This question has been migrated from MSE by a moderator as well as closed for being off-topic. As I recall, MSE consider to have in principle a scope that encompasses the scope of MO (i.e. all math questions), so that in principle no question would have to be migrated this way. There certainly are questions which should be migrated to MO since they would fit best here, but in this case it feels not very appropriate. Should we migrate it back -but it would feel a bit rude? Is there a way to discuss between the two sides what to do with such a question? Do I just fail to see why this question is suitable for MO?
Just close the question as off-topic, this will reverse the migration. The original will stay closed, but you can flag it for moderator attention on MSE to get it reopened.
Closing is the main method to reject migrations as intended by Stack Exchange, using it will cause the least amount of trouble.
Remigration is possible, but to be honest this is a really, really scary option. I'm a moderator on two other SE sites, and I have no idea what would happen if I tried to remigrate a question. I know from previous cases that all kinds of stuff can go wrong in multiple migration cases, the software is simply not designed for this case.