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Xpath expression with multiple predicates How to define a bijection between $(0,1)$ and $(0,1]$?   Or any other open and closed intervals?   If the intervals are both open like $(-1,2)\text{ and }(-5,4)$ I do a cheap trick (don't know if that's how you're supposed to do it): I make a function $f : (-1, 2)\rightarrow (-5, 4)$ of the form $f(x)=mx+b$ by \begin{align*} -5 = f(-1) &amp;= m(-1)+b \\ 4 = f(2) &amp;= m(2) + b \end{align*} Solving for $m$ and $b$ I find $m=3\text{ and }b=-2$ so then $f(x)=3x-2.$  Then I show that $f$ is a bijection by showing that it is injective and surjective. Recently I have installed an older version of Ubuntu on my old machine. Whenever I try to install any software, I get an error saying it couldn't be found:  $ sudo apt-get install vlc Reading package lists... Done                Building dependency tree        Reading state information... Done     E: Couldn't find package vlc Someone wants to send me cash by DHL. How's this scam supposed to work? This raises a bunch of red flags (also confirmed by googling the email address it came from), but I don't quite understand how this scam is actually supposed to work. So, here it goes:   I put up an ad (for a guitar, 90€) on LeBonCoin (the French equivalent of Craigslist). Someone contacts me by text message (anonymous with no actual phone number), saying they want to continue by email. I then get an email, with roughly the following content (rough translation):     OK, I will buy it, but I need to send you cash by "express DHL letter". The DHL driver will pick up the item when he gives you the cash. I need the following information: full name, postal address, price of the item, and your phone number.    I'm guessing that this works by them requesting some "delivery fee" in advance, but does anyone know how this actually works? Points of a Measure Zero Sets Covered by Intervals Infinitely Many Times Given a measure zero set $E$, by definition we have forall $\varepsilon &gt; 0$, a covering of $E$ by intervals whose lengths sum to $&lt; \varepsilon$.  I want to prove that we can cover $E$ in intervals such that the sum of the lengths the intervals is $&lt; 1$ and each point in $E$ is contained in infinitely many of the intervals.  Do you know how to prove this? Thank you We know that $221 = 17*13$. So we can check if the system has roots to both of those equations separately, which it does:  $x^{5} \equiv 2$ mod $13$ has the solution $6 + 13n$ and $x^{5} \equiv 2$ mod $17$ has the solution $15 + 17n$.   I got these numbers from wolfram, I have no idea how to solve this problem WITHOUT a calculator. And even after finding these numbers. How would one obtain a solution modulo $221$? I was thinking the Chinese Remainder Theorem but I am under the assumption that CRT only applies to problems with powers of $x$ which are $1$.  Thanks. Hill cipher, unknown letter value I've been struggling on this problem for a while now : the Hill cipher is well-known to be vulnerable to known-plaintext attack due to its linearity. Given a key matrix $K$ of size $n\times n$, one can retrieve the key with as little as $n^2$ plaintext/cipher text couples.  All examples found on the Internet assume an alphabet where $A=0, B=1 \dots Z=25$, but how could we break the Hill cipher using a know-plaintext attack without any clue over the alphabet permutation nor the key matrix ?  I've tried several things such as  the whole set of equations together, however it requires several divisions in $\bf{Z}_{26}$ which aren't possible (i.e. division by even numbers or 13) Double orthogonal complement is equal to topological closure How do question bookmarks work? What does bookmarking a question mean? How do you bookmark a question? Do users get updates on their bookmarked questions? Are my bookmarks publicly visible? How do you see your own bookmarks? How do you see other users' bookmarks? How do you see which users have bookmarked a particular question? What impact does the bookmark system have on the voting system? Can I &quot;un-bookmark&quot; a question? Can I search within my bookmarks? Intellisense in IntelliJ no longer working Syntactic characterisation of the intersection of CTL and LTL PDF of product of variables Could anyone please indicate a general strategy (if there is any) to get the PDF (or CDF) of the  product of two random variables, each having known distributions and limits?   My particular need is the following: Let $w :=u \cdot v$. The PDF of $u$ is $$\frac{1}{\pi u}\frac{1}{\sqrt{u^2-0.25}},$$ for $z&gt;0.5$ and the PDF of $v$ is $$\exp\left(-\frac{v}{v_0}\right),$$ for $v_1&lt;v&lt;v_2$. What's the PDF of $w$? How can I configure the Ubuntu Dock to show windows only from the current workspace? I really get confused when to use say and when to use tell. Which is appropriate in the following:   What did he tell? / What did he say? What are you saying? / What are you telling? Why is everything floating in space so cold? Of course there is pretty hot stuff too. For example Suns.  But isn't the vacuum of space the perfect containment for heat? And shouldn't the rare collision with particles even heat up an object that floats around in space?  Why aren't they hot from time to time? Is it because they loose their heat by radiant heat?  And if so does radiant heat emission always occur until the object is the same temperature as the surrounding or does it depend on other factors? How do I manage (add/remove) all the startup applications in Ubuntu 11.10 and up?  Startup Applications doesn't show all the applications which are started during boot. I am a EE grad. student who has had one undergraduate course in real analysis (which was pretty much the only pure math course that I have ever done). I would like to do a self study of some basic functional analysis so that I can be better prepared to take a graduate course in that material in my university. I plan to do that next fall so I do have some time to work through a book fully. Could some one recommend some good books to start working on this?   Thanks in advance Is there a way to do an "upside down" \widehat? I've come across this in a book I'm looking through, but I can't find the code for it. I want it to look exactly like the \widehat and go above text like \widehat does...I just want it to be (vertically) flipped! nvidia-libopencl1-331 has to be removed before installing WINE How do I install and manage GNOME Shell extensions? How do I install GNOME Shell extensions?  Is it possible to update, remove and manage the extensions?