I am getting Null pointer exception when i run my app 2nd time
What is a NullPointerException, and how do I fix it?
Solutions of $p!q! = r!$ The title says it all, more or less. Obviously, there are infinitely many "trivial" integral solutions of the form $p=n, q=(n!-1), r= n!$. How many non-trivial solutions are there?  I came across this about ten years ago; as far as I can tell, it hasn't appeared here before, so I thought that it might be of interest. I'm actually most interested in finding whether there was any progress made since By viewing the polynomials as a difference of two squares, factorise the following polynomial: $$x^4+x^2+1.$$ I searched but couldn't find a way to solve this  Edit: By using Hans Lundmark hint, I get:  $$(x^2+1)^2-x^2$$   Is it fully factorized? How do I instantiate a string array object into a Spring.Net container? Can Quantum Entanglement and Quantum Superposition be considered the same phenomenon? Quantum entanglement is known to be the exchange of quantum information between two particles at a distance, while quantum superposition is known to be the uncertainty of a particle (or particles) being in several states at once (which could also involve the exchange of quantum information for a particle that is known to be in several locations simultaneously).  I was wondering if all of this was nothing more but the exchange of quantum information between different masses, and if this could clear up all the confusion in terms of how quantum systems connect in this field of science.  A clear explanation for how both of these quantum phenomena work, and if they really are connected (the exchange of quantum information?) would be much appreciated. How do I install KDE? I want to have both KDE and Gnome shells on Ubuntu. Ubuntu only has Gnome installed by default. How do I install KDE? How to use my aliases in my crontab? What are the differences between mocks and stubs on Rhino Mocks? I haven't play enough with this and usually use mocks, but I wonder what are the differences between this two and when to use one or the other on Rhino Mocks.  Update:  I also found the answer to my question in :  The difference between stubs and mocks You can get the actual definition of the these terms in this article: . I want to focus on the difference from the point of view of Rhino Mocks.  A mock is an object that we can set expectations on, and which will verify that the expected actions have indeed occurred. A stub is an object that you use in order to pass to the code under test. You can setup expectations on it, so it would act in certain ways, but those expectations will never be verified. A stub's properties will automatically behave like normal properties, and you can't set expectations on them.  If you want to verify the behavior of the code under test, you will use a mock with the appropriate expectation, and verify that. If you want just to pass a value that may need to act in a certain way, but isn't the focus of this test, you will use a stub.  IMPORTANT: A stub will never cause a test to fail. Huawei E3272 Ubuntu I have a problem with Huawei E3272 modem. Ubuntu 14.04 does not detect it as a mobile broadband modem. If I put my SIM card into my old modem(E3131) everything works, E3272 is just not detected. I have checked lsusb, and here I can see the system detects it as  12d1:14fe Huawei Technologies Co., Ltd.  but I cannot choose it in my Network Connections.  I tried adding usbserial vendor=0x12d1 product=0x14fe in /etc/modules, but it did not change anything. The same goes for adding   # Huawei E3272  ATTR{idVendor}=="12d1", ATTR{idProduct}=="157c", RUN +="usb_modeswitch '%b/%k'"  in /lib/udev/rules.d/40-usb_modeswitch.rules. I also tried installing Modem Manager, but it did not help.  Have you got any idea what could I do, so my system would detect my new modem? I would like to be able to use LTE. :) LM317 Heat sink, how big? Keyboard layout/language changes in every boot How to show $(a^b)^c=a^{bc}$ for arbitrary cardinal numbers? One of the basic (and frequently used) properties of cardinal exponentiation is that $(a^b)^c=a^{bc}$.  What is the proof of this fact?  As Arturo pointed out in his comment, in computer science this is called . Should operator<< be implemented as a friend or as a member function? That's basically the question, is there a "right" way to implement operator&lt;&lt; ? Reading  I can see that something like:  friend bool operator&lt;&lt;(obj const&amp; lhs, obj const&amp; rhs);   is preferred to something like   ostream&amp; operator&lt;&lt;(obj const&amp; rhs);   But I can't quite see why should I use one or the other.  My personal case is:  friend ostream &amp; operator&lt;&lt;(ostream &amp;os, const Paragraph&amp; p) {     return os &lt;&lt; p.to_str(); }   But I could probably do:  ostream &amp; operator&lt;&lt;(ostream &amp;os) {     return os &lt;&lt; paragraph; }   What rationale should I base this decision on?  Note:   Paragraph::to_str = (return paragraph)    where paragraph's a string. Let $W^{1,p}(U)$ be the Sobolev space. Suppose that $U$ is connected bounded domain in $\mathbb{R}^n$ and $u \in W^{1,p}(U)$ satisfies $Du=0$ a.e. in $U$. How can I prove that $u$ is constant a.e. in $U$? How do you collect valuable information from regular "User Group" meetings? The scenario:   You work on a SaAS application.  Some of your users use your application heavily each day. Every 2 weeks (or month or whatever), a "User Group" of your most avid users gets together to talk shop and see how their progress is going in the context of a larger project they are working on. You can participate and potentially direct some group activities aimed at improving the application.   How do you gather valuable user information in this group setting? resolv.conf not working properly with Ethernet in Ubuntu I have a Lenovo X200 laptop on which I am running Ubuntu 9.10. Recently, (I assume after some updating, but I really don't know) my ethernet port stopped working under Linux. A more tech/linux-savvy friend of mine was able to temporarily fix the problem by manually editing the resolv.conf file with the DNS servers he found through his connection. However, after rebooting, the problem came back and now I am looking for a more permanent solution. grub2 wont load Ubuntu on dual boot Having an issue with a dual boot setup that is being run from Windows boot manager. I can successfully load into grub2 from Windows boot manager, however it seems that grub2 cannot locate to the Kernel or something, as it is dropping me right into a shell with the following output:  [ Minimal BASH-like line editing is suported. For the First word, TAB list the posible command completion. Anywhere else tab list the posible completions of a device/filename,]  grub&gt;   I have used Easybcd in the past and successfully loaded a Linux grub through Windows bootloader, but this was when both operating systems were on the same partition, but since my OS installationd are now on separate partitions it seems EasyBcd cant work its magic.   ONe encouragement is that I was able to get Easybcd to load a working Grub if I used the Neo grub bootloader and edit the confg with:  title Ubuntu 14.04 find --set-root /boot/vmlinuz-3.19.0-61-generic kernel /boot/vmlinuz-3.19.0-61-generic ro root=/dev/sdc initrd /boot/initrd.img-3.19.0-61-generic   however this loads in Grub4DOS which is very slow, and as of today this method stopped working See  for details).  Here is the output of EasyBcd Settings for all the different methods I have tried for the Ubuntu 14.04 installation:  Default: Windows 7 Timeout: 30 seconds Boot Drive: C:\  Entry #1 Name: Windows 7 BCD ID: {current} Drive: C:\ Bootloader Path: \Windows\system32\winload.exe  Entry #2 Name: Ubuntu 14.04 Legacy BCD ID: {a4f127cf-3150-11e6-8aaf-408d5cb9e442} Drive: C:\ Bootloader Path: \NST\nst_linux.mbr  Entry #3 Name: Ubuntu 14.04 Grub2 BCD ID: {a4f127d0-3150-11e6-8aaf-408d5cb9e442} Drive: C:\ Bootloader Path: \NST\AutoNeoGrub0.mbr  Entry #4 Name: Ubuntu Neo Grub BCD ID: {a4f127d1-3150-11e6-8aaf-408d5cb9e442} Drive: C:\ Bootloader Path: \NST\NeoGrub.mbr   *It seems that none of the paths seen above are pointing to my dev/sdc2/ partition which would be considered DISK1 on my Windows Volume manager.     EDIT - In the confusion of attempting to get a working bootmanager for Ubuntu/Windows, you will see that grub and Windows Boot manager have all been installed in numerous locations. Below are the present locations and contents of all bootmanager &amp; Grub installations:  dev/sdb Windows7 drive  /dev/sdb1 - 512 MB fat32 partition which is currently empty  /dev/sdb2 - 110 GB ntfs partition containing Windows7 installation. This partition contains a 'Boot' folder which contains BCD files and a whole wack of langauage folders. THIS IS The FOLDER THAT WINDOWS BOOTLOADER uses.  /dev/sdb4 - 121.53 ntfs partition containing storage for media    dev/sdd - Ubuntu Drive  dev/sdd1 - 512 MB partition containing a 'EFI' folder, inside of which are two folders 'grub' and 'Ubuntu', both of which contain the exact same files (grub.cfg, grubx64.efi, MokManager.efi, shium64.efi)  dev/sdd2 - 48.83 GB ext4 partition that contains the '/' folder and Ubuntu instalation.   dev/sdd3 - 69.91 GB ntfs partition containing storage for media  what do I need to do so that the Grub shell I am being dropped into will load up Ubuntu? HOw can I get this machine to dual boot out of Grub? On a plane, there are two congruent circles, $O_1, O_2$. One line intersects $O _1$ and A and B and $O_2$ at C and D such that AB=BC=CD=14. Another line intersects $O _1$ and E and F and $O_2$ at G and H such that EF=FG=GH=6. What is the radius of the circle.   The configuration was hard to draw. . You can reflect the lines over the centers, but it is still the same. Cleary, $\widehat{AB}=\widehat{CD}$ and $\widehat{EF}=\widehat{GH}$. I have no idea how to proceed. I have an android phone to which i have plugged an earphones. So at the top of the phone, I get the headphone symbol which indicates that the earphone is connected (In other words, the circuit at the 3.5 mm jack is closed).   Then I cut the two earphones (transducers) from it, and still the headphone symbol shows. When I later cut this cable, below where it branches out, even then it shows circuit completion.   So my question is this:   How does the phone detect circuit completion at the 3.5 mm jack and thus trigger all sound and music to be directed through the 3.5mm jack? Testing whether two regression coefficients are significantly different (in R ideally) If this is a duplicate question, please point to the right way, but the similar questions I've found here haven't been sufficiently similar. Suppose I estimate the model $$Y=\alpha + \beta X + u$$  and find that $\beta&gt;0$. However, it turns out that $X=X_1+X_2$, and I suspect $\partial Y/\partial X_1 \ne \partial Y / \partial X_2$, and in particular, that $\partial Y/\partial X_1 &gt; \partial Y / \partial X_2$. So I estimate the model $$Y=\alpha + \beta_1 X_1 + \beta_2 X_2 +u$$and find significant evidence for $\beta_1,\beta_2&gt;0$. How can I then test whether $\beta_1&gt; \beta_2$? I considered running another regression $$Y=\alpha +\gamma(X_1 - X_2) + u$$ And testing whether $\gamma&gt;0$. Is this the best way?  Also, I need to generalize the answer to many variables, i.e. suppose we have $$Y=\alpha + \beta^1 X^1 + \beta ^2 X^2 + \dots + \beta^nX^n + u$$ where for each $j=1,\dots,n$, $X^j=X_1^j+X_2^j$, and I would like to test for each $j$ whether $\partial Y/\partial X_1^j \ne \partial Y / \partial X_2^j$.  By the way, I am primarily working in R. I'm learning calculus, specifically derivatives and applications of MVT, and need help with the following exercice:      Show that, for all $n &gt; 1$ $$\frac{1}{n + 1} &lt; \log(1 + \frac1n) &lt; \frac1n.$$    I tried to follow the below steps in order to prove the RHS inequality:   Proving that $f &lt; g$ on $I$ from $a$ to $b$:  Step $1$. Prove that $f' &lt; g'$ on $\operatorname{Int}(I)$.  Step $2$. Show that $f(a) \leq g(a)$ or that $f(a^+) \leq g(a^+)$  Following the above steps, let $f(x) = \log(1 + \frac1x)$ and $g(x) = \frac1x$, for all $x &gt; 1$. One has  $$f'(x) =  -\frac{1}{x^2 + x} \quad \text{and} \quad g'(x) = -\frac{1}{x^2}.$$  We note that, for every $x &gt; 1$, $f'(x) &gt; g'(x)$. Moreover, $f(1^+) = \log(2) &lt; 1 = g(1^+).$   My problem is that I got the wrong inequality sign in Step $1$.    Looking at the solution in my textbook, the author suggests using the MVT but I don't know how to apply it in this case.