The second field in /etc/shadow shows "!" for user "bin" , "uucp" and few other service accounts as well. Can anyone explain the reason for "!" ?
If the second field of the /etc/shadow file is set to !!, does it mean that the account is disabled? Or does it mean that the account is active with no password assigned?
Let $x = (a, b), y = (c, d) \in \mathbb{Z}^2$. What is the condition on $a, b, c, d$ so that ${x, y}$ is a basis?   My answer: $ad\neq bc$ and $gcd(a, c) = gcd(b, d) = 1$.  The first condition ensures that they aren't the same vector; the second ensures that we can actually "get" all of the integer values/lattice points.  Is this correct?  Thanks. A coworker and I are locked in a debate over the value of SEO keywords in the URL of a page. She wants to change all the filenames of the HTML pages of a fencing company so they look like residential-home-chicago.html, contact-chicago-contractor.html, and so on. She is convinced that because Google highlights keywords in URLs in search results, that means that putting keywords here is more valuable.  My position is that these do not improve SEO, since Google doesn't seem to give keywords in the URL any more weight than keywords in the body of the page, and . In the meantime, they make it harder for me to find the pages I want when its time to edit them, and the site as a whole looks cheap and spammy.   suggests to me that yes, keywords in URLs are useful, but not superior, and that they are more useful for human readability than search engine rankings.  I'm looking for authoritative sources that support either position, not blog articles from SEO optimization companies trying to promote themselves. Linked and Related sections contain duplicate links to questions Proving the cross product matrix tranformation identity with an alternative solution I'm solving a problem from the book, Mathematics for 3D Game Programming and Computer Graphics, Third Edition, by Eric Lengley. The problem goes:  Let $N$ be the normal vector to a surface at a point $P$, and let $S$ and $T$ be tangent vectors at the point $P$ such that $S \times T = N$. Given an invertible 3 $\times$ 3 matrix $M$, show that $(MS) \times (MT) = (\text{det}M(M^{-1})^{T}(S \times T)$, supporting the fact that normals are correctly transformed by the inverse transpose of the matrix $M$.  The author provided a hint stating we can represent $(MS) \times (MT)$ as $$ (MS) \times (MT) = \begin{bmatrix} 0 &amp; -(MS)_{z} &amp; (MS)_{y} \\  (MS)_{z} &amp; 0 &amp; -(MS)_{x} \\  -(MS)_{y} &amp; (MS)_{x} &amp; 0  \end{bmatrix} MT $$ We then find a matrix $G$ such that $$ GU = \begin{bmatrix} 0 &amp; -(MS)_{z} &amp; (MS)_{y} \\  (MS)_{z} &amp; 0 &amp; -(MS)_{x} \\  -(MS)_{y} &amp; (MS)_{x} &amp; 0  \end{bmatrix} M $$ where $$ U = \begin{bmatrix} 0 &amp; -S_{z} &amp; S_{y} \\  S_{z} &amp; 0 &amp; -S_{x} \\  -S_{y} &amp; S_{x} &amp; 0  \end{bmatrix} $$ and show that $G = (\text{det}M)(M^{-1})^{T}$ to solve the problem. I am aware that there is an  to this problem, but I would like to solve it through the hints provided. Unfortunately, I am only able to go as far doing: $$ G = \begin{bmatrix} 0 &amp; -(MS)_{z} &amp; (MS)_{y} \\  (MS)_{z} &amp; 0 &amp; -(MS)_{x} \\  -(MS)_{y} &amp; (MS)_{x} &amp; 0  \end{bmatrix} M U^{-1} $$ At this point, I do not know how to proceed with showing that $G = (\text{det}M)(M^{-1})^{T}$. How would you proceed? I'd like to ask for hints on solving the problem. ,  and 1 still show the old bar. 1 See Can I successfully bake previously-frozen bread dough? I prefer my bread freshly baked (who doesn't?), but my mixer prefers kneading two loaves at a time to just kneading one. I have tried to freeze the second loaf before baking it, but when I thaw it, it does not rise well before baking. I've tried thawing it entirely in the refrigerator and then letting it rise on the counter, but that did not seem to work. Does anyone have a good technique or suggestion for this? Why is the ROC curve two-dimensional instead of three-dimensional? How to typeset the symbol “^” (caret/circumflex/hat) I need to display the symbol '^'  d &lt;- dist(fascores, method = "euclidean")^2   How do I do that? Show that $a^{q-1} \equiv 1 \pmod{pq}$ Assume that $p$ and $q$ are distinct add primes such that $p-1\mid q-1$. If $\gcd(a,pq)=1$ ,show that: $$a^{q-1} \equiv 1 \pmod{pq}$$   I have tried as follows: $$a^{q-1} \equiv 1 \pmod{q} \quad \text{and} \quad a^{p-1} \equiv 1 \pmod{p}$$ $$\implies a^{(q-1)(p-1)} \equiv 1 \pmod{q} \quad \text{and} \quad a^{(q-1)(p-1)} \equiv 1 \pmod{p}$$ $$\implies a^{(q-1)(p-1)} \equiv 1 \pmod{pq}$$ But then I am stuck - please help. Why didn't Escobar's hippos introduced in a single event die out due to inbreeding Today I read a  about how Pablo Escobar had once imported 4 hippos (1 male, 3 female) into his estate in Colombia for his private zoo. After his downfall, while other species were shipped out, hippos were considered too big to move and expected to not survive. However, to the surprise of all the hippos are thriving and are so numerous that there have been calls to cull them. From the report:  Numbers are projected to only get bigger. [Colombian biologist Nataly] Castelblanco and her peers say the population will reach over 1,400 specimens as early as 2034 without a cull - all of them descended from the original group of a male and three females. In the study, they envisaged an ideal scenario in which 30 animals need to be culled or castrated every year to stop that happening.  My understanding is that since there was only 1 male, the gene pool would be limited and lead to lot of inbreeding in the descendants. This would cause population to not explode because some individuals would be unfit to survive. Why has this not happened in case of hippos? Is it because there are 3 females (probably unrelated to each other) which keeps gene pool large enough? Or can mutations explain this phenomenon? Would results have been different if originally 2 females had been moved and only 1 retained? EDIT - I have just found that  is maybe less than 100 individuals which though big is not massive. EDIT2 - I have edited the question title to keep focus on the hippos although a general answer would be welcome. I have got this problem on limits  $$ \lim_{x \rightarrow 0}\left(\frac{\sin(x)}{x}\right)^\frac{1}{x^2}$$ What I am doing here is that taking log and then applying L-Hospital's Rule $$y= \lim_{x \rightarrow 0}\left(\frac{\sin(x)}{x}\right)^\frac{1}{x^2} $$ then $$\ln(y)=\lim_{x \rightarrow 0} \frac{1}{x^2}\ln\left(\frac{\sin(x)}{x}\right)$$ now it becomes $\frac{0}{0}$ type , but problem is how to take derivative here? when differentiating $\ln\left(\frac{\sin(x)}{x}\right)$ it will become $\frac{x}{\sin(x)}$ then we'll have to differentiate one more time like we do in case of $f(f(x))$ ? $\sin(x)$ and $x$ will be separately differentiated or together ? I'm running Fedora 25 and I need to take a few particular screen captures so I've read that shutter is a great tool for that. Sadly I am unable to use it for some reason. Whenever I try to take a screenshot this is the result: Uniform convergence with two sequences of functions For, $n\ge 1$ let, $g_n(x)=\sin^2(x+1/n)$ , $x\in [0,\infty)$ and $f_n(x)=\int_0^xg_n(t)\,dt.$  Then, which is(/are) correct ?  (A) $\{f_n(x)\} $ converges pointwise to a function $f$ on $[0,\infty)$ but does not converge uniformly on $[0,\infty)$.  (B) $\{f_n(x)\} $ does not converge pointwise to any function on $[0,\infty)$.  (C) $\{f_n(x)\} $ converges uniformly on $[0,1]$.  (D)  $\{f_n(x)\} $ converges uniformly on $[0,\infty)$.  I found that, $$f_n(x)=\frac{1}{2}\left[x-\frac{1}{2}\sin(x+1/n)+\frac{1}{2}\sin(1/n)\right].$$  So, $f(x)=\lim_{n}f_n(x)=\frac{x}{2}-\frac{1}{4}\sin x$.  Now, $|f_n(x)-f(x)|\le 3/4$. So, by M-test, $f_n(x)$ is not uniformly convergent, but point-wise convergent. So option (A) is correct..Am I right?? There are three random variables, $x,y,z$. The three correlations between the three variables are the same. That is,  $$\rho=\textrm{cor}(x,y)=\textrm{cor}(x,z)=\textrm{cor}(y,z)$$  What is the tightest bound you can give for $\rho$? Why is my dual-core CPU recognized as a quad-core one? My processor is an . As you can see, it's 2 cores and 4 threads.  Although, both Ubuntu 11.04 and Windows 7 seem to think it's a quad-core CPU:      Why is this? Is it because there are 2 threads per core, dividing the CPU in 4 processing units? I am travelling to Calgary from Delhi. I have an Air India flight from Delhi to Dubai then British Airways flight from Dubai to London and Air Canada flight from London to Calgary. My query is "Do I require a transit visa." Plz help as I am travelling in four days What are the options for traveling to the airports in Paris? Username and Password not recognized when logging into Minecraft How to reconnect a logically disconnected USB device? "States Party to" or "State Parties to" or "States parties to"? When discussing a treaty or international agreement, which is correct?  "There are 100 states party to the treaty." (for example, as used )  Or:  "There are 100 state parties to the treaty." (for example, as used )   Or: "There are 100 states parties to the treaty."  (for example, as used )