I asked a question on Network Engineering. Question has been put on-hold after less than 10 minutes after creation. While I do not contest the utility of the "on-hold" flag I suggest allowing some time (hours or days) before flagging the question. If Stack-Exchange aspires to be a real open to discussion and open to opinions platform it should give an equal  chance to both parties. Flagging a question shortly after creation kills all chances to be answered by a late reader and demoralizes the asking person who expects something positive, constructive not a door-slam in face. There is a hunt-for-questions and a competition for answering-question-rewards on this site. My personal feeling is the first arrived, if it doesn't know the answer and have enough privileges will flash the question "on-hold" closing all later attempts to answer it. It is detrimental for everyone and for the knowledge base.
So I had  closed within eight minutes of asking.  Is this a little fast? Not only do I object to the reason it was closed (not a real question, come on I'm asking for decent books!), but it was closed insanely fast. Should there be a grace period for questions (diamond mods exempt if so) before they can be voted on to be closed?
I am trying to prove the following:  In $C[0,1]$ the functions that preserve the rationals (i.e. $f(\mathbb{Q})\subseteq \mathbb{Q}$) are dense.  So far, I have not made much progress- I am aware that $A$ is dense in $B$ iff $A$ has non-empty intersection with each open set $U$ in $B$ but am not sure how to apply this definition. I have also thought about showing that the closure of $X$ (where $X$ is the set of functions that preserve the rationals) is equal to $C[0,1]$ by a double-inclusion proof but I have also made no progress with this approach. Am I on the right track or should I change my approach? I have a vocabulary like this:   Parent 1  Child 1 Child 2 Child 3   Parent 2  Child 1 Child 2    Now I would like to use a view to create a list of a Parent term with its Child terms. I try to do this with Views UI, but only show the term parent... No the child term's. It is very confusing for me.  Can anyone help with this case? Thanks :D I am an Indian citizen, holding a residence permit in Norway valid for 2 years. I want to know about the 90/180 rule.   Can I travel to Portugal for one week, come back to Norway and travel again for vacation for 2 weeks to Italy, and again come back to my residence country?  Does time spent in my residence country also count against my total stay in the Schengen area? Does Iit mean that the clock stops tickkng when person comes back to their residence country? I feel that I might have a complete misunderstanding here. If DNA has two strands, how does the machinery of RNA transcription determine which one to transcribe? In SQL Server, how do I "floor" a DATETIME to the second/minute/hour/day/year?  Let's say that I have a date of 2008-09-17 12:56:53.430, then the output of flooring should be:   Year:  2008-01-01 00:00:00.000 Month: 2008-09-01 00:00:00.000 Day: 2008-09-17 00:00:00.000 Hour: 2008-09-17 12:00:00.000 Minute: 2008-09-17 12:56:00.000 Second: 2008-09-17 12:56:53.000 I'm using latex on a small netbook, and with average-sized files (~150 pages at the moment) compilation is already pretty slow. So I am looking for every way to speed up the compilation.  In this search, I found  explaining how to "precompile a preamble", or, latexically-speaking, how to make a custom format file. That works fine enough.  But my preamble isn't as static as it should be, I keep adding or modifying definitions in there. To mix the two solutions, at the moment, I use a Makefile together with latexmk, the makefile handling the regeneration of the .fmt file if needed, and latexmk taking good care of the rest.  But I have two problems with this :   integration with emacs to compile the whole document when working on a single file (make needs to be called in the proper directory) latexmk doesn't recompile the file every time the preamble is modified, because it doesn't count as a dependency.   So I'd like to find a way to get rid of this Makefile, and get all the stuff done by latexmk.  I edited my .latexmkrc to include this :     pdflatex = 'pdflatex -fmt main %O %S';  add_input_ext('pdflatex','fmt');  add_cus_dep('tex', 'fmt', 1, 'compilepreamble'); sub compilepreamble {     print "Preamble compiling...\n";     $command = '&amp;.pdflatex ./fmt/preamble.tex\dump';     system("pdflatex -ini -jobname='$_[0]' '$command'"); };      But with this, if the file main.fmt is not present or if fmt/preamble.tex is updated, the main.fmt file won't be regenerated. Actually, in the first case, I get an error from pdflatex saying the it can't find main.fmt format file. So I understand latexmk doesn't even try to build main.fmt, it's probably not a problem in my custom dependency. However, I think I've added everything necessary for this to work.  So... Did I forget something? Is there a way round? Or is it a hopeless quest? $\DeclareMathOperator{\card}{card}$From Problem-Solvers Topology  Prove the following: CANTOR'S THEOREM If $A$ is a set, then $$\card A &lt; \card \mathcal{P}(A)$$ where $\card A$ stands for the cardinality of set $A$.  My Answer If $A = \emptyset$, then $\card A =0$ and $\card \mathcal{P}(A)=1$. If $A = \{a\}$, then $\card A=1$ and $\card \mathcal{P}(A)=2$. So suppose that $A$ has at least two elements. Define a function $f: A \rightarrow \mathcal{P}(A)$ such that $f(x) = \{x\}$ for all $x \in A$. Then $f$ is injective. But it cannot be surjective, because for any two distinct elements $a,b \in A$, there is no element in $A$ that is sent to the set $\{a,b\}$ in $\mathcal{P}(A)$. Therefore, there is no bijection, and $\card A &lt; \card\mathcal{P}(A)$. Do you think my answer is correct? Thanks in advance When I reinstall Windows XP by mistake I deleted all five partitions. I did not format them yet though.  How can I recover the partitions? When I attach the hard disk to another PC it just says that the partition is unformated and asks "Would you like to format it?". I'm new to feature selection and I was wondering how you would use PCA to perform feature selection. Does PCA compute a relative score for each input variable that you can use to filter out noninformative input variables? Basically, I want to be able to order the original features in the data by variance or amount of information contained. When traveling to a country with a different currency, how should you take your money? Recently my wife and I traveled to Italy and before we left we got about 85% of our money in travelers checks and the other 15% we carried on in cash. When we arrived there, no one would take our travelers checks and many places did not accept credit cards. Eventually, we found some currency exchange places that would take the travelers checks and give us Euro's, but at a 7-15% fee, which was not really a very fun surprise.   While I was under the impression that it is safer to use travelers checks, it seems that they were not really accepted (the usual explanation being fraud). So the question is, is it best to just exchange your currency at home and carry all of it with you when you travel or are there other alternatives to carrying cash that are safer and cost effective? What is an uninterruptible process? Sometimes whenever I write a program in Linux and it crashes due to a bug of some sort, it will become an uninterruptible process and continue running forever until I restart my computer (even if I log out).  My questions are:   What causes a process to become uninterruptible? How do I stop that from happening? This is probably a dumb question, but is there any way to interrupt it without restarting my computer? When would you call java's thread.run() instead of thread.start()? When would you call Java's thread.run() instead of thread.start()? Does going outside of range or line of sight after casting a spell have any effect? Does going outside the range of the spell or breaking line of sight between the caster and target break concentration or otherwise affect control over the spell?  For example, a Heat Metal spell has a range of 60 ft. If I cast the Heat Metal spell on a an armored foe, and that foe runs outside the range of the spell (60 ft), or around the corner, does that break concentration and end the spell? Heat Metal also allows you to use a bonus action to cause the 2d8 of damage — would exercising that option be prevented by being out of range or line of sight? Finding $\tan t$ if $t=\sum\tan^{-1}(1/2t^2)$ I am solving this problem. If $$\sum\limits_{i=1}^{\infty} \tan^{-1}\biggl(\frac{1}{2i^{2}}\biggr)= t$$ then find the value of $\tan{t}$.   My solution is like the following: I can rewrite: \begin{align*} \tan^{-1}\biggl(\frac{1}{2i^{2}}\biggr) &amp; = \tan^{-1}\biggl[\frac{(2i+1) - (2i-1)}{1+(2i+1)\cdot (2i-1)}\biggr] \\\ &amp;= \tan^{-1}(2i+1) - \tan^{-1}(2i-1) \end{align*}  and when I take the summation the only term which remains is $-\tan^{-1}(1)$, from which I get the value of $\tan{t}$ as $-1$. But the answer appears to be $1$. Can anyone help me on this. Trailing characters stripped from titles bypass length minimum Editing Photoshop (PSD) files on Ubuntu? I was wondering that is it possible to edit PSD files on Ubuntu (like with Gimp). Actually, I need to edit layers. I haven't managed to do it. It would be nice if there is a chance! :) Do you have to pass delete the same pointer that was returned by new, or can you pass it a pointer to one of the classes base types?  For example:  class Base { public:     virtual ~Base();     ... };  class IFoo { public:     virtual ~IFoo() {}     virtual void DoSomething() = 0; };  class Bar : public Base, public IFoo { public:     virtual ~Bar();     void DoSomething();     ... };  Bar * pBar = new Bar; IFoo * pFoo = pBar; delete pFoo;   Of course this is greatly simplified.  What I really want to do is create a container full of boost::shared_ptr and pass it to some code that will remove it from the container when it is finished. This code will know nothing of the implementation of Bar or Base, and will rely on the implied delete operator in the shared_ptr destructor to do the right thing.  Can this possibly work? My intuition says no, since the pointers will not have the same address.  On the other hand, a dynamic_cast&lt;Bar*> should work, so somewhere the compiler is storing enough information to figure it out.  Thanks for the help, everybody who answered and commented. I already knew the importance of virtual destructors, as shown in my example; after seeing the answer I gave it a little thought, and realized the whole reason for a virtual destructor is this exact scenario. Thus it had to work. I was thrown by the absence of a visible means of converting the pointer back to the original. A little more thinking led me to believe there was an invisible means, and I theorized that the destructor was returning the true pointer for delete to release. Investigating the compiled code from Microsoft VC++ confirmed my suspicion when I saw this line in ~Base:  mov eax, DWORD PTR _this$[ebp]   Tracing the assembler revealed that this was the pointer being passed to the delete function. Mystery solved.  I've fixed the example to add the virtual destructor to IFoo, it was a simple oversight. Thanks again to everyone who pointed it out. What is the connection between Poisson brackets and commutators? The Poisson bracket is defined as:  $$\{f,g\} ~:=~ \sum_{i=1}^{N} \left[  \frac{\partial f}{\partial q_{i}} \frac{\partial g}{\partial p_{i}} - \frac{\partial f}{\partial p_{i}} \frac{\partial g}{\partial q_{i}} \right]. $$  The anticommutator is defined as:  $$ \{a,b\} ~:=~ ab + ba. $$  The commutator is defined as:  $$ [a,b] ~:=~ ab - ba. $$  What are the connections between all of them?  Edit: Does the Poisson bracket define some uncertainty principle as well? multiple Open Drain UART over long distance I have 9 microcontrollers  and I have connected the RX and TX lines of 8 of them in parallel. My configuration is in master-slave. The master microcontroller send data to one of the 8 RX lines but only one of them can respond at a time. The cable length is about 3 to 5 metres between each node.  I have two questions. My test was done with 2 slaves instead of 8. UART at 9600bps.  If I setup the TX lines of the slaves as Open Drain it works fine, but the signal doesn't have steep ramps. I thought this happens due to Pull-up resistors. And I also think that is going to be better with 8 slaves on the bus, right?  Why doesn't that topology work if the TX lines of the slaves are not Open Drain?  I managed to make it work by disabling the transmit of the UART in all of them except one, and it worked fine. I had two slaves, so I disabled the one and left the other. Also I noticed that the edges of the signal was sharper. Why are they sharper in that case?  Regarding resistors: The value of the resistors for both RX and TX lines are 2.7KΩ. However the Open Drain topology would parallel the internal pull-up resistors. According to the datasheet that resistor is about 20KΩ at minimum. So, using a 2.7KΩ in the output would give us a total resistance of 2.52KΩ (20+2.7)/9. And the Open Drain would make a voltage divider 2.52KΩ + 2.7KΩ (Schematic 2). Which means with 8 slaves and Open Drain topology the signal will not go lower than 1V which is the threshold of the low level. Is that correct?  Wouldn't it be better to disable all the internal pull up resistors and place two (let's say: 4.7KΩ) only at the master side?  Distance: What is the maximum baud rate for such distance? I'm a little worried about the signal integrity. The test was done with 9600bps but I didn't notice any significant difference in 4800bps. The distortion of the signal didn't have a difference.  Voltage values are 3.3V for the UART bus.  Schematic 1. (all resistors are 2.7KΩ)        &ndash; Schematic created using   Schematic 2. (U4, U5, U6, U7, and U8 are omitted for simplicity).          EDIT: Thank you all very much about your answers so far. I wanted to point out that this is a question about UART only. I am not looking for alternative solutions. I'm just trying to find some answers about the above implementation.  EDIT2: Thank you very much for your answers! I ended up modifying a little my schematic. I disabled the internal pullup resistors of the microcontrollers and I placed external ones at the master's board. Their value would be around 1 KOhm to 3 KOhm. I have to do some tests more to decide the final value. I also replaced all series resistors with 130 Ohm.   I tested 5 slaves over 20 metre cable (with pullup 3 KOhm and 130 Ohm series resistors) and it worked fine.   Would it be better to add open drain/open collector line drivers on every TX lines? How does a new user begin using Stack Overflow/Stack Exchange? As a new user, I find it impossible to contribute to the community or to even begin contributing. I know why the reputation system is in place and I respect it, but at the same time it is too hard on new users.  It started with me wanting to upvote a question...     "Vote Up requires 15 reputation"   Just to appreciate a good question that helped me I need 15 rep? OK..., so I find out how I can get reputation points. More or less, I need to ask a question for +5 and add an answer for +10. I don't have any questions at the moment and I don't want to spam a random question, but I did happen to have a slightly different answer to the earlier question that I wanted to upvote. So I try to answer...     This question is protected to prevent "thanks!", "me too!", or spam answers by new users. To answer it, you must have earned at least 10 reputation on this site.   Now... I know this doesn't apply to every question, but at the same time, this means I'm forced to answer yet another question, and I don't want to go around looking for random questions and spam irrelevant answers...  Now I'm stuck. So I try to talk to some other users in the chat rooms about how to go about getting reputation points and how they first started. I entered into a "casual chat room".     You must have 20 reputation on Meta Stack Overflow to talk here.   Yes... I can understand why this is enforced, because we don't want new users to spam chat rooms, but all I wanted to do was ask for help. So I can't even talk to anyone about my newbie problems. So okay, I'll just settle for upvoting comments in the question that helped me. I don't see an upvote hover-over button so I can't even do that.    So now I'm stuck at 1 reputation point, unable to cross that 15 rep barrier, and even after posting this question I'll only have 6 rep points and still unable to do anything, and I'll be forced to look for questions to answer. Yes, I should be contributing to the community, but I shouldn't need to begin answering or making questions to upvote questions and answers. This only spurs more spam/irrelevant answers if anything.   Very, very, very new-user unfriendly.