I believe this is different from other similar questions such as   My question is not about encouraging politeness, it is about providing a simple automated way to increase positive, constructive feedback.  My suggestion is that a down-vote could be required to have an associated comment so that the down-voter is identified.  You might object that this would encourage revenge voting. Not so IMO. The revenge voter would also have to provide sensible reasons for their down-votes. It would be obvious if someone was doing it vindictively.  Please discuss both the benefits and drawbacks of this idea.  Thank you.
Note: If you're looking for a simple explanation as to why comments aren't required on downvotes, see .   I used to get "upset" (though that is too strong a term) when I got downvoted without comment. If my answer isn't good enough then I'd like to know why. Not only does it improve the answer for the OP but it improves my knowledge too.  Where the down-vote has been explained I've found it useful &amp; it has improved my answer, or forced me to delete the answer if it was totally wrong.  So is there any way we can encourage people to leave a comment? Perhaps they don't lose rep if they explain their down-vote?  I must admit though that I haven't always explained my down-vote either so you could call me a hypocrite. I've also grown a thicker skin over the months of using SO (it seems to have come with the higher rep score ;-)), so I'm less bothered about this now.
Show that $\mathbb{Z}/n^2\mathbb{Z}$ is not isomorphic to other group I'm currently working on a proof, but didn't get the right idea, a hint would be appreciated:  Show that $\mathbb{Z}/n^2\mathbb{Z}$ is not isomorphic to $\mathbb{Z}/n \times \mathbb{Z}/n$  So, lets assume there is an isomorphism between these groups, my first idea to was to construct a contradiction with the homomorphism property. But I don't know how, there should be an element in  $\mathbb{Z}/n \times \mathbb{Z}/n$ which is not in $\mathbb{Z}/n^2$ to reach this kind of contradiction. Of course there is $n \in \mathbb{Z}/n^2$ which is not in the other group but I don't know if this helps.  Thanks Arrays, linked lists and other data structures in cmd.exe (batch) script How to mount PCIe SSD from MacBook Air in another computer I got this SSD out of an Apple laptop that's broken from liquid damage.  What is this connection called and how do I mount this storage device to retrieve data from it? It looks like PCIe. Does Apple sell a part to mount this via USB adapter or something? What are the rules about take a bike on the London Tube? Can I take a bike on the Tube (underground) in London, if so how much does it cost? "The antipodal map of $S^2$ is the isometry $\bar m$ sending each point $P$ to its opposite point $-P$" (from textbook)  Show that antipodal points remain antipodal under any isometry of $S^2$. In other words, for each isometry f of $S^2$, if $P$ and $P'$ are antipodal, then so are $f(P)$ and $f(P')$  I am unsure how I can define the $P$ and $P'$ so that they are opposites of each other in the antipodal map, and am I supposed to apply the traditional isometries (translation, reflection and rotation) to the points? I don't understand this problem. How do I use Conditional Formatting based on due dates in Excel 2013? I am trying to format cells in Excel 2013. I want the cells to get colored as green, yellow, and red based on how close they are with the target date (January 13, 2015 in this case).  I am unable to use conditional formatting as Excel says that relative formatting is not permitted within conditional formatting.   Target:   06-01-2015  01-01-2015 02-01-2015 03-01-2015 04-01-2015 05-01-2015 06-01-2015 07-01-2015 08-01-2015 09-01-2015 10-01-2015   If the date in the cell is more than 2 days of the target date, I want the cell to change color to Green. I used =DATEDIF($N12,$N$9,"d")&gt;=3 and the format changed to green.  If the date in the cell is exactly 2 days near the target date, I want the cell to change color to yellow. I used =DATEDIF($N12,$N$9,"d")=2 and the format changed to yellow.  If the date in the cell is within 2 days of the target date, I want the cell to change color to red. I used =DATEDIF($N12,$N$9,"d")&lt;2 and the format changed to red only for 5th and 6th January. Apparently, it does not recognize negative values.  How can I get this to match the required colors? Doing principal component analysis or factor analysis on binary data I have a dataset with a large number of Yes/No responses. Can I use principal components (PCA) or any other data reduction analyses (such as factor analysis) for this type of data? Please advise how I go about doing this using SPSS. $(m, m+2)$ is twin prime, iff $4((m-1)! + 1) \equiv -m \pmod {m(m+2)}$ The Wiki page on  says     The pair $(m, m+2)$ is twin prime, iff $4((m-1)! + 1) \equiv -m \pmod {m(m+2)}$.   This is obviously connected to Wilson's Theorem. Can anybody provide a proof for that, along the lines of thought given here, ? Array with labeling columns Why can't the end code of an environment contain an argument? Display each sub-directory size in a list format using one line command in Bash? I want to get a list of the directories and their sizes in a list format like how you get when you do a ls -l.  The thing is that is there a one line command that can do this? I see others have long commands just to output this. That's just too long.  What command can do this or combination of commands that can be easily typed? du -h gives it, but it displays all of the sub-folders which is not what I want. just the current directories folders. 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? How to read and write into file using JavaScript? Can anybody give some sample code to read and write a file using JavaScript? I'm on SO pretty much every day researching something to do with programming, and over the last month or two, I find I'm growing frustrated and a little disgusted at how many interesting questions have been closed as "not a good fit" or "too subjective".  I think there is a confusion between "subjective" and "calls for judgement". To me one of the most valuable things about SO is the tremendous breadth and depth of experience of the members, and experience translates into smart decisions and advice, and that advice cannot always be asked for, or expressed in terms of, bald facts and data.  It's going to be 'subjective'.  I don't think SO should be Wikipedia. I think that if I could ask a question to a group of skilled programmers sitting around a table at lunch, and get interesting, valuable answers, then I don't care whether the question or the answers are 'subjective', I think that question should equally well be fair game for Stack Overflow.  I would like to see a mechanism whereby people who vote to close a question lose reputation when somebody expresses interest in that question or its responses.  How can I, or can I, influence this aspect of SO? Is a strong background in maths a total requisite for ML? I'm starting to want to advance my own skillset and I've always been fascinated by machine learning. However, six years ago instead of pursuing this I decided to take a completely unrelated degree to computer science.   I have been developing software and applications for about 8-10 years now, so I have a good handle but I just can't seem to penetrate the maths side of machine learning/probabilities/statistics.   I start looking at learning material and on the first page it might include something which confuses me and immediately sets up a barrier in my learning.   Is a strong background in maths a total requisite for ML? Should I try and fill in the blanks of my maths before continuing with ML? Can self learning really work for just a developer without any hard computer science background?    Related question: Can a shadow move faster than the speed of light? I'm just a layman with no real science background  (a) I'm not really sure what a shadow is. Some people are saying that it's the absence of something (a photon?) and therefore it is not subject to laws of physics. But I know when I'm in a shadow and when in sunlight... that's "information", and I thought that information flow can't break the speed of light limit.  (b) Some others use the laser pointer/flicking wrist example to talk about laser dots moving faster than light speed. But that can't be right, can it? The photons from the laser point have to travel from A to B and (after wrist flick) from A to C ... but the photons are all moving at the speed of light - there's no law-breaking in moving the laser dot from point B to point C because the photons are moving either from A to B or A to C ... the distance from B to C is not material (is it?).  (c) Also, can a shadow be thought of as a gap in a photon flow? E.g. if I interrupt a beam of single photons then can I think of it as e.g. PPPSSSPPPSSSPPPSSSPPP, etc where P = photons and S(shadow) = no photon &amp;, if so, the "SSS" is effectively travelling at the speed of light ... no faster. Re-entering the US for the third time in a year with total of 240 Days I went to the US for 90 days on an ESTA in August to November. Then back to my home country for 1 month and then back again mid December until mid February and now I am going back again after 10 days for another 90 days. Now I can see that some people refer to 180 day rule in a year. My stay will be 240 days in one year with short gaps between coming back. Is the 180 day rule accurate? Some people write that you can do unlimited trips to The US as long as you don't stay more than 90 days. Is that correct? My reason for going there the first time was to visit and travel with an American friend.  After my first visit (she is my girlfriend) moved with me to my home country but has ended up in a custody battle in the US so she needed to return urgently so I returned only to be with her and support her, not to be a tourist.  Then after my second visit I needed to go back to my sick Dad and to rent out my apartment since we left in a rush to go back to the US.  Now, my third visit is to continue to be with my girlfriend and support her in her difficult situation.   My question is, if I do end up in trouble should I tell them this whole story? Or should I just say I am visiting a friend for a third time because I want to see more of the country and I have some more time off before I go back to my job.  I have proof of money in my bank account and I stay and eat with her for free Documented proof that I have a job to return to in my home country My return ticket printed and an address to stay at in the US and I have proof I have an apartment in my home country Where do I ask where to ask? If I have a specific question I want to ask about, but I don't know which Stack Exchange site would be on topic for asking that question, where can I ask where to ask that question?  Is there any generic place where we can ask for advice? Is it here?  This is not about a specific question but in general.  For example: "Where should I ask for advice about fishing?", or if I need to know where I should ask "What would be the best fishing rod to fish for salmon?" (regardless of how good/bad these examples are) I need to graph a rectangle on the Cartesian coordinate system. Is there an equation for a rectangle? I can't find it anywhere. How are ETF prices calculated after the market is closed For example:  Because the NASDAQ: DAX is tracking the index INDEXDB: DAX should its trading value be completely flat when the market the markets are open in the US, but closed in Germany? I'm not seeing that artifact in any of the charts I can find.