Why is there a NullPointerException in the output?
What is a NullPointerException, and how do I fix it?
Why is AES considered to be secure? The security of RSA is based on the integer factorization problem, which is a very well defined and understood mathematical problem. This problem must be solved in order to fundamentally break RSA.   What about AES (and others based on the same idea)? Why is it difficult to break? Is there any mathematical principle that ensures its security? Integration of elementary rational fractions $$\int{\frac{Mx+N}{x^2+px+q}}dx$$  When all the coefficients are given, it can be easily solved using substitution but I have no idea how to find a solution in a standard form.  What are the first steps? I just upgraded from Ubuntu 10.04 to Mint 16. The installer did not detect my existing RAID 1 array, and I can't access it anymore.  My drives are:  SDA - First RAID Disk SDB - Second RAID Disk SDC - Non-RAID where the OS is installed.   The disks are full of stuff I want to keep, so I don't want to create a new RAID.   On the old machine, the two raid drives became MD0, which was a physical disk for Logical Volume Management.  How do I put the two halves together and get it to mount at boot? (I know how to mount single drives and how to add fstab entries for single drives, but the part I don't know is how to mount the two drives as one disk as a RAID. I have downloaded and extracted the packet tracer file, then I used this command as root  chmod +x install   then I ran (as root)  ./install    Then I used this command   ./set_ptenv.sh   but whenever I run this command   packettracer    it just doesn't start Benefits & Practicality of Virtual Machines I have been living under a rock for the past 5 - 8 years, and am just now getting up to speed with virtualization, virtual machines and platforms like VMware or VirtualBox.  I think I am starting to get them, but there are two concepts about VMs that I'm just choking on for some reason; one has to do with their intrinsic benefit to the enterprise, and the other has to do with their practicality when being used.  So I understand that you can have multiple VMs running on the same physical machine, all doing their own "thing" as if they were separate machines. So let's say I want to make a nice little virtual network of servers in my home office so I can have a big sandbox to play inside of for all my projects. I set up 2 physical machines as servers, and on each of them I have, say, 4 virtual machines running (1 might be an app server, another might be a RDBMS, another might be a message broker, etc.).   Besides saving me money (as I only have to provide 2 physical machines instead of buying 8 servers), what instrinsic benefit/purpose do these VMs serve here? I don't care how clever virtual machines are, if my computer only has 8GB of RAM, and I have 4 VMs running on it, those VMs only get 8GB of RAM to share between them. Putting 4 VMs on my server doesn't endow it with 4 times that amount of RAM! So, as far as I see it, VMs save you money on hardware, but just end up bogging down that hardware as you have multiple machines competing over the same pool of resources. What am I missing here? Can VMs communicate with each other over your network using high-level protocols like TCP/IP, HTTP, FTP, etc.? Otherwise, it just doesn't make sense to implement all these VMs if you have to feed them separate and independent chunks of data/requests to process. Real valued analytic function defined on a connected set is constant lstlisting directly after section start removes margin before next section If I place an lstlisting directly after the start of a (sub)section, the margin before the following (sub)section (title) disappears.  Here is a an example document showing the issue:  \documentclass[twocolumn]{article} \usepackage{listings} \begin{document}  \section{Lorem ipsum} Consectetur adipiscing elit.  \begin{lstlisting} Lorem \end{lstlisting}  Sed quis ante nec mi volutpat congue eu id leo.  \section{Dolor sit amet} Suspendisse at lorem mattis, semper massa id, euismod risus.  \newpage  \section{Lorem ipsum} \begin{lstlisting} Lorem \end{lstlisting}  Sed quis ante nec mi volutpat congue eu id leo.  \section{Dolor sit amet} Suspendisse at lorem mattis, semper massa id, euismod risus.  \end{document}   And you can see the (annotated) result here:    Why is there no margin before the fourth section (title)? Is this a bug in my LaTeX environment (MacTeX-2013), or is this the expected behavior? Migration stats not consistent between sites Apparently, as a result of  it appears that Yannis and myself see different statistics when going to the programmers.SE.com migration stats tool and the stackoverflow.com tool respectively.    :        while I see:        Why are the rejection rates so radically different?  Which is correct?  (Or am I simply misinterpreting the data somehow and these are representing different statistics) Writing a git post-receive hook to deal with a specific branch How does one attack a two-time pad (i.e. one time pad with key reuse)? My question might appear the same as the question , but actually I did read all the answers and none of them helped me with the details I need.  I am new to cryptography and my problem is with two time pad attacks on OTP. The problem I had in my course was that I have 10 ciphertexts encrypted with the same key $K$. I am then given another ciphertext that I should decrypt.  I know that XOR-ing two ciphers gives me the XOR of their original messages.  My question is what is the correct thing to do after that?  I tried to take 3 ciphertexts $C_1, C_2$ and $C_3$.  Then get $S_1 = C_1 \oplus C_2  \oplus $' ', also get $S_2 = C_1 \oplus C_3 \oplus$ ' '.  After that I compared all corresponding characters in $S_1$ and $S_2$,  and if $S_1[i] = S_2[i]$ then I calculate $S_1[i] \oplus C_2[i]$ to get $K[i]$.  I tried this on paper before coding and it worked, but I might be missing something.  Is this the right approach? Why does it work? In the  a line is delivered at around 1:41 where Han Solo says "a 190 years old!? You look great" and I was wondering if there is a defined concept of a year in the Star Wars universe or would Han Solo just be familiar with the duration of a  solar orbit. How to prove $D^n/S^{n-1}\cong S^n$? In my textbook it is said that the quotient space $D^n/S^{n-1}$ is homeomorphic to $S^n$. I can imagine it for $n=2$, but fail to make a mathematical proof for any dimension.  Can anyone provide a rigorous proof? I am doing an analysis with 2 groups of patients (control and disease) whose blood pressures have been measured continuously for 24 hours. My first goal is to test the effect of GROUP on BP. In the beginning, I built the mixed effect model with TIME as random effect variable and group as fixed effect model. The model I used is listed below:  mod &lt;- lme(BP ~ factor(GROUP), random=~1+ TIME| ID, data=gdata)   However, the experimenter wants to know the effects of GROUP on the slope of blood pressure changing per hour. In another words, we need to have blood pressure slope, which is the change of blood pressure by time, as response variable in the model. And I generated another model as bellow:  mod &lt;- lme(BP ~ factor(GROUP)*TIME, random=~1+ TIME| ID , data=gdata)   However, in the new model, the time works both as random effect variable and fixed effect variable. I am not sure if this is allowed in the mixed effect model. Which rationals can be written as the sum of two rational squares? Which rational numbers can be written as the sum of two rational squares?  That is, for which rational numbers $a$, are there rational numbers $x$ and $y$ such that $a = x^2 + y^2$.  It is a famous theorem that if an integer can be written as the sum of two rational squares then it can be written as the sum of two integral squares, and then the solution is the famous one by Fermat, but I didn't find anything about the general case. Error while updating debug log trace flag I am trying to update the debug log trace flag for my user but getting the following error     Having an active trace flag triggers debug logging. You have 259 MB of the maximum 250 MB of debug logs. Before you can edit trace flags, delete some debug logs.   I have removed all existing debug logs but it is still throwing the error. Tag hierarchy to browse through tags Possible Duplicate:          Since so many thousands of tags and topics exist, it would be really useful to visually browse through tags by organizing them into a , or hierarchical map. This drill-down map would make it easier to find the tag/subtopic you want or just generally browse within the topics you're interested in.  So we could nest all "visual-studio-2008" and "visual-studio-2003" tags into "visual-studio", which would be under "dev-tools". Something like how Wikipedia's categories have sub-categories. How many elements does the ring $ℤ[X]/(X^2-3,2X-4)$ have? Describe the structure of this ring. I'm making exercises to prepare for my ring theory exam:     How many elements does the ring $ℤ[X]/(X^2-3,2X-4)$ have? Describe the   structure of the this ring.   I find it always difficult if the ideal is generated by two elements.   I thought about someting like this. $X=2$ and $X^2=3$. Therefore $4=3$ therefore $1=0$. As $2=1$ then $X=0$. And then everyting seems to become $0$.   I'm not sure if I'm allowed to see it this way, but this is the first thing that comes to my mind. Mapping Russia - Int'l dateline splitting polygon I am having the issue where the International Dateline is splitting a polygon I am working with.  I have a layer of the "Federal Districts" of Russia, and the eastern most federal district is being "cut" by the international dateline.   I currently have the data frame's coordinate system set to Asia Lambert Conformal Conic and this places the two "cut" polygons next to each other (how they should be).  I really need the polygon to not be cut and show as a continuous polygon.   I've researched up and down, tried changing the projection/coordinate system, but with no luck.  I've attached a screenshot of the one multi-part polygon that is being split by the international dateline.  I would like it so that there is no cutting of the polygon, and therefore no boundary line going through the polygon.  I've even tried transforming this multi-part polygon into separate, single-part polygons, then merge them back together, but anytime any part of any polygon crosses this line, it gets cut.  Layout View:    Data View: The Time That 2 Masses Will Collide Due To Newtonian Gravity My friend and I have been wracking our heads with this one for the past 3 hours... We have 2 point masses, $m$ and $M$ in a perfect world separated by radius r.  Starting from rest, they both begin to accelerate towards each other. So we have the gravitational force between them as:  $$F_g ~=~ G\frac{Mm}{r^2}$$  How do we find out at what time they will collide?  What we're having trouble with is this function being a function of $r$, but I have suspected it as actually a function of $t$ due to the units of $G$ being $N(m/kg)^2$.  I've tried taking a number of integrals, which haven't really yielded anything useful.  Any guidance?  No, this is not an actual homework problem, we're just 2 math/physics/computer people who are very bored at work :) C# Check if a decimal has more than 3 decimal places?