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How do I reverse a list using recursion in Python? I want to have a function that will return the reverse of a list that it is given -- using recursion. How can I do that? Ubuntu touch - remote access to device in our company we developed an android app installed on nexus 7 tablets and on tablet we release to several customers. This app stays in foreground active 24/7 but we have problems in monitoring  the devices due to the restrictive access policies of unrooted Android devices.   We can't for example reboot the device remotely.  We are evaluating the possibility of installing Ubuntu touch to replace android on this tablets. Does Ubuntu touch offer more freedom for the remote access of the device? May I do remotely everything I could do normally with the tablet?   Moreover, are the apps developed with ubuntu sdk stable enough to run 24/7?  Thanks proving the inequality $\triangle\leq \frac{1}{4}\sqrt{(a+b+c)\cdot abc}$ Counterexample for $A,B\triangleleft G,G/A\cong B \Rightarrow G/B\cong A$ How to combine various armatures? How to compute rational or integer points on elliptic curves This is an attempt to get someone to write a canonical answer, as discussed in this . We often have people come to us asking for solutions to a diophantine equation which, after some clever manipulation, can be turned into finding rational or integer points on an elliptic curve. See , , , , . (This list is biased towards questions I have answered because I remember them best; other people have answered such questions as well.) I would like an answer to which, after one of us has explained the cleverness, we could direct the OP.  An ideal answer would address    How to find both rational and integer solutions Good software solutions. Ideally, it would nice to have a walkthrough for doing these things with , so that people could find solutions without even installing anything. References for how to transform some standard presentations of elliptic curves into Weierstrass form, so that we don't have to write out the algebra every time. I'm thinking of  a cubic in $\mathbb{P}^2$ with a rational point that is not a flex,  $y^2 = \mbox{degree 4 polynomial}$, a $(1,1)$ curve on $\mathbb{P}^1 \times \mathbb{P}^1$, or an intersection of two quadrics in $\mathbb{P}^3$. Which features of a wizard's familiar, if any, are considered magical? The  spell has a duration of instantaneous, which implies that the familiar itself is not an ongoing magical effect. However, the spell provides a number of ongoing features that are definitely supernatural:    The familiar disappears when it drops to 0 HP. The wizard can temporarily or permanently dismiss the familiar. The wizard can communicate with the familiar telepathically. The wizard can observe through the familiar's eyes and ears. The familiar can deliver touch spells on behalf of the wizard.   Which of these features are considered magical? For example, would any of them show up to a detect magic spell or be suppressed by an antimagic field? In addition, is my assumption above correct that the familiar itself is not considered magical? Is there a single word to describe a solution that hasn't been optimized? I am working with some code, and I would like to describe the difference in its performance when it is optimized versus when it is not optimized. Unfortunately, I can't find a word to describe the code before it is optimized; "sub-optimal" might work, but I want to emphasize that this is the first iteration of the code. If "unoptimized" were a word, I would use that, but I haven't found any references to it in any dictionary.  Perhaps "raw" or "original"? I seem to remember from first year physics that we can calculate the RMS speed of a stationary, ideal gas with $v=\sqrt{\frac{3RT}{M}}$. Does a similar equation exist for liquids? Can't install any app, App Store asks for credit card I haven't been installing anything on my iPad Mini for the past few weeks. But previously, I just had to tap on "Install", enter my Apple ID password and go.  Now, for every new application, even those free ones, I have to confirm each time, that I'm 17 or over. And then App Store asks to enter my credit card information. If I disagree and hit "Cancel", installation process is interrupted and I can't install any application, even free one.  I don't trust Apple at all and I'm not willing to share my credit card information, especially that I'm installing only free applications.  I'm pretty sure that I haven't changed anything in the configuration of my iPad Mini for ages.   Am I missing some significant change to the App Store or this is some kind of bug or virus? Is there anything I can do in this situation? How can I install a free application from the App Store, without having to share my credit card information with Apple? What does the origami represent in Blade Runner? Throughout Blade Runner we see Gaff (Edward James Olmos) making origami figures at various points in the movie. He makes a chicken, matchstick men and at the end of the film there is a unicorn waiting outside Deckard's apartment.    There is a   of  out there regarding the unicorn in particular, and how it indicates that Deckard is a replicant. I think an answer regarding the use of origami in general will be interesting new ground to cover.  Why origami? What does the origami represent in Blade Runner? Preventing hyphenation of specific short, repeated, hyphenated words If I run:  sudo apt-get --yes install postfix    or  sudo bash -c 'yes | apt-get --yes install postfix'   an interactive prompt appears to configure postfix. I need to automate the installation of postfix (I can modify the configuration files after the install).  Is there some magic that lets me install postfix (and other package) without human intervention? Has drupal abandoned drush? I've been away from drupal for 2-3 years. I just came back around and installed drush via composer () and now every time I try to use drush I get a message telling me that I should be using composer:  ~/sites/d7 $ drush version  Drush version : 9.0-dev-gd4a3872 ~/sites/d7 $ drush dl drupal-7.x  [notice] dl has been deprecated. Please build your site using Composer. Add new projects with composer require drupal/[project-name]. Use https://www.drupal.org/project/composer_generate to build a composer.json which represents the the enabled modules on your site.   I downloaded drupal 8 following these , but when I try to use drush in that project I get this message:  ~/sites/cfbtv/web $ drush dl devel This codebase is assembled with Composer instead of Drush. Use `composer update` and `composer require` instead of `drush pm-updatecode` and `drush pm-download`. You may override this error by [error] using the --pm-force option.   Is there something wrong with my local env, or is drush a thing of the past now? It can be proved that there are two irrational numbers $a$ and $b$ such that $a^b$ is rational (see ) and that for each irrational number $c$ there exists another irrational number $d$ such that $c^d$ is rational (see ).  My question is: Is there an irrational number $a$ such that $a^a$ is rational (and how could you prove that)? I have the following data structure (a list of lists)  [  ['4', '21', '1', '14', '2008-10-24 15:42:58'],   ['3', '22', '4', '2somename', '2008-10-24 15:22:03'],   ['5', '21', '3', '19', '2008-10-24 15:45:45'],   ['6', '21', '1', '1somename', '2008-10-24 15:45:49'],   ['7', '22', '3', '2somename', '2008-10-24 15:45:51'] ]   I would like to be able to   Use a function to reorder the list so that I can group by each item in the list. For example I'd like to be able to group by the second column (so that all the 21's are together)  Use a function to only display certain values from each inner list. For example i'd like to reduce this list to only contain the 4th field value of '2somename'    so the list would look like this   [      ['3', '22', '4', '2somename', '2008-10-24 15:22:03'],       ['7', '22', '3', '2somename', '2008-10-24 15:45:51'] ] Google Spreadsheets Enter/Return Key Behavior In Microsoft Excel, the default behavior of pressing the Enter (return) key is this:     If you are in a cell, and tab over as   you input data, at the end of the   series when you click Enter, the focus   will go to the cell directly under the   cell that you began the series of   tabs.      For example, if you are in cell A1,   tab over to C1, and then click Enter,   the focus will go to cell A2.   Is there any way to get this behavior in Google Spreadsheets? grep a file, but show several surrounding lines? I would like to grep for a string, but also show the preceding five lines and the following five lines as well as the matched line. How would I be able to do this? "add/show comments" should dismiss notification Is the gradient a surface normal vector or does it point in the direction of maximum increase of f I'm having some trouble trying to visualize and physically understand what's happening with the gradient.  I understand that the following is true:   The gradient of f (grad(f)) points in the direction of maximum increase of f   However, later on, we are told that a gradient of a surface f (grad(f)) gives us the surface normal vector (i.e pointing away). How can that be? From the first statement, I thought that the gradient must be pointing in the direction of maximum increase of f - surely the direction of maximum increase of f should be a vector pointing in some direction on f itself. How can it be pointing outward as a surface normal vector?