Can't download software's from Ubuntu Software center ( 14.04 LTS )
"Unable to locate package" while trying to install packages with APT
While my code is compiling, I usually like to go through some reviewing and especially the late answers.  What I do see a lot in answers to old questions is the self-defense mechanism     ""   As if there is something fundamentally wrong with answering old questions. This leads me to two questions/thoughts:   Should we edit phrases like that out of the answer? (Given it's a good/valid answer to begin with, without any of the likely "old answer" problems. I realize the example linked to has its flaws.) Should we perhaps explicitly encourage answering old unanswered questions somewhere in the FAQ? (I'm unaware of any such recommendation, but might have read over it. Let me know if I did) Should black holes be considered two-dimensional or three-dimensional? Another layman blackhole question, pulling one end of a string out from behind the event horizon No long explanation is needed,  What would happen if I were to allow one end of a rope to fall past the event horizon of a black hole while I held the other end?  Would I be able to pull it out? Would the rope feel extremely (infinitely?) heavy? "This computer currently has no detected operating systems" when installing In some books the word "if" is used in definitions and it is not clear if they actually mean "iff" (i.e "if and only if"). I'd like to know if in mathematical literature in general "if" in definitions means "iff".  For example I am reading "Essential topology" and the following definition is written:     In a topological space $T$, a collection $B$ of open subsets of $T$ is said   to form a basis for the topology on $T$ if every open subset of $T$ can be written as a union of sets in $B$.   Should I assume the converse in such a case? Should I assume that given a basis $B$ for a topological space, every open set can be written as a union of sets in $B$?  This is just an example, I am not asking specifically about this sentence. Proof of dividing fractional expressions For dividing two fractional expressions, how does the division sign turns into multiplication? Is there a step by step proof which proves  $$\frac{a}{b} \div \frac{c}{d} = \frac{a}{b} \cdot \frac{d}{c}=\frac{ad}{bc}?$$ I have Ubuntu server 12.04 installed, so I have no GUI. When I do the command ifconfig, I cannot find my internal IP address. It says inet addr: 127.0.0.1. Here is the output of ifconfig -a: eth0   link encap:Ethernet  HWaddr 00:06:4f:4a:66:f0     BROADCAST MULTICAST  MTU:1500  Metric:1     RX packets:0 errors:0 dropped:0 overruns:0 frame:0     TX packets:0 errors:0 dropped:0 overruns:0 carrier:0     collisions:0 txqueuelen:1000     RX bytes:0 (0.0 B)  TX bytes:0 (0.0 B)  eth1   link encap:Ethernet  HWaddr 00:16:ec:05:c8:9c      BROADCAST MULTICAST  MTU:1500  Metric:1     RX packets:0 errors:0 dropped:0 overruns:0 frame:0     TX packets:0 errors:0 dropped:0 overruns:0 carrier:0     collisions:0 txqueuelen:1000     RX bytes:0 (0.0 B)  TX bytes:0 (0.0 B)  lo     Link encap:Local Loopback     inet addr 127.0.0.1  Mask:255.0.0.0     inet6 addr:  ::1/128 Scope:Host     UP LOOPBACK RUNNING MTU:65536  Metric:1     RX packets:1800 errors:0 dropped:0 overruns:0 frame:0     Tx packets:1800 errors:0 dropped:0 overruns:0 carrier:0     collisions:0 txqueuelen:0     RX bytes:143896 (143.b KB)  TX bytes:143896 (143.8 KB)  here are the contents of etc/network/interfaces: # The loopback network interface auto lo iface lo inet loopback  # The primary network interface auto eth0 iface eth0 inet dhcp  If someone could edit this for me, the contents of etc/network/interfaces should be on separate lines. The output of host askubuntu.com was: ;; connection timed out; no servers could be reached.  I set up owncloud and webmin a few months ago and was using them for a month with no problems. I think the power went off one day 2 months ago and I never turned the server back on until yesterday. I haven't done anything that would have affected the internet setup So i'm not sure why it doesn't work anymore. As far as my network topology goes, I have a pci-e network card for the pc. The ethernet line goes from the network card to a switch, and then to a modem/router from there. I am encountering two definitions of the special orthogonal lie algebra, and I would like to know if they are equivalent, and if there are advantages to working with one over the other.  If we begin with an $n$-dimensional vector space $V$ over a field $k$ and a chosen basis, we can define a bilinear form on $V$ by a matrix $S\in M_n(k)$, ie, let $\langle v,w\rangle=v^tSw$ for all $v,w\in V$.  Now $g\in GL_n(k)$ preserves the form ($\langle g(v),g(w)\rangle=\langle v,w\rangle$) if and only if $g^tSg=S$, so all such $g$ form a linear algebraic group $G$.  The tangent space at the identity of $G$ will be contained in that of $GL_n(k)$, so $T_eG\subset T_eGL_n(k)=M_n(k)$, and in fact, $T_eG=\{B\in M_n(k)\mid B^tS+SB=0\}$.  $T_eG$ becomes a lie algebra, $Lie(G)$, if we define the bracket to be the commutator of two matrices.  Now, if $S=I_n$, it follows that $G=O_n(k)$ is the orthogonal group of matrices satisfying $g^tg=I_n$, and $Lie(G)=\mathfrak{so}_n$ is the lie algebra of antisymmetric matrices.  In Humphrey's Introduction to Lie Algebras and Representation Theory, he defines $\mathfrak{so}_n$ to be all matrices $B$ satisyfing $B^tS+SB=0$, where $$ S=\begin{pmatrix} 1&amp;0&amp;0\\ 0&amp;O&amp;I_l\\ 0&amp;I_l&amp;O \end{pmatrix} \hspace{.5in}\text{or}\hspace{.5in} S=\begin{pmatrix} O&amp;I_l\\ I_l&amp;O \end{pmatrix} $$ depending on the parity of $n$.  The matrices obtained in this way are not antisymmetric, nor is the group $G$ preserving the form defined by $S$ the orthogonal group $O_n(k)$.  Are the two groups obtained from considering different $S$ isomorphic?  Are the two lie algebras isomorphic?  If so, why would one prefer one form to the other? How to prove there are no more positive integers that are products of 2 and 3 consecutive numbers? How do I resolve unmet dependencies after adding a PPA? Occasionally, when I'm installing stuff, I get an error like the following:  Some packages could not be installed. This may mean that you have requested an impossible situation or if you are using the unstable distribution that some required packages have not yet been created or been moved out of Incoming. The following information may help to resolve the situation:  The following packages have unmet dependencies:  package1 : Depends: package2 (&gt;= 1.8) but 1.7.5-1ubuntu1 is to be installed E: Unable to correct problems, you have held broken packages.   How can I resolve this? From some comments in the answers for  (now deleted, 10k only), there's confusion around the usage of I vs. me:  While the sentence, "the other attendees are myself and Steve," is agreed to be incorrect, there's confusion about whether the correct form is "the other attendees are me and Steve" or "the other attendees are Steve and I."  (I've always used the heuristic of removing the other people from the sentence, so I always thought "the other attendee is me" would be correct, instead of "the other attendee is I." Is this true, or am I using a flawed heuristic?) Adding words to links generated by hyperref \documentclass{article} \usepackage{hyperref} \usepackage{lipsum} \usepackage{polski} \begin{document} \begin{figure} . \caption{A figure} \label{fff} \end{figure} \lipsum[1]  Rysunek~\ref{fff} jest zabawny, bo na rysunku~\ref{fff} nic nie ma. \end{document}   In this sample, a clickable link is added to output PDF for 1 and 1. I want to be able to change them to Rysunek 1 and rysunku 1. I want to be able to put any other text + number of the figure into a hyperlink. The word rysunek can't be set in preamble as a default because there is a declension in Polish. The same question goes for tables, listings, equations, pages (pageref) etc (basically any reference).  HTML equivalent of my MWE:    Rysunek &lt;href="fff"&gt;1&lt;/a&gt; jest zabawny, bo na rysunku &lt;a href="fff"&gt;1&lt;/a&gt; nic nie ma.   I want it to be:    &lt;href="fff"&gt;Rysunek 1&lt;/a&gt; jest zabawny, bo na &lt;a href="fff"&gt;rysunku 1&lt;/a&gt; nic nie ma. How do I enumerate the properties of a JavaScript object? How do I enumerate the properties of a JavaScript object?  I actually want to list all the defined variables and their values, but I've learned that defining a variable actually creates a property of the window object. In the standard PrintDialog there are four values associated with a selected printer: Status, Type, Where, and Comment.  If I know a printer's name, how can I get these values in C# 2.0? How many solutions $X^{10} - I=0$ has in $M_2(\mathbb{R})$? How many solutions $X^{10} - I=0$ has in $M_2(\mathbb{R})$? Where $M_2(\mathbb{R})$ denotes the set of $2 \times 2$ real matrices.  I absolutely have no idea of where I should start from. $I$ and $-I$ solve the equation, but are there any other non-trivial solutions? Are there infinitely many solutions? Does "Going Infinite" necessarily end the game? Say I have 12 and have to factorize it in 2 factors. Now possible factors are 1, 12 2, 6 3, 4   Now 3,4 is such that value of maximum factor i.e. 4 is minimized among all pairs.  How to find the solution in generic case where a number N is to be divided in K factors minimizing maximum value. How to remove a vector QgsVectorLayer from QGIS python plugin I managed to add QgsVectorLayer in QGIS through a python plugin, for example:  vl = self.iface.addVectorLayer(uri.uri(), layerName, self.dbConn.getProviderName())   But how can I remove it? One problem I have found when learning calculus is that there are many different ways to denote the derivative. If $y=f(x)=x^2$, then we could write \begin{align} f'(x)&amp;=2x \\ y'&amp;=2x \\ \frac{df}{dx}(x)&amp;=2x \\ \frac{df(x)}{dx}&amp;=2x \\ \frac{d}{dx}f(x)&amp;=2x \\ \frac{dy}{dx}&amp;=2x \end{align} And this is just Lagrange and Leibniz's notations alone. What I find troubling is that they all seem to be suggesting subtly different things about what the derivative actually is. Is it a function, a limit of a quotient, or both? In the interests of keeping my post brief, I'll focus my attention on $f'(x)=2x$ and $\frac{dy}{dx}=2x$, as these seem to be the most common notations.  $$ f'(x)=2x $$  It does make sense to think of the derivative as the gradient function: $$ f'\colon x\mapsto\lim_{\Delta x \to 0}\frac{f(x+\Delta x)-f(x)}{\Delta x} $$ In this case the limit expression is equal to $2x$, and so we can write $$ f' \colon x \mapsto 2x $$ However, this notation seems a little counter-intuitive when we consider what it means to differentiate a function with respect to a variable other than $x$. If I ask what is the derivative of $f(x)$ with respect to $\frac{x}{2}$, does this question make sense? Is it simply $f'(\frac{x}{2})$? Or do we have to express $x^2$ in terms of $\frac{x}{2}$? And how can we can express this derivative using Lagrange's notation?  $$ \frac{dy}{dx}=2x $$  There are many things which are nice about Leibniz's notation, including the fact that it is explicit which variable you are differentiating with respect to. However, in this case, it is unclear whether we are talking about a function, or something else entirely. There are other issues. Some people say they dislike the Leibniz formulation of the chain rule $$ \frac{dy}{dx}=\frac{dy}{du}\frac{du}{dx} $$ saying that they find it to be inaccurate. I don't really understand why this is the case. Could someone please elaborate? Java Generics (Wildcards) I have a couple of questions about generic wildcards in Java:   What is the difference between List&lt;? extends T&gt; and List&lt;? super T&gt;? What is a bounded wildcard and what is an unbounded wildcard?