Why does the sum of inverse squares equal $\pi^2/6$?
Different methods to compute $\sum\limits_{k=1}^\infty \frac{1}{k^2}$ (Basel problem)
Quick directory navigation in the bash shell I would like to frequently switch between directories that are in totally unrelated paths, for example /Project/Warnest/docs/ and ~/Dropbox/Projects/ds/test/.  But I don't want to type cd /[full-path]/ all the time. Are there any shortcut commands to switch to previously worked directories?  One solution I could think of is to add environment variables to my bash .profile for the frequently used directories and cd to them using those variables.  But is there any other solution to this? Lighter nuclei liberate energy when undergoing fusion, heavier nuclei when undergoing fission.  What is it about the nucleus of an Iron atom that makes it so stable?  Alternatively: Iron has the greatest nuclear binding energy - but why? I am trying to implement the following K-map with a table    AB C     00 01 11 10       ______________     0| 1  0  1  0     1| 0  1  1  0   I implemented it like this:   \begin{table}[htdp] \begin{center} \begin{tabular}{c|c|c|c|c|}  &amp;00 &amp; 01&amp; 11&amp; 10 \\ \hline 0&amp;1&amp;1&amp;1&amp;1\\ 1&amp;0&amp;0&amp;0&amp;0\\ \end{tabular} \end{center} \label{default} \end{table}   However on the top row I have vertical lines that I'd like to not see. I went with this approach because if i was to use a one row with the information so that I wouldn't have to worry about aligning the headings. So is there a way I can get the vertical lines not to appear in the heading row? List comprehension rebinds names even after scope of comprehension. Is this right? Is there a way to make an Android phone (HTC Desire HD) act as Bluetooth speakers or headset?  I'd like to use the phone as wireless headphones whilst watching a movie on a computer (running Ubuntu). A screen of mine burnt out, and was replaced with a screen of lower resolution. It works in safe mode as that is in 800x600 resolution, but when I try to boot to normal mode, it fails. Is there a way to change the resolution for normal mode in safe mode? Have we ever observed two drosophila lineages that evolved reproductive isolation in labs? Background  The standard definition of species refers to the concept of . If two lineages are found to be reproductively isolated, then we consider these two lineages to belong to different species. My question concerns evolved reproductive isolation in Drosophila sp. following labs due to experimental evolution.  Question  Have we ever demonstrated that two Drosophila sp. lineages that could initially interbreed (in nature or in labs) evolved through artificial selection (and drift and mutations) in labs to finally not be able to interbreed anymore either due pre- or post- zygotic isolation (see )? Or, in other words, have we ever demonstrated that two drosophila lineages evolved to become different species (reproductive isolation definition) in labs experiments?  If not, have we ever observed some partial reproductive isolation such as inbreeding depression for example?    Note: This question is motivated by @LotusBiology that could not receive the answers she/he was waiting for because he/she failed to ask questions that are possibly answerable! So I wanted to ask this question that somehow addresses  question he/she asked (now on-hold)  For more information about the concept of species, please have a look at There was a book (it actually had a second book as well) where a guy gets paid with a group of others to go to a planet that is enclosed in a metallic sphere.   There are many species in this planet. I don't recall if they actually met the races in the first or the second book but there an incident where an alien in the first book ate fruit from a tree that removed any signs of gender from it. If humans were able to catch all sun energy reaching the earth for their use, will the climate change? I guess that energy will be used up and, at the end, will contribute to heat the earth, so I see no big differences...  please explain your point of view. Are "w/o", "w/", "b/c" common abbreviations in the US? I have seen many ways to make a USB an installer for about any Windows OS. But how can I (using grub or something like that) make it bootable to all editions on Windows XP and 7?   I have tried Googling it and researching, I've even tried to do it, But I dont understand much about GRUB or linux at all.  Please keep in mind that I am not very good with linux, so please use as many details as possible. Scale settings for exporting to STL (for 3d printing) I've set my Units (in scene properties) to Metric, and the Scene Scale to 0.01 hoping to get the Blender dimensions it mm's. While the (normal) scale is 1 for x,y,z, the standard cube has the dimensions 2x2x2 cm. When I export this to an STL and open it in a slicing program (Netfabb, Slic3r or Cura) they all give the dimensions: 2x2x2 mm. While exporting, the scale is 1.  What am I doing wrong? I hope you can help me.  Thanks! Schengen Visa Refusal: Justification for the purpose and conditions of the intended stay was not reliable Schengen refusal formulae can be difficult to understand sometimes.  Sometimes an applicant receives a refusal with:       Justification for the purpose and conditions of the intended stay was not reliable.     often accompanied by:       Your intention to leave the territory of the Member States before the expiry of the visa applied for could not be ascertained.   What is meant by this refusal reason?  What are they trying to say? Where can I find Dogmeat after dismissing him to Sanctuary Hills? I recently dismissed Dogmeat in favor of Codsworth, and I'm fairly sure I sent him to Sanctuary Hills. However, I've had a fairly detailed look around the place and I haven't found him yet. Does anyone know where he is? Function pointers, Closures, and Lambda I am just now learning about function pointers and, as I was reading the K&amp;R chapter on the subject, the first thing that hit me was, "Hey, this is kinda like a closure." I knew this assumption is fundamentally wrong somehow and after a search online I didn't find really any analysis of this comparison.  So why are C-style function pointers fundamentally different from closures or lambdas? As far as I can tell it has to do with the fact that the function pointer still points to a defined (named) function as opposed to the practice of anonymously defining the function.  Why is passing a function to a function seen as more powerful in the second case, where it is unnamed, than the first where it is just a normal, everyday function that is being passed?  Please tell me how and why I am wrong to compare the two so closely.  Thanks. How can a singularity in a black hole rotate if it's just a point? I guess nobody really knows the true nature of black holes, however, based on everything I know about black holes, there is a "singularity" at their center, which has finite mass but is infinitely small and thus infinitely dense.  So a black hole is really just a point mass that dramatically warps the space-time around it so that an event horizon is achieved.  What doesn't make sense to me is how a true mathematical "point" can rotate, or at least how it is even possible to determine if a point is rotating (or if it should even make a difference about whether or not it is rotating, for the same reason it would be impossible to tell if a perfect, featureless sphere were rotating (I know it would have higher rotational energy but lets assume you can't measure that because a true "point" can't attain rotational energy since it has no radius)).  The only conceptual solution I have to describe a rotating black hole would be to compare it to a rotating whirlpool, where the surface of the water is the fabric of space-time... but this is different because there is no "singularity" in a whirlpool that causes the water around it to rotate, whirlpools form from flow of water around a point (put into motion perhaps by some kid swirling water in a 2-liter bottle... it's due to an external cause), but the only thing that could cause rotation around a black hole would be the singularity itself, which doesn't make sense because, once again, how can we know if a true point is actually rotating?  Aside from everything I've said, here is what I am actually asking:  Why/how do some black holes rotate... what is the mechanism of their rotation (is analogous to a spinning point or a 3-d whirlpool, or something different)? I am trying to connect my Galaxy Tab to the Internet through an ad-hoc network made via my PC. The PC ad-hoc settings are fine because I can use it from my iPhone and other devices. Regex to strip lat/long from a string Anyone have a regex to strip lat/long from a string? such as:  ID: 39.825 -86.88333 How to prove the divisibility rule for $3\, $ [casting out threes] The divisibility rule for $3$ is well-known: if you add up the digits of $n$ and the sum is divisible by $3$, then $n$ is divisible by three.  This is quite helpful for determining if really large numbers are multiples of three, because we can recursively apply this rule:  $$1212582439 \rightarrow 37 \rightarrow 10\rightarrow 1 \implies 3\not\mid 1212582439$$ $$124524 \rightarrow 18 \rightarrow 9 \implies 3\mid 124524$$  This works for as many numbers as I've tried.  However, I'm not sure how this may be proven.  Thus, my question is:     Given a positive integer $n$ and that $3\mid\text{(the sum of the digits of $n$)}$, how may we prove that $3\mid n$? Mac Sierra won't boot - SmartBattery: finished polling type 4