I know the differences between mathoverflow and math-stack but I have been wondering:

  • Are there legitimate mathematical questions that would not get answered at math-stack because there is no expertise around and yet would be considered trivial (not research level worthy for mathoverflow?)

  • I agree that the fact that a question that is not answered at math-stack should not be "upgraded" to mathoverflow but is there some other site that covers a middle ground? Is there a place one can ask clarification questions about material covered in PhD math classes?

If you allow me to illustrate, I posted this question https://math.stackexchange.com/questions/554397/effect-of-pullback-of-differential-forms-on-an-ideal that I think is perhaps too trivial for the overflow crowd but that despite a bounty about to expire is languishing at math-stack. I'm afraid there is no good site for type of questions that are appearing in my self-study. Thanks for any feedback.

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    $\begingroup$ From MO's What question can I ask here: "[...] ask and answer research level math questions, the sorts of questions you come across when you're writing or reading articles or graduate level books." (my emphasis) In principle such a q is on-topic on MO (your q is really not in my field so I do not want to make any definite statement). In general, good graduate level q are welcome, only sloppy and/or completely standard ones are put on hold. Now, it can likely happen one thinks it is good but turns out to be 'comlpetely standard' to someone, but then this is not that big a problem. $\endgroup$ – user9072 Nov 17 '13 at 11:07
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    $\begingroup$ Dear Sergio, Your question seems too advanced for math-stack and appropriate to mathoverflow. $\endgroup$ – Gil Kalai Nov 17 '13 at 15:55
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    $\begingroup$ I would argue that, in principle, there's not supposed to be anything that is "too advanced" for math.SE; that is, math.SE is supposed to welcome question at all levels. In practice, it is likely that advanced questions will receive attention from an audience more likely to answer them in mathoverflow than in math.SE (because of the composition of the readerships of both groups), but that is a different question from whether it would be on-topic in math.SE. (cont) $\endgroup$ – Arturo Magidin Nov 17 '13 at 22:53
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    $\begingroup$ (cont) It used to be common that if a question seemed hard and was not getting any answers in math.SE, then it would be suggested that it be crossposted to mathoverflow (I don't know if migration is now possible that the two sites are on the same platform). Again, this would be different from saying it was off-topic or inappropriate for math.SE. My general impression is that the set of questions appropriate for mathoverflow are a proper subset of those that are appropriate for math.SE; again, this is different from whether a question is more likely to get useful answers here or there. $\endgroup$ – Arturo Magidin Nov 17 '13 at 22:56
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    $\begingroup$ Math.SE now has such a huge number of questions now, that I think it's hard for advanced technical questions to be seen by the right people even if they're browsing m.SE now and then. $\endgroup$ – Noah Snyder Nov 18 '13 at 0:08
  • $\begingroup$ @NoahSnyder: Fair enough; haven't taken a look at math.SE in a while (I sometimes get inadvertedly logged-in when I look click at a question in MO that gets migrated). $\endgroup$ – Arturo Magidin Nov 18 '13 at 2:12
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    $\begingroup$ I think it depends very much on what you are interested in. Advanced questions on general topology or set theory for example are usually answered quite fast at MSE. btw: Users cannot migrate questions from MSE to MO, only moderators can. $\endgroup$ – Michael Greinecker Nov 18 '13 at 15:22
  • $\begingroup$ @Michael: Yes, I know; users can flag questions for migration. But back when I was reading math.SE, migration was not possible, period, so people were encouraged to repost (after a suitable wait), with cross-references. $\endgroup$ – Arturo Magidin Nov 18 '13 at 17:18
  • $\begingroup$ Some other websites can be found here: Useful mathematical fora. Although I am not sure to which extent they would be useful in the situation described by the OP. In connection with the example of a question which was not answered on MSE but might be unsuitable here, it is worth mentioning that now this thread exists: Interesting (and not sufficiently answered) questions on math.SE. $\endgroup$ – Martin Sleziak Dec 4 '16 at 7:54
  • $\begingroup$ This discussion on meta.MO seems related, too: A StackExchange website aimed at graduate students? $\endgroup$ – Martin Sleziak Feb 11 '17 at 7:07

The reddit website has several forums dedicated to mathematics. The main math forum has a large audience but there are several others subs such as /r/learnmath, /r/homeworkhelp, /r/cheatatmathhomework, /r/askmath, /r/MathHelp which are somehow more "finegrained" than mathoverflow/mathstackexchange.

See this link for an extensive list of subreddits dealing with mathematics.

Interesting math subreddits not covered by the stackexchange websites include /r/mathpics, /r/mathgifs, /r/mathriddles, /r/crypto, /r/mathpuzzles, /r/WomenInSTEM, /r/badmathematics. There are also subreddits dedicated to specific courses like /r/Math2240.

And of course the mandatory /r/catMath.

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How about art of problem solving? http://www.artofproblemsolving.com/Forum/index.php?

I saw a lot of hard problems there but somehow i would hesitate to post such ones to MO. My feeling is that this site is in between MO and Math.SE.

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    $\begingroup$ The orderings are not quite linear. I have found much interesting mathematics on Math.SE, but art of problem solving seems a bit more specialized (as the name suggests of course). $\endgroup$ – Lucia Nov 18 '13 at 4:48

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