Present perfect vs. Past perfect for finished actions
Present perfect for past action with present effect
I added a comment to  but something strange is happening. The end code reads "); but SO decided to change that to ";);.    I did not add a ; in front of the ).  To further explain what's happening here, the following:  "http://example.com/." http://example.com/. "http://example.com." http://example.com. "http://example.com" http://example.com   renders as: The "Very Low Quality" (VLQ) flag reason is really annoying me.   It is the analog to the mommy mommy flag (which my 1.5-year-old child raises quite frequently). It provides no real clue as to what the flagger really wants us to do with the post.   Does she want us to close the question?  Does she want us to edit it into shape?  Does she want us to delete the question?  Does she want us to ban the user?  Does she want us to jump around and have a party?    Should we get rid of this flag reason?   Is it just me? Should and can we replace it with something better?     EDIT I accepted Jeff's answer. The tougher wording has heavily decreased the amount of very low quality flags. We now see only about 25% of the amount we used to and they are usually valid. Additionally, we now only allow very low quality on posts with a score of 0 or lower. After logging in on terminal with user name and password, how do I start the GUI? I am new to Ubuntu, and I am working on a Mac. The Ubuntu installation was done by someone else. I logged in with the user name and password, but I'm not sure what next to do in the terminal to get to the graphical interface. Could someone assist or point me in a direction where I can get useful guidelines? I have done research, but nothing I found helped me.   When I tried to start the lightdm greeter, I got the following message:  lightdm.service not found     Update: My understanding is that Ubuntu Server is installed. Spatial reference for dummies? Can anyone recommend a good reference or tutorial that explains the principles of spatial references, projections, and coordinate systems?   We are a small team of programmers working with GIS technology, and we often get snagged when working with data in different SRS/CS because we don't understand the concepts. How does Cantor's diagonal argument work? I'm having trouble understanding Cantor's diagonal argument.  Specifically, I do not understand how it proves that something is "uncountable".  My understanding of the argument is that it takes the following form (modified slightly from the  article, assuming base 2, where the numbers must be from the set $ \lbrace 0,1 \rbrace $):  \begin{align} s_1 &amp;= (\mathbf{0},1,0,\dots)\\ s_2 &amp;= (1,\mathbf{1},0,\dots)\\ s_3 &amp;= (0,0,\mathbf{1},\dots)\\ \vdots &amp;= (s_n \text{ continues}) \end{align}  In this case, the diagonal number is the bold diagonal numbers $(0, 1, 1)$, which when "flipped" is $(1,0,0)$, neither of which is $s_1$, $s_2$, or $s_3$.  My question, or misunderstanding, is: When there exists the possibility that more $s_n$ exist, as is the case in the example above, how does this "prove" anything?  For example:  \begin{align} s_0 &amp;= (1,0,0,\mathbf{0},\dots)\ \ \textrm{ (...the wikipedia flipped diagonal)}\\ s_1 &amp;= (\mathbf{0},1,0,\dots)\\ s_2 &amp;= (1,\mathbf{1},0,\dots)\\ s_3 &amp;= (0,0,\mathbf{1},\dots)\\ s_4 &amp;= (0,1,1,\mathbf{1},\dots)\\ s_4 &amp;= (1,0,0,\mathbf{1},\dots)\ \ \textrm{ (...alternate, flipped } s_4\textrm{)}\\ s_5 &amp;= (1,0,0,0,\dots)\\ s_6 &amp;= (1,0,0,1,\dots)\\ \vdots &amp;= (s_n \text{ continues}) \end{align}  In other words, as long as there is a $\dots \text{ continues}$ at the end, the very next number could be the "impossible diagonal number", with the caveat that it's not strictly identical to the "impossible diagonal number" as the wikipedia article defines it:     For each $m$ and $n$ let $s_{n,m}$ be the $m^{th}$ element of the $n^{th}$ sequence on the list; so for each $n$,      $$s_n = (s_{n,1}, s_{n,2}, s_{n,3}, s_{n,4}, \dots).$$      ...snip...      Otherwise, it would be possible by the above process to construct a sequence $s_0$ which would both be in $T$ (because it is a sequence of 0s and 1s which is by the definition of $T$ in $T$) and at the same time not in $T$ (because we can deliberately construct it not to be in the list). $T$, containing all such sequences, must contain $s_0$, which is just such a sequence. But since $s_0$ does not appear anywhere on the list, $T$ cannot contain $s_0$.      Therefore $T$ cannot be placed in one-to-one correspondence with the natural numbers. In other words, it is uncountable.   I'm not sure this definition is correct, because if we assume that $m = (1, \dots)$, then this definition says that "$s_n$ is equal to itself"&mdadsh;there is no "diagonalization" in this particular description of the argument, nor does it incorporate the "flipping" part of the argument, never mind the fact that we have very clearly constructed just such an impossible $T$ list above.  An attempt to correct the "diagonalization" and "flipping" problem:  $$s_n = (\lnot s_{m,m}, \lnot s_{m,m}, \dots) \quad \text{where $m$ is the element index and} \quad\begin{equation}\lnot s_{m,m} = \begin{cases}0 &amp; \mathrm{if\ } s_{m,m} = 1\\1 &amp; \mathrm{if\ } s_{m,m} = 0\end{cases}\end{equation}$$  This definition doesn't quite work either, as we immediately run in to problems with just $s_1 = (0),$ which is impossible because by definition $s_1$ must be $ = (1)$ if $s_1 = (0)$, which would also be impossible because...   Or more generally, with the revised definition there is a contradiction whenever $n = m$, which would seem to invalidate the revised formulation of the argument / proof.  Nothing about this argument / proof makes any sense to me, nor why it only applies to real numbers and makes them "uncountable". As near as I can tell it would seem to apply equal well to natural numbers, which are "countable".  What am I missing? Sum of a power series $n x^n$ I would like to know: How come that  $$\sum_{n=1}^\infty n x^n=\frac{x}{(x-1)^2}$$  Why isn't it infinity? Is this sentence grammatically correct?  Anyone who loves the English language should have a copy of this book in their bookcase.  or should it be:  Anyone who loves the English language should have a copy of this book in his or her bookcase. Get width of a given text as length I have the problem, that PHP replaces all spaces with underscores in POST and GET variables. For example if I have the URL: http://localhost/proxy.php?user name=Max the browser will convert it to http://localhost/proxy.php?user%20name=Max. But if I give the $_GET parameters out, the key is not user name but user_name (note the underscore)! Is there any possibility to change this behaviour? Callback after the record was created using e.force:createRecord Are there a way to add callback after the user created a record and clicked the Save button?  I am using a custom tab and custom components and open the Create a record modal window using e.force:createRecord, but I was not able to find out how to attach callback to the event.  I even found :     Currently force:createRecord event does not accept any callback method or any after save event to handle custom action. Current force:createRecord always redirects to the newly created record. But in case if you want to just close the force:createRecord popup after save and refresh some other related component using the refresh is not possible.    So, is it really impossible? If so, what are current workarounds here? I connect my phone to a Bluetooth speaker to play music. When engage Siri or try to use dictation, the phone wants to use the microphone on the Bluetooth device, which is either too crappy or too far away for it to understand what I am saying.  So, I would like a way to restrict the iPhone to using its own microphone even when connected to a Bluetooth speaker.  I am open to doing this via a setting, buying an app, or jailbreaking or whatever.  Is there a way to do this? Option clash for package xcolor How to get into Walker in the Wastes as a level 3 Wizard I am building a Wizard character that focuses on using Greater Drought from the Walker in the Wastes prestige class.  The campaign is starting at level 12, so I need to figure out a way to build a Wizard 3/Walker in the Wastes 9.  I am planning on using Southern Magician and Extra Spell to get around the spell requirement, but I'm having trouble qualifying for Heat Endurance by level 3.  We are using a variant rule where, at every level divisible by 3, we get 3 feats instead of 1. At level 3, I'll have 1 feat from level 1, and 3 feats from level 3.  How can I build a character with 11 total levels of Wizard spellcasting and the Greater Drought Walker in the Wastes ability by level 12?  My current character sheet can be found . What is the current state of apple device (iPhone, iPod, etc.) support? "Startup Applications" not working I need to learn basic optics for a high school academic competition. Does anyone know any good places, preferably free and online, to learn the basics of optics, like lenses, angles of incidence, etc.? I can't seem to find an MIT OpenCourseWare course addressing the issue. Is there a way to get the count of rows in all tables in a MySQL database without running a SELECT count() on each table? I need to have one column as the primary key and another to auto increment an order number field. Is this possible?  EDIT: I think I'll just use a composite number as the order number. Thanks anyways. Etiquette for revision comments Assume that $A$ is an $n\times n$  real matrix, i.e. $$A^T=-A.$$  Since $\det(A-\lambda I)=\det(A^T-\lambda I)$, $A$ and $A^T$ have the same eigenvalues. On the other hand, $A^T$ and $-A$ also have the same eigenvalues. Thus if $\lambda$ is an eigenvalue of $A$, so is $-\lambda$. If $n$ is odd, $\lambda = 0 $ is an eigenvalue.  A curious search in Google returns  the nonzero eigenvalues of $A$ are all pure imaginary and thus are of the form $iλ_1, −iλ_1, iλ_2, −iλ_2,$ … where each of the $λ_k$ are real.   Here is my question:     How can I prove the fact that "the nonzero eigenvalues of $A$ are all pure imaginary"?