong? It seems that &gt;&gt;&gt; is confusing for Stack Overflow. I've tried to escape it with backslashes, and it was working fine in preview but doesn't work after the question is rendered.  I was able to observe that problem on: recent Chrome (Windows, Mac), Safari 4 (Mac).  Here is  I wasn't able to make right.  Funny, I think the snippets are rendered just fine on Meta Stack Overflow. Here is one:  &gt;&gt;&gt; def throw(): raise Exception()  ...  &gt;&gt;&gt; a=1 &gt;&gt;&gt; a=throw() Traceback (most recent call last):   File "&lt;stdin&gt;", line 1, in &lt;module&gt;   File "&lt;stdin&gt;", line 1, in throw Exception &gt;&gt;&gt; a 1 Are global variables bad? Commenting out multiple lines of code, specified by line numbers, using vi or vim I have learned from  that it is possible to use vi/vim to comment out a specified range of line numbers.  For example, suppose I have the following bash script:  #!/bin/bash  This is my very very great script   Now suppose that I want to comment out line numbers 6 through 8 (which contain the words very, very, and great) using the # comment character.  In vi/vim, I can simply type :6,8s/^/# to obtain the following:  #!/bin/bash  This is my #very #very #great script   which comments out lines 6 through 8.  My question is, is it possible to type a similar one liner that will remove the # comment character from lines 6 through 8 (but not any other commented lines in the file)?  Having said this, I realize that there is some debate about whether I am actually using vi or vim.  In practice, I open a file script.sh with the command vi script.sh.  Also, when I type the command which vi, I obtain /usr/bin/vi.  Nevertheless, when I simply type vi and press Enter, I obtain this:  ~                              VIM - Vi IMproved ~ ~                               version 7.2.330 ~                           by Bram Moolenaar et al. ~                 Vim is open source and freely distributable ~ ~                           Sponsor Vim development! ~                type  :help sponsor&lt;Enter&gt;    for information ~ ~                type  :q&lt;Enter&gt;               to exit ~                type  :help&lt;Enter&gt;  or  &lt;F1&gt;  for on-line help ~                type  :help version7&lt;Enter&gt;   for version info   which seems to suggest that I'm actually using vim.  I am accessing a remote Ubuntu Linux cluster using SSH from my PC.  I am not using a Ubuntu Linux GUI. In MTG, what happens when a player loses in multiplayer? In Magic: The Gathering, the rules for a player losing in a multiplayer game are quite complex, so I'd like to try to understand them by example. I would be happy to ask each of these situations as a separate question if that is preferred, but they all seem related, and I would love to have one place to look up all of these answers.  Let's say I'm playing with two friends, Bob and Chloe, in a simple free for all. Unfortunately, I lose. When I lose, suppose the following:   I controlled an  which has exiled Bob's , does he get his bear back? I have a  on the stack targeting Chloe, does Chloe take the 2 damage still? Earlier in the turn I cast  to prevent damage to one of Bob's . If it later takes 3 damage, will that damage be prevented? Something I controlled gave Bob a 1/1 soldier token. What happens to the token? Something Chloe controlled gave me a 1/1 soldier token. What happens to the token? I had a  enchanting one of Bob's army of . What happens to the bear? I had also played  on Bob's final . What happens to the last bear? Chloe had  my . What happens to the Clone? Are there any other situations specifically addressed by the rules? I'm wanting to use Xen Hypervisor to be able to run both Ubuntu 12.04 and Windows 7 at the same time using virtualization. I can find plenty of XCP tutorials but I'm not looking to install the cloud stack, I just want the server confined to my machine.  All I need is a link to a decent tutorial for this purpose. I've read a ton of articles that lead me to believe I'm on the right track, but not enough to feel comfortable getting it right if I just dive in. I'm getting caught up on whether I need to install Ubuntu and then install Xen into it, or do I need to install Xen on a small partition and then leave the rest of the hard drive open for the virtualization... I dunno.  Basically, I need the virtualization of Windows 7 for Adobe Premiere and After Effects (until Lightworks for Linux comes out and I can afford NUKEX). If I didn't need those, I'd be rocking out a straight up Ubuntu machine or just using VirtualBox for my virtualization. However, I need Windows to run as an HVM so it runs like I would expect rather than having the issues I may run into with VirtualBox (video drivers, etc).  So, any suggestions? How to zoom in/out an UIImage object when user pinches screen? I want to give a client access to my server, but I want to limit those users to their home directories. I will bind-mount in any files I want them to be able to see.  I've created a user called bob and added him to a new group called sftponly. They have a home directory at /home/bob. I've changed their shell to /bin/false to stop SSH logins. Here is their /etc/passwd line:  bob:x:1001:1002::/home/bob:/bin/false   I've also changed the /etc/ssh/sshd_config to include the following:  Match Group sftponly         ChrootDirectory /home/%u         ForceCommand internal-sftp         AllowTcpForwarding no   When I try to log in as them, here's what I see   $ sftp bob@server bob@server's password:  Write failed: Broken pipe Couldn't read packet: Connection reset by peer   If I comment out the ChrootDirectory line I can SFTP in but then they have free rein over the server. I have found that ChrootDirectory /home works, but it still gives them access to any home directory. I have explicitly tried ChrootDirectory /home/bob but that doesn't work either.  What am I doing wrong? How can I limit bob to /home/bob/?  ----EDIT-----  Okay so I just had a look at /var/log/auth.log and saw this:  May  9 14:45:48 nj sshd[5074]: pam_unix(sshd:session): session opened for user bob by (uid=0) May  9 14:45:48 nj sshd[5091]: fatal: bad ownership or modes for chroot directory component "/home/bob/" May  9 14:45:48 nj sshd[5074]: pam_unix(sshd:session): session closed for user bob   I'm not entirely sure what's going on there,  but it suggests something is wrong with the user directory. Here is the ls -h /home output:  drwxr-xr-x 26 oli      oli      4096 2012-01-19 17:19 oli drwxr-xr-x  3 bob      bob      4096 2012-05-09 14:11 bob Windows 8.1 fresh install with Windows 8 licence I've been hearing some rumors from friends, mostly anecdotal indicating that skills, when gone past 10 (such as initially having 10 in one skill, then finding a bobblehead or skillbook), actually improve past 10.     I can't find any proof of this yet, and my current character right now is slightly incapable of maxing any stats soon to 10 and testing it out. So I'd like to understand if SPECIAL stats increased past 10 show any benefit?  Can SPECIAL stats be increased past 10? Hi I don't know how to show that $\langle 2,x \rangle$ is not principal and the definition of a principal ideal is unclear to me. I need help on this, please.   The ring that I am talking about is $\mathbb{Z}[x]$ so $\langle 2,x \rangle$ refers to $2g(x) + xf(x)$ where $g(x)$, $f(x)$ belongs to $\mathbb{Z}[x]$. What are Java command line options to set to allow JVM to be remotely debugged? I know there's some JAVA_OPTS to set to remotely debug a Java program.  What are they and what do they mean ?