Which Doctor Who (classic) episodes aren't available today?
What original Dr Who episodes are left when not watching unfinished stories?
Let $G$ be a finite semi-group with identity such that it has only one idempotent.Is $G$ a group?  It only remains to show that for any $a\in G$ $\exists b\in G$ such that $ab=ba=e$ where $e$ is the identity of $G$  Also $e$ is the only idempotent of $G$ .How to proceed next? Why is "No new release found" when upgrading from a LTS to the next? Prove $A\cup (A\cap B) = A$ Is there a characterization of the nonconstant entire functions $f$ that satisfy $|f(z)|=1$ for all $|z|=1$?   Clearly, $f(z)=z^n$ works for all $n$. Also, it's not difficult to show that if $f$ is such an entire function, then $f$ must vanish somewhere inside the unit disk. What else can be said about those functions?  Thank you. I'm sure there is a really simple explanation to this but haven't found any pointers on this forum or elsewhere. I seem to have lost the 'processing' menu from QGIS and can't figure out how/why!? Any ideas what I might have done to make this happen?  I'm running QGIS 2.8 built against GDAL 1.10 on Ubuntu 14.04. Is it possible to append to innerHTML without destroying descendants' event listeners? In the following example code, I attach an onclick event handler to the span containing the text "foo". The handler is an anonymous function that pops up an alert().  However, if I assign to the parent node's innerHTML, this onclick event handler gets destroyed - clicking "foo" fails to pop up the alert box.  Is this fixable?  &lt;html&gt;  &lt;head&gt;  &lt;script type="text/javascript"&gt;    function start () {     myspan = document.getElementById("myspan");     myspan.onclick = function() { alert ("hi"); };      mydiv = document.getElementById("mydiv");     mydiv.innerHTML += "bar";   }   &lt;/script&gt;  &lt;/head&gt;   &lt;body onload="start()"&gt;    &lt;div id="mydiv" style="border: solid red 2px"&gt;      &lt;span id="myspan"&gt;foo&lt;/span&gt;    &lt;/div&gt;  &lt;/body&gt;  &lt;/html&gt; Inspired by a , here's a list of the 'etymology' for the  hats. For pictures and a list of criteria, see  instead.  The list is a community wiki; feel free to edit it if you have more information about a hat. If you like to do some research, most hats have been featured in earlier editions, so you might be able to find more information in old Meta Stack Exchange questions. Story ID: Boy who is stuck at 5 years old with robot nanny Here's what I remember:  The boy is 5 years old, and has been for some time, because there's a bit in which he recalls turning 5 (celebrating his 5th birthday) many different times  There's a robot nanny in the apartment with him that he learned to disable so that he can hang out with the girl, who he isn't supposed to know. (In my mind the nanny bears somewhat of a resemblance to Muthr from the Wondla series... however I definitely read this story well before reading Wondla)  There's a neighbor girl who teaches him to read (I think she's like 9), and when they first meet she asks how old he is (5 is the last age at which growth/development can be successfully (and safely?) suspended) and he lies, saying that he's 6, but doesn't know why he does.  When a child whose aging process has been stopped learns how to read, they begin aging normally again, so the parents don't want them to learn to read.  The children who have stopped aging are kept apart from other children who grow normally-- they have separate playgrounds and the like  The injection/serum used to halt the aging process was developed by a scientist, who when he first shared his findings had a 5 year old son and injected him, then told the onlookers/potential buyers/other parents to come back in 20(?) years. When they did, their children were adults and no longer needed them or were ungrateful layabouts, and the scientist's son was still 5 years old, cute, needy, etc.  The main character has a discussion about how he wants to turn 6 with his parents and they agree but only because he's already learned to read and it's irreversible.  It's kinda dark and emphasizes the need that parents have to feel wanted. Please help! I remember it so vividly and I'd love to read it again! Does there exist a function $f: \mathbb{R} \to \mathbb{R}$ having the following properties?   $f(x) = 0$ for all $x \le 0$. $f(x) = 1$ for all $x \ge 1$. For $0 &lt; x &lt; 1$, $f$ is strictly increasing. $f$ is everywhere $C^\infty$. The sequence of $L^\infty$ norms $\langle \left\lVert f \right\rVert_\infty, \left\lVert f' \right\rVert_\infty, \left\lVert f'' \right\rVert_\infty, \dots \rangle$ is bounded.   If we impose only the first four conditions, there is : for $0 &lt; x &lt; 1$, define $f(x)$ by $$ f(x) = \frac{e^{-1/x}}{e^{-1/x} + e^{-1/(1-x)}} = \frac{1}{1 + e^{1/x - 1/(1-x)}} $$ However, the derivatives of this function appear to grow quite rapidly. (I'm not sure how to verify this, but it seems at least exponential to me.)  If such a function does not exist, what is the smallest order of asymptotic growth that the sequence $\langle \left\lVert f \right\rVert_\infty, \left\lVert f' \right\rVert_\infty, \left\lVert f'' \right\rVert_\infty, \dots \rangle$ can have? Dominoes and induction, or how does induction work? I've never really understood why math induction is supposed to work.  You have these 3 steps:   Prove true for base case (n=0 or 1 or whatever) Assume true for n=k.  Call this the induction hypothesis. Prove true for n=k+1, somewhere using the induction hypothesis in your proof.   In my experience the proof is usually algebraic, and you just manipulate the problem until you get the induction hypothesis to appear.  If you can do that and it works out, then you say the proof holds.  Here's one I just worked out,  Show $\displaystyle\lim_{x\to\infty} \frac{(\ln x)^n}{x} = 0$  So you go:   Use L'Hospital's rule. $\displaystyle\lim_{x\to\infty} \frac{\ln x}{x} = 0$. Since that's $\displaystyle\lim_{x\to\infty} \frac{1}{x} = 0$. Assume true for $n=k$. $\displaystyle\lim_{x\to\infty} \frac{(\ln x)^k}{x} = 0$. Prove true for $n=k+1$.  You get $\displaystyle\lim_{x\to\infty} \frac{(\ln x)^{k+1}}{x} = 0.$   Use L'Hospital again: $\displaystyle\lim_{x\to\infty} \frac{(k+1)(\ln x)^{k}}{x} = 0$.  Then you see the induction hypothesis appear, and you can say this is equal to $0$.  What I'm not comfortable with is this idea that you can just assume something to be true ($n=k$), then based on that assumption, form a proof for $n=k+1$ case.  I don't see how you can use something you've assumed to be true to prove something else to be true. i know that|$S_n$|=$n!$ and |$A_n$|= $\frac{n!}{2}$ but i'm not sure where to go from there. Thanks. I have a vps using Ubuntu 12.04 LTS and I my host corrupted my vps so now it won't boot up. When it boots up it says:  Booting from Hard Disk... error: file not found. grub rescue&gt;   How can I fix/repair this? How can I monitor all and any Internet traffic from my home PC? I would like to monitor all and any Internet traffic from my home PC to see what programs installed on my home machine are accessing the Internet.  I thought I could use  for this, but I noticed Firefox traffic doesn't show up by default; you need .  What options do I have to monitor all traffic? If $\cot A+\cot B+\cot C=\sqrt3$ then prove that the triangle is equilateral triangle.   Trial: I can counter check that this is true as $\cot 60+\cot 60+\cot 60=3 \frac{1}{\sqrt3}=\sqrt3$.Here I also know that $A+B+C=180$. But I am unable to show it. Please help. Is it better to initialize class member variables on declaration  private List&lt;Thing&gt; _things = new List&lt;Thing&gt;(); private int _arb = 99;   or in the default constructor?  private List&lt;Thing&gt; _things; private int _arb;  public TheClass() {   _things = new List&lt;Thing&gt;();   _arb = 99; }   Is it simply a matter of style or are there performance trade-offs, one way or the other? The  gives the recoil force, $\mathbf{F_{rad}}$, back on a charged particle $q$ when it emits electromagnetic radiation. It is given by:  $$\mathbf{F_{rad}} = \frac{q^2}{6\pi \epsilon_0 c^3}\mathbf{\dot{a}},$$  where $\mathbf{\dot{a}}$ is the rate of change of acceleration.  If a particle has a constant acceleration, $\mathbf{\dot{a}}=0$, then there is no reaction force acting on it. Therefore the particle does not lose energy.   Does this mean that a constantly accelerating charged particle does not emit electromagnetic radiation? Redirect non-www requests to www on Amazon Route 53 I want to redirect all requests from example.com to www.example.com.   Preferably, this should happen at DNS level. I tried using PTR records, but that simply fails, returning a 404.  wwww.example.com is an ALIAS for an Elastic Load Balancer.  What’s the simplest way to achieve this? Breakthrough at the Large Hadron Collider -Leptoquarks skepticism? There are many cards that can put multiple permanents onto the battlefield simultaneously, such as , , or .  How do these interact with the following types of abilities?  As [this permanent] enters the battlefield... You may have [this permanent] enter the battlefield as... [This permanent] enters the battlefield with... When [a permanent with certain characteristics] enters the battlefield...   Justification There have been many questions already about particular instances of this situation.  This question is intended to be a canonical question and answer to all questions such as:      Note to moderators: three of these five questions have already been marked as duplicates pointing to this one.  Can we do the same for the remaining two? Why does holding something up cost energy while no work is being done?