I've never figured out how "migration" works, but I've repeatedly seen its existence mentioned, and I've seen migrated questions. Thus when I posted a question about the shape of the earth to math (dot) stackexchange (dot) com someone decided it belonged in history (dot) stackexchange (dot) com and somehow it got "migrated" to that site. So now I see this question posted to mathoverflow, and I wonder why it doesn't just get migrated to math (dot) stackexchange (dot) com, which seems to be where it belongs. How is that done and why would one not do it in a case like that?
2$\begingroup$ Links to some basic info about migration can also be found in the migration tag-info here on meta. $\endgroup$– Martin SleziakJun 22, 2017 at 0:01
You can read all about migration and how it works here: What is migration and how does it work?
As explained at that post, migrations should be done with circumspection; for a variety of reasons it's not a good idea to migrate something which is likely to be closed at the receiving site. Therefore the people best placed to judge when migration is appropriate are those familiar with the culture of the other site and who can sense whether it would be well-received there. In the case of Math.SE, my understanding is that questions that look like homework (the linked question could very well be an example) and that don't give indication that the OP put much thought or effort into it are generally not well-received there. In such cases it's better just to point to Math.SE as a better place for such questions than MO, thus giving the OP a chance to adapt the question before posting it there. Whereas in other cases, it may be obvious that migration to Math.SE (or wherever) is in order.
$\begingroup$ It seems to me that we need to treat this kind of issue more seriously. For example, I have just seen this closed question without explicit reference to the MSE (and a suggestion to add more thoughts, say). $\endgroup$– user20948Feb 19, 2021 at 18:17
$\begingroup$ What do you mean, no explicit reference to MSE? It's there in the closure announcement. Perhaps you mean to change the message in that announcement? $\endgroup$– Todd Trimble ModFeb 19, 2021 at 18:43
$\begingroup$ If I remember correctly, there was an option "more appropriate for other sites like MSE". Now seemingly this disappears. Personally, I would guess that the current indication might confuse a new user, especially when there are mathematical comments left. $\endgroup$– user20948Feb 19, 2021 at 19:07
$\begingroup$ No, among community-based reasons, it's just this, or "This question does not appear to be about research level mathematics within the scope defined in the help center", or a custom-built reason (user-specified). The other reasons to close are duplicate, needs clarity or focus, needs more details, and opinion-based. That's it. Might you be thinking of a different site? $\endgroup$– Todd Trimble ModFeb 19, 2021 at 22:16