I have an old iPhone 4 that has been sitting unused since I got my new phone - it has now sold and I have to clean it out so it's ready for the next person.  Here's the problem:   The wifi doesn't work and the phone's inactivated (so I can't get online). It is asking for a restrictions passcode I don't ever remember setting.   I tried the default (0000) but no luck. What can I do to get this phone back to factory settings?
Is there a way to change the restrictions passcode on an iPhone without restoring the phone? I haven't forgotten the passocde; I just want to change it.
I'm trying to install the  IDE in my Ubuntu and in the process it's asking for my root password which I don't remember. I know my  user password. How can I recover my root password? I have a valid B1/B2 US visa expiring about a year from now. The passport it's in expired a few years ago, and I recently got a new one.   The passport number does not match, but the national ID number, mentioned in both, does (it used to be the same as your national ID, but not anymore)  Can I travel to the US carrying both passports? Or is it asking for trouble? (I read a few years ago that it was possible, but that might have changed)  Should I just get a new visa to avoid potential complications? Fact  The intersection of a closed set $A$ and a compact set $B$ is closed.  Background  I wanted to prove that the intersection of closed and compact sets is compact. There might be other ways, but I am not interested in them for now and am looking to prove the above statement.  Relevant Facts  I don't have much idea what to use. $A^c$ is open. Every cover $\cup_{i\ge1}U_i$ of $B$ contains a subcover $\cup_{i=1}^kU_i$. I might want to prove $A\cap B^c$ is open. Limit points may be another points to consider but I don't get any idea in that direction.  I also don't have that $B$ is closed otherwise it would be trivial.  Some Clarification  I am aware that compact sets are not closed in arbitrary topological spaces. The reason why I was motivated to believe tha intersection of closed and compact set is compact is true is: See Matt E.'s Answer      So if $K$ is compact in an arbitrary topological space (which is just to say that   it is a compact topological space when given its induced topology) and     $L$ is closed then $K \cap L$ is a closed subset of $K$ in its induced topology   Is it true if $A$ and $B$ are not whole spaces?  I would appreciate any hints. What is the yield keyword used for in C#? In the  question one of the answers had the following code snippet:  IEnumerable&lt;object&gt; FilteredList() {     foreach(object item in FullList)     {         if(IsItemInPartialList(item))             yield return item;     } }   What does the yield keyword do there?  I've seen it referenced in a couple places, and one other question, but I haven't quite figured out what it actually does.  I'm used to thinking of yield in the sense of one thread yielding to another, but that doesn't seem relevant here. This question is inspired by my own answer to a  which I tried to answer and got stuck at one point.     The question was:     HI DARLING.      USE MY ATM CARD, TAKE ANY AMOUNT OUT, GO SHOPPING AND TAKE YOUR FRIENDS FOR LUNCH.      PIN CODE: $\displaystyle \int_{0}^{1} \frac{3x^3 - x^2 + 2x - 4}{\sqrt{x^2 - 3x + 2}} \, dx $      I LOVE YOU HONEY.      Anyone knows? Are we gonna get an integer number?     My attempt:  Does this help?     $$\frac{3x^3-x^2+2x-4}{x-1}=3x^2+2x+4$$   (long division)   \begin{align*} I&amp;=\int\frac{3x^3-x^2+2x-4}{[(x-1)(x-2)]^{1/2}} dx = \\ &amp;=\int\frac{(3x^2+2x+4)(x-1)^{1/2}}{(x-2)^{1/2}} dx = \\ &amp;=\int 3(u^4-4u^2-4)(u^2+1)^{1/2}du \times 2 \end{align*}   after the substitution   \begin{gather*} (x-2)^{1/2}=u\\ du=\frac1{2(x-2)^{1/2}}dx\\ u^2=x-2\\ (x-1)^{1/2}=(u^2+1)^{1/2} \end{gather*}   Update:  may help us proceed.    I tried to proceed:     $$6\int (u^4-4u^2-4)(u^2+1)^{1/2} du = 6\int ((t-3)^2-8)t \frac{dt}{2u}$$   after $u^2+1=t$ and $dt=2udu$   \begin{align*} u^4-4u^2-4 &amp;= (u^2+1)^2-(6u^2+5) \\ &amp;= (u^2+1)^2-6(u^2+1)+1 \\ &amp;= ((u^2+1)-3)^2-8 \end{align*}   I wonder whether this question can be solved from here?    Update:   This has been getting a lot of views, and I think most people came for the sort of problem mentioned in the title (where I got stuck) rather than the original problem itself.  Keepin this in mind, I'm reopening the question and here's the kind of answers I expect — Solutions to the original problem are good, but I'd prefer solutions that continue from the part where I got stuck — the polynomial in $u$ — that's the sort of problem mentioned in the title. I want to prove that $\lim_{(x,y)\to (0,0)} \frac{\sin(x+y)}{x+y} = 1$  Is it sufficient to say that if $u = x+y$ then as $(x,y) \rightarrow (0,0)$ then $u \rightarrow 0$ and then evaluate $\lim_{u\to 0} \frac{\sin u}{u}$ which is 1 by L'Hôpital's rule?  I feel that this is somehow "cheating" and I would really like to prove it in a more rigorous way, i.e., by using an $\epsilon$ - $\delta$ argument.  I have tried for a long time to use the one variable version and prove that if $0 \lt \sqrt{x^2 + y^2} \lt \delta$ then somehow $0 \lt |x+y| \lt \delta$ and it would follow from the one variable case that $| \frac{\sin(x+y)}{x+y} - 1| \lt \epsilon$. However, because $|x+y| = 0$ when $y = -x$ I am not getting anywhere with this.  Any input on this would be helpful :) Mean absolute error OR root mean squared error? Why use Root Mean Squared Error (RMSE) instead of Mean Absolute Error (MAE)??    Hi  I've been investigating the error generated in a calculation - I initially calculated the error as a Root Mean Normalised Squared Error.  Looking a little closer, I see the effects of squaring the error gives more weight to larger errors than smaller ones, skewing the error estimate towards the odd outlier. This is quite obvious in retrospect.   So my question - in what instance would the Root Mean Squared Error be a more appropriate measure of error than the Mean Absolute Error? The latter seems more appropriate to me or am I missing something?  To illustrate this I have attached an example below:                    The scatter plot shows two variables with a good correlation,  the two histograms to the right chart the error between Y(observed ) and Y(predicted) using normalised RMSE (top) and MAE (bottom).     There are no significant outliers in this data and MAE gives a lower error than RMSE. Is there any rational, other than MAE being preferable, for using one measure of error over the other? Enumerated description list How can a list be created, whose items are both    enumerated (a,b,c,d,...)  and allow for setting a description via \item[texthere] ?   Im am using MikTex on Windows. Switch statement fallthrough in C#? Installing other fonts on wine How can I edit this trigger to only fire once? How can I prevent data theft from a company?  Are there any solutions to this problem?  Is there any software that encrypts important data (.doc, .xls, etc.) and prevents it from copying?  I need a solution where employees can read/write documents but can not copy then to a USB drive or e-mail them. Zero to the zero power – is $0^0=1$? Could someone provide me with a good explanation of why $0^0=1$?  My train of thought:  $x&gt;0$  $0^x=0^{x-0}=0^x/0^0$, so  $0^0=0^x/0^x=\,?$  Possible answers:   $0^0\cdot0^x=1\cdot0^0$, so $0^0=1$ $0^0=0^x/0^x=0/0$, which is undefined   PS. I've read the  on mathforum.org, but it isn't clear to me. Follow a question I've come across a few interesting questions over the last few months on Stack Overflow. They're not necessarily questions I need an answer to, but the questions are nonetheless very interesting, and now I want to know what the answers are.  I don't want to add this question to my favourites because I have my really handy posts in there.   Ideally I would like to have another kind of shortlist to which I can add these posts. I would get notifications just as if it were my own post, without having these as favourites.  What do you think of that? It would be available only for questions, and you would receive notifications just like the author, for new answers and comments on the question (not comments on the answers).  You could browse the posts you're following, as well as stop following them.   The thing is sometimes I come across a semi interesting question and upvote and maybe even comment, but I never hear about it again, because sometimes it's just not interesting enough to remember.  Like I said in the comments, I don't feel like the favourite list is really for this. I want in-your-face red dot notifications for new answers on brand new interesting questions I find. The favourites is for that post with an answer that you know you'll need again. Find a real function $f:\mathbb{R}\to\mathbb{R}$ such that $f(f(x)) = -x$? Is there an optimal loss function for dealing with imbalanced classes? I'm aware that there are many ways of dealing with datasets where there is a strong class imbalance in the target variable: downsampling the more prevalent and less important class, over-weighting the valuable minority class, etc. Are there any loss functions that are ideally suited for these types of problems? I've heard of AUC, but AUC sounds like a means of evaluating a model once its training is complete (correct me if I'm wrong). Log loss and squared loss penalize a model's over confidence, but overconfidence and class imbalance seem unrelated. Is that true? Deleting Lines Containing a Specified String in Different Files Suppose there are two files in web/ named foo.php and bar.php. The 1st line of foo.php is "sdajgeSTRINGdsad" and the 10th line of bar.php is "gdfhu98324STRING". The task is to first locate these two files, then delete 1st line of foo.php and 10th line of bar.php given that the specified string is "STRING". sql server 2008 management studio not checking the syntax of my query Why are packages in the official Ubuntu repositories older than the latest (upstream) versions from Debian Sid, PPAs, the authors, etc.? I've been wondering this for a while, why is there free space at the end of a drive that was formatted to NTFS in windows?  You can only see the unallocated space in Linux. So why is it there? Does it serve a purpose?