For how many pairs of positive integers n and m is the statement $mn - 8m + 6n =0$ true?
Show that there are exactly 16 pairs of integers $(x,y)$ such that $11x+8y+17=xy$.
What is `$?` ? Is it a variable? The question is pretty straight forward. I can get the exit code of the previous command by using $?. But what exactly is it?  I don't think it is a variable, because you can't have any special characters in the name of variable other than _.  foo?foo=10   will result in the error: foo?foo=10: command not found  So if it is not a variable, what is it? Are there others like it? Error: Account is Not Allowed To Suggest Edits I get this message while trying to edit a post on SuperUser while using the iOS mobile app. I have at least 101 reputation on SU, more than enough to be able to edit posts. This appears to be a bug. I add js file with drupal_add_js in hook_form_alter, it is ok and I see it in list of js loaded in page, but when I submit form and form not validate and return to that form (after validation failed) I don't see js files that added to form, why this happened? and how can add my js file in that form after validation failed ?   some part of my code   function mymodle_form_alter(&amp;$form, $form_state, $form_id) {   if ($form_id == 'news_node_form') {     drupal_add_js(drupal_get_path('module', 'mymodule') . '/js/mymodulejsfile.js'); //, array('scope' =&gt; 'footer','weight' =&gt; 10));   } }   code work correctly in add and edit form, but when form submit and validation failed it show validation error but my js file not loaded in page (I checked html source and check firebug script files), but there is not my js file, I use a Asus laptop with dual-boot Ubuntu and Windows 8.1  Wifi is on but I am not able to detect Wifi networks.  When I run :  lspci -knn | grep Net -A3; rfkill list   This is what happens:    I get  acer-wireless: Wireless LAN     Soft blocked: yes Not getting email alerts on answers on Stack Overflow Not getting email alerts on answers on my questions on stackoverflow.com   Used to get it earlier (>1 month back), but not getting alerts in recent times Email field in profile is filled  In prefs, "I'd like email notifications of activity on questions I've asked" is ticked.   What is missing?  Is it possible that signing up on another Stack Exchange community (say, Server Fault), on which by default, "I'd like email notifications of activity on questions I've asked" is unchecked affects same preference on stackoverflow.com? If so, it is a bug. How do I get help for "echo" or other bash commands? man echo works. but out of curiosity, does echo have something similar to a --help flag/parameter? Show $x^p-t$ has no root in the field $\mathbb{F}_p(t)$ I don't think I fully understand.  Let's say there is a root $x_0 \in K=\mathbb{F}_p(t)$, where $p$ is a prime number.  Then $x_0 = \frac{P(t)}{Q(t)}$ for some polynomials $P,Q \in \mathbb{F}_p[t]$. We can assume $\gcd(P,Q)=1$  and $x_0^p-t= \frac{(P(t))^p}{(Q(t))^p}-t= \frac{P(t)^p-tQ(t)^p}{Q(t)^p} =0$, so coefficients of $P(t)^p, tQ(t)^p$ must be identical, which contradicts $\gcd(P,Q)=1$,   hence such $x_0$ does not exist and the polynomial has no root in $K$.   am I about right?    anyway, I would appreciate an explanation about $\mathbb{F}_p(t)$, what is $t$? what is the meaning of a variable which does not belong to any specific "world"? I cannot use $t$ as if it was a member of $\mathbb{F}_p$ and hence cannot assume $t^{p-1} = 1 \pmod p$... Reoccurring Bent Axles, any reasons or upgrades? Earlier this month I had a issue with a broken axle.  It was the original that was sold with the bike and lasted around 5,000 miles.  I went to my LBS and bought a new axle (unbranded, labeled, etc, paid $13, came with cones, spacers, and hardware).  I did the install myself and had everything adjusted nicely.  The next time I rode it was on my commute to work, but I didn't get 4 miles into it when I found that I bent the axle.    I took the wheel to the shop but they wouldn't do anything about it since I didn't have them do the work.  However, I figured this was a fluke and asked them for a new axle and to true the wheel.  Once again I did the install of the axle and only got 15 miles out of it this time before bending it.  Before I got home I felt the wheel get stiff, but it stayed true.  The question is what will be a cause of bending 2 axles in 20 miles?    I think the guy at the shop said it was a cro-moly axle (while talking to another person today at the shop, these may not be), and he couldn't believe that I bent it.  If so, where would such an axle fall in the quality hierarchy?  Or is my hub toast?  While the wheel was being repaired, I swapped to a new original wheel for about 2 weeks.  I have not had any issues with it, but I wanted to keep it as new as possible.   Reading other questions I saw a comment that said not to use a hub where the bearing was catching.  I did have one in this wheel but I figured since it is a cheap wheel I would just burn it up and move on to a better one.  Besides, it has lasted 5k miles without issue to this point.  The bike in question is a 2008 Schwinn High Timber with Joytech freewheel hubs.  I do use a trailer with this bike using a Burley axle hitch.  While I had the hitch installed on both axles I bent, I did not have either trailer hooked up to them.  Something I would like to note is that when I removed the first bent axle (axle #2) that the cones on it had a wavy bearing track.  I didn't think too much of it, and I put them on axle #3.  Only having a few miles on them shouldn't have made a difference? I notice that modern C and C++ code seems to use size_t instead of int/unsigned int pretty much everywhere - from parameters for C string functions to the STL.  I am curious as to the reason for this and the benefits it brings. I have an array in Perl:  my @my_array = ("one","two","three","two","three");   How do I remove the duplicates from the array? I'm glad the outside overlay scrollbars are finally gone.  But the scrollbar is still a razor thin line that becomes a less razor thin scrollbar, but still thin scrollbar, when the mouse hovers.  How can I get a normal scrollbar? I.e., like in other OSes, like in Firefox, a scrollbar that doesn't change depending on what the mouse points to, and with a nice width.  I already tried  gsettings set com.canonical.desktop.interface scrollbar-mode normal   but no luck. Gradient descent and many other methods are useful for finding local minima in cost functions. They can be efficient when the cost function can be evaluated quickly at each point, whether numerically or analytically.    I have what appears to me to be an unusual situation. Each evaluation of my cost function is expensive. I am attempting to find a set of parameters that minimize a 3D surface against ground truth surfaces. Whenever I change a parameter, I need to run the algorithm against the entire sample cohort to measure its effect.  In order to calculate a gradient, I need to change all 15 parameters independently, meaning I have to regenerate all the surfaces and compare against the sample cohort way too many times per gradient, and definitely way too many times over the course of optimization.  I have developed a method to circumvent this problem and am currently evaluating it, but I am surprised that I have not found much in the literature regarding expensive cost function evaluations.  This makes me wonder if I am making the problem harder than it is and that there might be a better way already available.  So my questions are basically this: Does anyone know of methods for optimizing cost functions, convex or not, when evaluation is slow?  Or, am I doing something silly in the first place by rerunning the algorithm and comparing against the sample cohort so many times? While the etiquette of dealing with duplicate answers , there hasn't been a great way to deal with these, "Uh, yeah, I said this, too" types of behavior than the almighty downvote.   It would be nice to have a "Duplicate Answer" option on the flag menu. Five flags auto-deletes the answer, as with any other flag. In order to prevent abuse, the flagged answer would have to have the last owner edit (including original post) be younger than the original answer's original posting by at least 2 minutes.  Also, this ability should be a 5,000+ rep privilege, since we currently have no fanfare of rep from 3,001 to 9,999, and a power like this has a bit more umph than closing a question.  Thoughts? 20GB of Temporary Files, Only 50MB in Temp Folder I recently discovered that the Settings app is showing around 20GB of space used by temporary files. But when I go to remove them I can only remove about 28MB. When I open the temp folder by using Run and then %temp% it only shows around 40-50MB of files right after boot. How can I remove these 20GB of invisible files? I only have a 240GB SSD so space is quite scarce. Here are a few photos:    I run Windows 10 Home Activated. I have tried Disk Cleanup but this removed only around 30MB of files.  UPDATE: After turning on hidden items I found a folder name WINDOWS-BT, after doing some research it turned out to be a folder for the previous windows version. I deleted it using TakeOwnerShip pro and this removed around 12GB of temporary files, but 8GB still remain. I am currently downloading TreeSize free to find out where the other 8GB is.  UPDATE 2: Hi, I downloaded TreeSizeFree and I didn't really find anything there.   The biggest folder is 87GB of Program Files which mainly consists of my GTA V install. The Program Files (x86) folder was mainly made up of my Steam games (CS:GO, Besiege, etc). The 5.5GB [3 Files] location was noted as being my pagefile.    The Windows folder shows up as 25GB when in Settings it is only 18GB, this could explain where the other 8GB are coming from. What are some things I should look for in there?  FINAL UPDATE: After deleting the Windows-BT folder it reappeared a few hours later. I decided to check from system updates and surely enough one was downloaded and pending to be installed. After installing the update the Windows-BT folder disappeared on it's own and a new folder taking up 20GB called Windows.old was created. I then followed this  to remove the .old folder and I had all my disk space back! NOTE: Disk Cleanup wasn't able to find the Windows.old folder, this is why I had to delete it from the settings app. How to find collisions in a hash function knowing its coefficent? Say I have a simple hash function where $x$ is a sequence of integers:  $$h(x)=(a_1\cdot x_1+...+ a_n\cdot x_n) \bmod N=\sum_i a_i\cdot x_i \bmod N$$  where $a_1,a_2,a_3,...,a_n$ is the coefficient known to the attacker, size of $x$ can be anything smaller than $N$.   How to find 2 different inputs that hashes to the same value, i.e $0$?  I feel like this has something to do with linear combination / euclid algorithm but I'm not sure how to proceed. US doctor working in Tripoli wants me to open online account I am compiling LaTeX files with Texmaker on Ubuntu 12.04 system.   When I run pdflatex and then view the pdf file (internal), it does not show me the exact place where I am editing on Texmaker i.e. the generated pdf file does not show me the position of the cursor.   But some days before it was working fine. I don't know what causes this problem.  I already tried uninstalling and then reinstalling, I did update and upgrade too. But the problem still persists. Merging two pictures Is there a tool in Ubuntu to automatically merge two images having a common strip? I need to scan a large image, which will not fit at once in the scanner. I was thinking about scanning it in two parts and hand-editing the result in Gimp to join them. But probably an automatic tool can do the same more quickly and better.  EDIT I think I may have not made my point clear. My difficulty is not aligning two images. My problem is that one has to compensate for the rotation or distortion introduced by the manual process of putting the sheet into the scanner, and an automatic tool should obtain more precise results at analyzing common patterns in the two images to rotate and align them seamlessly. How do electric currents make light? In an incandescent light bulb, electrons flow through wires to the bulb, where is passes through a filament, and the filament lights up.  Does this work because of several high energy electrons losing some of their energy to disturb the electromagnetic field, creating photons? The electrons just leave the filament afterward and are replaced by more high-energy electrons from the negative terminal. Or does it work differently?  As a secondary question, is there a type of light bulb that directly converts electron flow to photons? What would be a good term for continuous testing post-deployment? I'm thinking of live production sites here, where there is a continuous community or much content being updated, sort of like a newspaper site or community site (the one I work at is a healthcare improvement company) and we have news articles, programs and so on.  While the underlying code is tested and released there are often production issues that come up, maybe certain pages stop working, style sheets suddenly don't appear like they did due to a service patch or some tweak done to improve production security or performance.  There is Acceptance Testing for when a deployment goes out to make sure things work right, Regression Testing to make sure nothing has been broken since previous releases.  Considering we've done these types of tests and things are working fine what would you call testing, or checking, the site to make sure things still work fine?  To be sure new pages are being crawled for search indexing?  To check that newly added pages are using plug-ins correctly?  Or so on.  My manager and I were discussing some issues that have cropped up and decided we needed to do something like this, but then to sort of note it in our status reports we didn't really have a name for it.  Anyone done this and have a term that might be good?  Or would you consider it covered by something like Continuous Acceptance Testing or some such term?