I'm wondering how this is typically done at a high level. Please correct me if I'm wrong, but from what I understand, normally you could have some sort of validation. But if you have access to the source code, anyone could see how to send data to the server (or make your own server for that reason). Is there a viable solution for this?
I'm playing around with a native (non-web) single-player game I'm writing, and it occured to me that having a daily/weekly/all-time online highscore list (think Xbox Live Leaderboard) would make the game much more interesting, adding some (small) amount of community and competition. However, I'm afraid people would see such a feature as an invitation to hacking, which would discourage regular players due to impossibly high scores.  I thought about the obvious ways of preventing such attempts (public/private key encryption, for example), but I've figured out reasonably simple ways hackers could circumvent all of my ideas (extracting the public key from the binary and thus sending fake encrypted scores, for example).  Have you ever implemented an online highscore list or leaderboard? Did you find a reasonably hacker-proof way of implementing this? If so, how did you do it? What are your experiences with hacking attempts?
Both sides differentiation expression with integral I have a problem with derivation of some function. This is related to computing the Poincare Map of Logistic Population model with Periodic harvesting. This function is:  $\phi(t,x_0) = x_0 + \int_{0}^{t} f(s,\phi(s,x_0)) ds $,  where $\phi(t,x_0)$ is a function: $\mathbb{R}  \times \mathbb{R} \rightarrow \mathbb{R}$.  I'm trying to differentiate  equation of this function both sides  with respect to $x_{0}$. In book from this equation comes from the result of this differentiation is:  $\frac{\partial \phi}{\partial x_0} (t,x_0) = 1 + \int_0^{t} \frac{\partial f}{\partial x_0} (s, \phi(s,x_0)) \cdot \frac{\partial \phi}{\partial x_0} (s, x_0)ds$.  I can't understand expression under the integral. In this book it is explained that it is clear from chain rule.Could someone explain to me this transformation in easy way? I will be grateful for your help Best regards What does $$ mean in the shell? Someone who just thinks they know what they're talking about I have used the code shown below to draw a static CMOS inverter. I have tried to use the TikZ library calc to calculate the point at which drains of the PMOS and the NMOS devices meet; and the mid-point of the line which connects the gates of the two devices. Apparently calc gets the y coordinates of the points right. However, calc seems to get the x coordinates of the points wrong. What could I be doing wrong?  Here is the TikZ code:  \documentclass{standalone} \usepackage[americaninductors]{circuitikz} \usetikzlibrary{calc} \begin{document} \begin{circuitikz} \draw % Draw MOSFETs: (0,0) node [pigfete,yscale=-1,anchor=S] (M1) {} (M1.D) node[nigfete,anchor=D] (M2) {} (M1.G)--(M2.G) % The line connecting the gates of the MOSFETs. ; % Draw the power rails: \draw ($(M1.S)-(2.0,0)$) -- (M1.S) -- ($(M1.S)+(1.2,0)$); \draw ($(M2.S)-(2.0,0)$) -- (M2.S) -- ($(M2.S)+(1.2,0)$); % Input and output nodes: \node (I) at ($0.5*(M2.G)+0.5*(M1.G)$) {}; % The mid-point between the gates. \node (O) at (M1.D) {}; % The point at which the drains of the MOSFETs are conneted. % The Input and output lines. \draw ($(I)-(.8,0)$) -- (I); \draw (O) -- ($(O)+(0.8,0)$); \end{circuitikz} \end{document}   Here is the figure that I get: Note that that input and the output lines are not connected to the rest of the circuit. Can we attribute the problem of the lines not being connected, to the rest of the circuit, to the way I am using calc? I have a very large folder that I am trying to create a tar archive of. The issue is I don't have enough extra free space to store the entire archive so I want to create say 100-200GB chunks of the archive at a time and transfer those individually to cloud storage. I need to be able to control when new chunks are created so my HDD doesn't fill up but all of the commands i've found to create split tarballs always create it all at once, in the same directory. The closest solution I found was from  but all the responses base the archives on number of files, not size which is important for my use case as my file sizes are unevenly distributed. I´d like to enable the ssh root access on Ubuntu 14.04. Editing the /etc/ssh_config (PermitRootLogin yes) didn't affect anything. Is there a way to set up a default folder for Windows Explorer? I often use Win + E to pop up Windows Explorer, and then I usually have to navigate down into a few network folders to find my primary directory. Is there a way to set this folder as the default so that Windows Explorer always opens to this folder? Prove that $\frac{a_1^2}{a_1+a_2}+\frac{a_2^2}{a_2+a_3}+ \cdots \frac{a_n^2}{a_n+a_1} \geq \frac12$ Let $a_1, a_2, a_3, \dots , a_n$ be positive real numbers whose sum is $1$. Prove that $$\frac{a_1^2}{a_1+a_2}+\frac{a_2^2}{a_2+a_3}+ \ldots +\frac{a_n^2}{a_n+a_1} \geq \frac12\,.$$  I thought maybe the Cauchy and QM inequalities would be helpful. But I can't see how to apply it. Another thought (might be unhelpful) is that the sum of the denominators on the left hand side is $2$ (the denominator on the right hand side). I would really appreciate any hints. Update: Now that it's 2016 I'd use PowerShell for this unless there's a really compelling backwards-compatible reason for it, particularly because of the regional settings issue with using date. See @npocmaka's     What's a Windows command line statement(s) I can use to get the current datetime in a format that I can put into a filename?  I want to have a .bat file that zips up a directory into an archive with the current date and time as part of the name, for example, Code_2008-10-14_2257.zip. Is there any easy way I can do this, independent of the regional settings of the machine?  I don't really mind about the date format, ideally it'd be yyyy-mm-dd, but anything simple is fine.  So far I've got this, which on my machine gives me Tue_10_14_2008_230050_91:  rem Get the datetime in a format that can go in a filename. set _my_datetime=%date%_%time% set _my_datetime=%_my_datetime: =_% set _my_datetime=%_my_datetime::=% set _my_datetime=%_my_datetime:/=_% set _my_datetime=%_my_datetime:.=_%  rem Now use the timestamp by in a new ZIP file name. "d:\Program Files\7-Zip\7z.exe" a -r Code_%_my_datetime%.zip Code   I can live with this, but it seems a bit clunky. Ideally it'd be briefer and have the format mentioned earlier.  I'm using Windows Server 2003 and Windows&nbsp;XP Professional. I don't want to install additional utilities to achieve this (although I realise there are some that will do nice date formatting). Determining ethnicity according to ancestry? My sister, cousins, and other relatives speculate that my mom's grandmother was Jewish (for various reasons). I've heard people say at times things like, for example, "I'm 50% German" or "I'm 25% Swedish." This has to do with their ancestry. But, how exactly is this calculated? Say for example that my mother's paternal grandmother was indeed 100% Jewish, what would that make me and my siblings, and how is it calculated? How do I use formatting in comments? I see some people write comments that are for example - in italics.  I do not see any way of doing that while writing my comments. Why is it so? Is there an easy way to calculate the residue at one of the poles of a rational expression of the form $\frac{1}{1+z^n}$?  I end up having to add a bunch of polar-form complex numbers in the denominator which I have no idea how to do except to convert them to rectangular coordinates, which becomes really messy.  I feel like there ought to be simple pattern/rule for such a canonical expression. When I am writing I often come across the words "around" and "round." I was wondering what is different between them, and how they would be used in different contexts. Per User Named Credential and External Identity Users My question is almost similar to one mentioned in 'Auth Provider with per-user Named Credential' (). I am trying to authenticate and make a API call when a new or existing Salesforce community users try to log in. The Authentication is provided by 3rd party (Cognito). So, the requirement is :  User to be authenticated using the 3rd party system for login. After successful authentication and before community page is displayed, the API call is to be made to 3rd party system to get more details about the user.  In order to achieve this:  Created an Open ID Connect Auth. Provider Use the registration handler on the Auth. Provider to create/link a new/existing user. Use a login flow for API call. Use Per User Identity Type Named Credential to make API calls.  No, where I am having issue is once the user has authenticated successfully, how do I allow the user to make API call without authenticating it again. I am getting the error: An Apex error occurred: System.UnexpectedException: You don't have permission to view this data, enter credentials for the named credential in your personal settings. Or ask your Salesforce admin for help. I believe this error is due to the fact that the user is not listed under 'Authentication setting for External Systems'. My requirement doesn't allow me to authenticate the user again, so how do I achieve this using either APEX or any other way? Why are "south" and "southern" pronounced with different vowels? I was wondering why we pronounce the vowels in the words south and southern. They seem to be very closely related to each other. Both refer to the same direction. South is a noun and Southern is an adjective, which I don't think caused them to have a different vowel. Oxford Dictionary shows that south is /saʊθ/ and southern is /ˈsʌð(ə)n/. In short, south has a long vowel and southern has short. I am starting to see a lot of people claiming that convential current is 'wrong' because Franklin made an error when he first started investigating electrostatics, and that later scientists didn't bother correcting the mistake, but preferred to keep the 'convention' (here is a classic example: )  I always thought he didn't get it wrong. He said that current is positive in the direction that positive charge flows, and vice-versa. He of course had no way to know which side of two sticks behind rubbed actually gained or lost mass. So he wasn't wrong. What were you taught?  P.S. I can't help but feel we are lucky that he got it 'backwards', because clearly many people are confused about electrostastics (including the author of that text book!) and believe electricity has to involve electrons (an unfortunate name... why couldn't they have been named negatrons...) I'm sorry if this seems a bit too basic, but I guess I'm just looking to confirm understanding here. I get the sense I'd have to do this in two steps, and I've started trying to grok correlation matrices, but it's just starting to seem really involved. I'm looking for a concise explanation (ideally with hints towards a pseudocode solution) of a good, ideally quick way to generate correlated random numbers.   Given two pseudorandom variables height and weight with known means and variances, and a given correlation, I think I'm basically trying to understand what this second step should look like:     height = gaussianPdf(height.mean, height.variance)    weight = gaussianPdf(correlated_mean(height.mean, correlation_coefficient),                          correlated_variance(height.variance,                          correlation_coefficient))    How do I calculate the correlated mean and variance? But I want to confirm that's really the relevant problem here.  Do I need to resort to matrix manipulation? Or do I have something else very wrong in my basic approach to this problem? How do I play a YouTube video in the background on my iPad? I like to 'listen' to YouTube video in the background while I am surfing on my desktop. However, I cannot do this on the iPad as, as soon as I switch from YouTube video to the browser, the video pauses. Is there a solution for this either in the form of an app or a website? According to the documentation, the  method uses a round-to-even algorithm which is not common for most applications. So I always end up writing a custom function to do the more natural round-half-up algorithm:  public static decimal RoundHalfUp(this decimal d, int decimals) {     if (decimals &lt; 0)     {         throw new ArgumentException("The decimals must be non-negative",              "decimals");     }      decimal multiplier = (decimal)Math.Pow(10, decimals);     decimal number = d * multiplier;      if (decimal.Truncate(number) &lt; number)     {         number += 0.5m;     }     return decimal.Round(number) / multiplier; }   Does anybody know the reason behind this framework design decision?  Is there any built-in implementation of the round-half-up algorithm into the framework? Or maybe some unmanaged Windows API?  It could be misleading for beginners that simply write decimal.Round(2.5m, 0) expecting 3 as a result but getting 2 instead. How to log commands within a "sudo su -"? If I:   [user@notebook ~] sudo echo 123456uu 123456uu [user@notebook ~]    Then I can see that in the logs:  [root@notebook /var/log] grep 123456uu * auth.log:Jan  9 17:01:51 notebook sudo: user : TTY=pts/3 ; PWD=/home/user ; USER=root ; COMMAND=/bin/echo 123456uu [root@notebook /var/log]    but if I:   [user@notebook ~] sudo su - [root@notebook ~] echo 1234567zz 1234567zz [root@notebook ~]    I cannot see it in the logs:   [root@notebook /var/log] grep 1234567zz * [root@notebook /var/log] echo $? 1 [root@notebook /var/log]    My question: How can I turn the logging on for the commands within the "sudo su -"?  OS is an Ubuntu 12.04 but the question is in general.   UPDATE#1:   [user@notebook ~] sudo su - [sudo] password for user:  [root@notebook ~] echo zizizi zizizi [root@notebook ~] cd /var/log [root@notebook /var/log] grep -iIR 'zizizi' * [root@notebook /var/log] grep 'COMMAND=/bin/su -' * auth.log:Jan 10 15:42:42 notebook sudo: user : TTY=pts/1 ; PWD=/home/user ; USER=root ; COMMAND=/bin/su - [root@notebook /var/log]