Currently the custom off-topic reasons for MathOverflow read:
This question does not appear to be about research level mathematics within the scope defined in the help center.
Questions about homework are generally off-topic. MathOverflow is for mathematicians to ask each other questions about their research.
There are some rather obvious overlaps between the two reasons, and I think we can perhaps optimize a bit.
As a moderator on Math.StackExchange, recently we have had some discussions about the closure of migrated questions. As you may already know: if a question is migrated to another site, and if it is closed there for anything other than "Duplicate", the question gets bounced back to the original site, the migration canceled, the question now just put on-hold as generically "off topic", and the user who asked the question originally may be none-the-wiser.
You may be less likely to know that recently, due in part to the increased volume of questions, and in part to many members' preference to teach mathematics instead of handing out homework solutions, Math.StackExchange has implemented a "Lack of context" close reason, trying to encourage users to include the source/motivation of the mathematical problems that they are asking about, as well as any partial work that they have done toward resolving it.
Already at times we close the question as off-topic, only to leave a comment stating that perhaps MSE is a better place for it. My proposal makes for less typing by the users.
We can keep the first off-topic reason (for questions that are not even mathematical), but change the second one to the following:
- MathOverflow is for mathematicians to ask each other questions about their research; general questions in mathematics should be asked at Math.StackExchange.
In the spirit of "teach a man to fish", I would advocate for using this custom close reason instead of direct migration when dealing with all but the highest quality (but unsuitable for MO) questions. This way the user learns to register a new account on MSE, and ask his questions there. And if his question is not of a sufficient quality for MSE, he will be told why by members of MSE. Ideally this will keep users who do not care about research level mathematics from coming back to MO time and again.
Math.SE is rather more tolerant of questions, overall, than many other sites. In the past three months, MSE tried to migrate 10 questions to Physics with a staggering 70% rejection rate [Edit: this I checked by hand, and the number is accurate. See comments below], whereas Physics dumped 40 questions on MSE with only 5% rejected.
MO is by far the largest migrator to MSE, with 185 questions in the past three months, of which
21% were rejected [Edit: this 21% may or may not be accurate. See comments below]. 11% were rejected (see comments below). The fact that there are still, on average, two questions per day (just shy of 10% of the questions asked per day) which are migrated from MO to MSE (and I am sure a few others that are just closed as off topic without migration), means that we are still not getting the message across about the difference between MO and MSE. Maybe this will help a tiny bit.