When I try to open this activity I'm getting nullpointerexception
What is a NullPointerException, and how do I fix it?
How can one cite theorems of references? I would like to cite the item (iii) of proposition 3 of a reference. How can I do it? Which ways do you suggest? This is a question not only about LaTeX, but also about style. One option I have seen to cite pages of a reference is \cite[p.~333]{Audet}. Is there something similar to cite theorems, propositions and so on? How do I use reflection to call a generic method? Short story, domed refuges Why are angles dimensionless and quantities such as length not? So my friend asked me why angles are dimensionless, to which I replied that it's because they can be expressed as the ratio of two quantities -- lengths.   Ok so far, so good.  Then came the question: "In that sense even length is a ratio. Of length of given thing by length of 1 metre. So are lengths dimensionless?".   This confused me a bit, I didn't really have a good answer to give to that. His argument certainly seems to be valid, although I'm pretty sure I'm missing something crucial here. Unwanted faint lines connecting dashed double line I've been learning C#, and I'm trying to understand lambdas.  In this sample below, it prints out 10 ten times.  class Program {     delegate void Action();     static void Main(string[] args)     {         List&lt;Action&gt; actions = new List&lt;Action&gt;();          for (int i = 0; i &lt; 10; ++i )             actions.Add(()=&gt;Console.WriteLine(i));          foreach (Action a in actions)             a();     } }   Obviously, the generated class behind the lambda is storing a reference or pointer to the int i variable, and is assigning a new value to the same reference every time the loop iterates.  Is there a way to force the lamda to grab a copy instead, like the C++0x syntax  [&amp;](){ ... } // Capture by reference   vs.  [=](){ ... } // Capture copies Raspberry Pi + IRFZ44N to control a 5050 RGB LED strip: current seems to be limited How to prove if $n$ is prime and $n | a^2$ then $n | a$? My professor assigned this for homework but I don't understand how to connect the dots. He suggested using the fact that $\gcd (x,y) \cdot \operatorname{lcm} (x,y) = xy$ but I'm not sure how that's relevant. How to change specific values in a List Calculating de Broglie wavelength How is the ability of a person who gained physical access to your computer to get root with those steps in any way secure?   When grub2 menu opens press  e  to edit the Linux start options Change:   "linux   /vmlinuz-2.6.35-23-generic root=UUID=e7f1e48d-0015-485f-be7d-836217a31312 ro   quiet splash"    to:  "linux   /vmlinuz-2.6.35-23-generic root=UUID=e7f1e48d-0015-485f-be7d-836217a31312 ro init=/bin/bash"  Now you have root access just do:   mount -o remount,rw / passwd user mount -o remount,ro /  sync  reboot the computer and you win.   My question is, how is being able to break into any Linux machine through grub2 secure? I don't understand that fact about Linux, thank you for your answers. Is it possible to generate transcendental numbers of arbitrary irrationality measure ? I am thinking of something like an infinite series which depends on a parameter fixing the irrationality measure.   All I have found so far is bounds on the irrationality measure of famous numbers, I have not been able to find something going the other way around. Can we have a red/status tag for "obsolete"? Given the changes that have been rolled out on Meta for the new close hold reasons, we've lost "too localized" here on meta.  However, "too localized" has a genuine use on meta (which is part change management system) to indicate those things which are truly obsolete (as evidenced by the text of the old close reason "refers to a specific point in time").  That said, there are things that are obsolete on meta, but we have no way of indicating that other than an "off topic" closure, which really sends the wrong message.  To mitigate that, I'd like a status-obsolete tag for meta sites which meta diamonds can apply to posts that have been made obsolete by other changes that were made to the system afterward. I've read a lot of conflicting advice on whether it's advisable to store eggs in the refrigerator. The case against seems twofold: (1) that eggshells are porous, and eggs can take on unwelcome flavours, or even spoil faster in the fridge, and (2) that cooking with cold eggs can be problematic. But lots of people refrigerate eggs through perceived necessity.  Do eggs last longer in the fridge, and is the shelf life gain worth it? Or should I just stick them in the cupboard? I need some help in relation to this exercise    "Prove that if a normal subgroup $H$ of $ G$ has index $n$, then $g^n \in H$ for all $g \in G$."  I'm not allowed to use quotient groups in the proof, because the exercise is in the chapter before.   I tried by induction on $n$. The case $n=1,n=2$ are obvious, but even  the case $n=3$ is giving me trouble so I give up studying the general case of the inductive step.  My other approach was studying left or right coset of $G$. But I only proved that if $g \in aH$ then $g^2 \notin aH$ if $a \notin H$, and I can't find a way to demonstrate that $g^n \in H$. (my starting idea was to prove that every power of $g$ is in a different coset but then I realize that in this way I don't handle several case, for example $g$ has period strictly lesser than $n$ and in conclusion it doesn't prove the exercise)  Maybe I'm missing something about indexes, and this is why I asked here for some help,  (I can't use quotient groups because they are introduced later than this exercise, forgot to add this info in the beginning) Thanks in advance :) There are several algorithms to calculate distance between two latitude/longitude points.     Hubeny     But there are not so many algorithms (actually I can't finally find it) which can calculate latitude/longitude of point B from another latitude/longitude point A with bearing and distance.      I'm glad to know if there are any algorithms which is lesser accurate, but faster than Vincenty's formulae. We add negative numbers and zero to natural sequence to make it closed under subtraction, the same thing happens with division (rational numbers) and root of -1 (complex numbers).  Why this trick isn't performed with division by zero? During U-substitution, is the $du$ that we use as the infinitesimal in $\int f(u) \ du$ changing size? I am familiar and comfortable with the following definition of an integral (defined as the the Riemann Sum where n approaches infinity).    I have circled in red two terms to emphasize a concept that I was taught: the $\frac{b-a}{n}$ term effectively becomes the $dx$ term. This intuitively makes sense to me.   Additionally, I am familiar with the following statement (as expressed here: )    However, I have boxed in red something that I find rather confusing when trying to interpret it through the understanding that $\frac{b-a}{n}$ sorta kinda equals $dx$...  Specifically, the $du$ value appears to be changing throughout the interval $[ g(a), g(b)]$. To reframe this in the "Riemann Sum" version,we would have something like: $\text{ "value that varies depending on where you are"} * \frac{g(b)-g(a)}{n}$  This is very confusing to me...because, if this is the correct interpretation, I'm not quite certain I understand why this is allowed. Log4Net "Could not find schema information" messages How to get over "device or resource busy"? I tried to rm -rf a folder, and got "device or resource busy".  In Windows, I would have used LockHunter to resolve this. What's the linux equivalent? (Please give as answer a simple "unlock this" method, and not complete articles like . Although they're useful, I'm currently interested in just ASimpleMethodThatWorks™)