If $x\in Z$ and divisible by $4$, then there are existing $a,b \in Z$ , so: $x=a^2-b^2$   I had been trying to figure out which way this should be solved.  First I found out that if $x=20$, then $a^2 = 6^2$ and $b^2 = 4^2$ (of course $20=36-16$ ).  now I'm not sure if i could place those examples as an answer, or I need to prove it with some "proving tricks". anyway, I thought about that $a^2 - b^2 = (a+b)(a-b)$ and $4\mid x$ is also $x=4k$ for some $k$ integer.
I want to prove this:  Prove that if $a \in \mathbb{Z}$ is divisible by $4$, then there exist $b$ and $c$ where $b,c \in \mathbb{Z}$ such that $a = b^2 - c^2$  I want to prove this directily:  $4\mid a $,  $a\in \mathbb{Z} \implies \exists b,c \in \mathbb{Z}, \quad$ s.t. $a=b^2-c^2$  Let $k\in \mathbb{Z}$. Then $4=a\cdot k$ and $k$ can divide $4$. Hence,  $\frac{4}{k} =b^2 - c^2 = (b-c)(b+c) \implies \dfrac{4}{k\cdot(b-c)} = b+c$  Is this proof correct?
Client-Side Rendering/JS Link documentation? Has there been any sort of documentation or anything like that released for SharePoint 2013 Client-Side Rendering/JS Link?  Anthing unofficial? Any dates for when documentation might be released? Shutdown Cause -128 This is not a duplicate question because I am looking for an explanation directed to a general audience as to the mistakes (if any) in  (reproduced below). (Numberphile is a YouTube channel devoted to pop-math and this particular video has garnered over 3m views.)   By general audience, I mean the same sort of audience as the millions who watch Numberphile. (Which would mean, ideally, making little or no mention of things that a general audience will never have heard of - e.g. Riemann zeta functions, analytic continuations, Casimir forces; and avoiding tactics like appealing to the fact that physicists and other clever people use it in string theory, so therefore it must be correct.)  Numberphile's Proof.  The proof proceeds by evaluating each of the following:  $S_1 = 1 - 1 + 1 - 1 + 1 - 1 + \ldots$   $S_2 = 1 - 2 + 3 - 4 + \ldots $   $S = 1 + 2 + 3 + 4 + \ldots $   "Now the first one is really easy to evaluate ... You stop this at any point. If you stop it at an odd point, you're going to get the answer $1$. If you stop it at an even point, you get the answer $0$. Clearly, that's obvious, right? ... So what number are we going to attach to this infinite sum? Do we stop at an odd or an even point? We don't know, so we take the average of the two. So the answer's a half."   Next:  $S_2 \ \ = 1 - 2 + 3 - 4 + \cdots$  $S_2 \ \ = \ \ \ \ \ \ \ 1 - 2 + 3 - 4 + \cdots$  Adding the above two lines, we get:  $2S_2 = 1 - 1 + 1 - 1 + \cdots$  Therefore, $2S_2=S_1=\frac{1}{2}$ and so $S_2=\frac{1}{4}$.  Finally, take  \begin{align} S - S_2 &amp; = 1 + 2 + 3 + 4 + \cdots           \\ &amp; - (1 - 2 + 3 - 4 + \cdots)          \\ &amp; =   0 + 4 + 0 + 8 + \cdots           \\ &amp; = 4 + 8 + 12 + \cdots           \\ &amp; = 4S \end{align}  Hence $-S_2=3S$ or $-\frac{1}{4}=3S$.  And so $S=-\frac{1}{12}$. $\blacksquare$ Is there a --parents option for mv like for cp? cp has a useful feature --parents that allows a file's directory structure to be copied to another directory:  $ cp -v --parents source/directory/from target/ ‘source/directory/from’ -&gt; ‘target/source/directory/from’   How do I similarly move a file? Both static_cast and reinterpret_cast seem to work fine for casting void* to another pointer type. Is there a good reason to favor one over the other? How to use paging in a WFS query? I am running a GetFeature request against a WFS server which does not support to download all data at once. Can I use ogc:PropertyIsGreaterThanOrEqualTo to split the dataset into chunks at download them step by step? I wonder how I can construct the actual URL to include the paging filter. Here is what I tried:  http://example.com/wfs.aspx?request=GetFeature&amp;service=WFS&amp;version=1.1.0 \ &amp;typeName=example:example&amp;maxFeatures=50000 \ &amp;FILTER=&lt;ogc:Filter xmlns:ogc="http://www.opengis.net/ogc"&gt; \ &lt;ogc:PropertyIsGreaterThanOrEqualTo&gt;&lt;ogc:PropertyName&gt;OBJECTID&lt;/ogc:PropertyName&gt; \ &lt;ogc:Literal&gt;50000&lt;/ogc:Literal&gt;&lt;/ogc:PropertyIsGreaterThanOrEqualTo&gt;&lt;/ogc:Filter&gt;   This is not a valid URI though... That's why uri-encoded the filter part:  http://example.com/wfs.aspx?request=GetFeature&amp;service=WFS&amp;version=1.1.0 \ &amp;typeName=example:example&amp;maxFeatures=50000 \     &amp;FILTER=%3Cogc%3AFilter%20xmlns%3Aogc%3D%22http%3A%2F%2Fwww.opengis.net \ %2Fogc%22%3E%3Cogc%3APropertyIsGreaterThanOrEqualTo%3E%3Cogc%3APropertyName%3EOBJECTID \ %3C%2Fogc%3APropertyName%3E%3Cogc%3ALiteral%3E50000%3C%2Fogc%3ALiteral \ %3E%3C%2Fogc%3APropertyIsGreaterThanOrEqualTo%3E%3C%2Fogc%3AFilter%3E   This actually works. Is there a "nicer" way to do this? Turn off text message (SMS) sync to Exchange email account I recently started using a Samsung Galaxy S4 and one of the features in a synchronization between text messages and email accounts.  When a text message comes in it throws a copy into my Exchange email accounts that are one the phone. I want to turn this feature off but cannot find a setting for it anywhere.  Any ideas? 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Possible Duplicate:          My HDD on a dock just started making noises like it's being accessed but I don't see any program accessing it.  Is there a program that shows which HDD is accessed by which program in percentages similar to how the task manager shows which programs use the most CPU? HTTP 401 twice before HTTP 200 when serving files $$\lim _{x \rightarrow 0} \left(\frac{ \sin x}{x}\right)^{1/x}$$  I have spent an hour on the above limit and have no work to show. I tried using L'Hopital's Rule, but just kept going around in circles. Any help would be appreciated. Thank you. arcpy.env.overwriteOutput = True not working for one dataset Antecedent Precedence? What tips and advice do you have for photographing the Aurora Borealis? A friend has been kind enough to lend me a DSLR for a few months.  It's my first foray into DSLR photography, or indeed any kind of photography beyond simply aiming my Canon IXUS at something and clicking the button.    Over the last few weeks, I've been busy studying, practising and trying to learn as much as possible ahead of a planned trip to Norway where I hope to see, and get some photos of, the Aurora Borealis.  I currently have:   Canon Rebel Xsi (450D) Canon 50mm f/1.4 L  Canon 18-55m kit lens Aluminium full height tripod with ball head Cable release Spare battery &amp; memory cards   I'm prepared to spend the night wrapped up outdoors, in the hope that they will make an appearance. I'd like some advice on how to get the most out of this kit for the trip:   How should I go about setting up to get pics of the Aurora?  How might I achieve optimum focus? From what I've read, I'll need to use the tripod and cable release, with IS turned off, on continuous shooting mode with long exposure times. But I'd like some more specific tips in terms of which aperture and ISO settings might work best? Should I use one of the lenses above, or would it be worthwhile renting a better, wide angle lens; if so, which would you recommend? filling area between 2 functions, with shading Does the Planck scale imply that spacetime is discrete? On a quantum scale the smallest unit is the , which is a discrete measure.  There several question that come to mind:   Does that mean that particles can only live in a discrete grid-like structure,  i.e. have to "magically" jump from one pocket to the next? But where are they in between? Does that even give rise to the old paradox that movement as such is impossible (e.g. )? Does the same hold true  for time (i.e. that it is discrete) - with all the ensuing paradoxes? Mathematically does it mean that you have to use difference equations instead of differential equations? (And sums instead of integrals?) From the point of view of the space metric do you have to use a discrete metric (e.g. the ) instead of good old Pythagoras?   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