/etc/apt/sources.list Permissions?
How do I restore the sources.list file?
Can black holes actually merge? If time stops at the event horizon, can we ever detect two black holes merging? In other words, if you are a short distance away, would you encounter a spherically symmetric gravitational field, or a dipole field? What is the official name for the "special" ASP.NET tags like this:  &lt;%# %&gt; &lt;%= %&gt; &lt;%@ %&gt; &lt;%$ %&gt;   I can't seem to figure out the conceptual or well known name for these, so I'm having trouble searching for more info.  As a bonus, can anyone give me a quick rundown of all of the possible "special tags" and what each one of them does (or point me to a resource)? Prove that if $k\mid m,$ then $Z_m$ has a subgroup of order $k.$ Prove that if $k\mid m$, then $Z_m$ has a subgroup of order $k.$  Ok, so this doesn't look like too hard of a problem. So do I just show that it is closed under multiplication and inverse? I just don't know how I would do that with a problem like this. Can anyone help me get started please? Finding the Mobius Transformation that maps open unit disk onto itself Hi I am trying to find all the Mobius transformations that map unit open disk onto itself i.e., if $|z|&lt;1$ then $|f(z)|&lt;1$ where $f(z)=\frac{az+b}{cz+d}$. I did so far \begin{align*} &amp;\Big|\frac{az+b}{cz+d}\Big|&lt;1\\ \Rightarrow &amp; |az+b|&lt;|cz+d|\\ \Rightarrow &amp; |az+b|^2&lt;|cz+d|^2\\ \Rightarrow &amp; |az+b||\bar{a}\bar{z}+\bar{b}|&lt;|cz+d||\bar{c}\bar{z}+\bar{d}|\\ \Rightarrow &amp; |a|^2|z|^2+|b|^2+2\text{Re }(az\bar{b})&lt;|c|^2|z|^2+|d|^2+2\text{Re }(cz\bar{d}) \end{align*}   After that I am stuck. Can anyone help me. I would be obliged...Thanks in advance. I've got a Canon EOS camera that takes a BP-511 battery.  The specs on the battery are 7.4V and 1100mAh.  The battery is dead and I want to buy a replacement, but I'm looking at a third party battery, because it's much cheaper than the Canon branded one.  I've found one with very high reviews online, and it seems identical except that it's rated at 1500mAh instead of 1100mAh.  So the question is: is this difference important to the camera or not? What is the proper way to extend the Fibonacci numbers to negative numbers? I have a feeling this question is a duplicate, but it's not coming up in "Questions that may already have your answer."  We all know very well that $F(0) = 0$, $F(1) = 1$ and $F(n) = F(n - 2) + F(n - 1)$ for all $n &gt; 1$.  I'm wondering about $n &lt; 0$. My first thought was $F(-n) = -F(n)$, which is appealing from a multiplicative point of view, as it seems to preserve certain identities, like $F(n)^2 = F(n - 1) F(n + 1) - (-1)^n$.  But it doesn't quite make sense from an additive point of view, it doesn't seem to work both "forwards" and "backwards." For example, it would give us $F(-1) + F(0) = -1 \neq F(1)$.  How do we extend $F(n)$ to negative $n$ so as to maintain both the related identities and the basic defining identity? In the following code I loop through a map and test if an element needs to be erased.  Is it safe to erase the element and keep iterating or do I need to collect the keys in another container and do a second loop to call the erase()?  map&lt;string, SerialdMsg::SerialFunction_t&gt;::iterator pm_it; for (pm_it = port_map.begin(); pm_it != port_map.end(); pm_it++) {     if (pm_it-&gt;second == delete_this_id) {         port_map.erase(pm_it-&gt;first);     } }   UPDATE: Of course, I then  which I didn't think would be related but answers my question. Parsing input with scanf in C I've been having a lot of problems trying to figure out how to use scanf().  It seems to work fine with integers, being fairly straight forward scanf("%d", &amp;i).  Where I am running into issues is using scanf() in loops trying to read input.  For example:  do {   printf("counter: %d: ", counter);   scanf("%c %c%d", &amp;command, &amp;prefix, &amp;input); } while (command != 'q');    When I enter in a validly structured input like c P101, it seems to loop again before prompting me.  This seems to happen even with a single:  scanf("%c", &amp;c)    in a while loop.  It'll do the loop twice before prompting me again.  What is making it loop twice, and how do I stop it? When I enter in less amount of input that programmatically wouldn't have another character or number such as q, pressing enter seems to prompt me to enter more.  How do I get scanf() to process both single and double character entries? Select all contents of textbox when it receives focus (Vanilla JS or jQuery) What is a Vanilla JS or jQuery solution that will select all of the contents of a textbox when the textbox receives focus? Is there a quick way to lock my Mac? On Windows you can hit the keys Windows+L to lock your computer if you step away.  I don't want to have to remember if sleep currently asks for a password, if there is a time out where my screen isn't locked during a short period. Even with Lion supporting resume of many apps, I don't want to log out.   I do want to be sure data is secure when stepping out for lunch or a smoke break. Whenever a question pops up on SO about Java synchronization, some people are very eager to point out that synchronized(this) should be avoided. Instead, they claim, a lock on a private reference is to be preferred.  Some of the given reasons are:    (very popular this one, also has an "accidentally" variant)     Other people, including me, argue that synchronized(this) is an idiom that is used a lot (also in Java libraries), is safe and well understood. It should not be avoided because you have a bug and you don't have a clue of what is going on in your multithreaded program. In other words: if it is applicable, then use it.  I am interested in seeing some real-world examples (no foobar stuff) where avoiding a lock on this is preferable when synchronized(this) would also do the job.  Therefore: should you always avoid synchronized(this) and replace it with a lock on a private reference?      Some further info (updated as answers are given):   we are talking about instance synchronization both implicit (synchronized methods) and explicit form of synchronized(this) are considered if you quote Bloch or other authorities on the subject, don't leave out the parts you don't like (e.g. Effective Java, item on Thread Safety: Typically it is the lock on the instance itself, but there are exceptions.) if you need granularity in your locking other than synchronized(this) provides, then synchronized(this) is not applicable so that's not the issue Does "FN" stand for or mean anything? Each time I open any application, it zooms to fill-up the entire desktop and so I am always forced to use "Unmaximize" using Alt+Space menu. Proof that any self-complementary graph has to have $4k$ or $4k+1$ vertex, for some $k \in \mathbb N$ Prove that $\sqrt{n} \le \sum_{k=1}^n \frac{1}{\sqrt{k}} \le 2 \sqrt{n} - 1$ is true for $n \in \mathbb{N}^{\ge 1}$ Prove that $r \in R$ such that $0 \lt r \lt 1$ then $\frac{1}{r (1 - r)} \ge 4$ What's the difference between \usepackage{ngerman} and \usepackage[ngerman]{babel} Is there something additional that \usepackage[ngerman]{babel} does that a simple \usepackage{ngerman} won't be able to do? For linear regression, we can check the diagnostic plots (residuals plots, Normal QQ plots, etc) to check if the assumptions of linear regression are violated.  For logistic regression, I am having trouble finding resources that explain how to diagnose the logistic regression model fit. Digging up some course notes for GLM, it simply states that checking the residuals is not helpful for performing diagnosis for a logistic regression fit.  Looking around the internet, there also seems to be various "diagnosis" procedures, such as checking the model deviance and performing chi-squared tests, but other sources state that this is inappropriate, and that you should perform a Hosmer-Lemeshow goodness of fit test. Then I find other sources that state that this test may be highly dependent on the actual groupings and cut-off values (may not be reliable).  So how should one diagnose the logistic regression fit? As a new user on a site, what's the best way to ensure your question will be positively received? Trisect a quadrilateral into a $9$-grid; the middle has $1/9$ the area