Mathematics : Physics = Grammar : Literature
While many writers may be very good at grammar, one would not ask a pure grammar question to a writer, but to a linguist.
There is a big difference between a question of mathematics as applied to physics and a question of pure mathematics. While many physicists are good at applying mathematics, only a few of them have the breadth or depth of knowledge that a mathematician has.
My fear is that pure mathematical questions would not receive as good answers here ans on math.se or mathoverflow and my very strong suspicion is that they would not receive a good peer review because of course pure mathematics is a larger set of concepts and skills as compared to the subset used in physics.
We want to maximize usefulness of the network and minimize effort of the communities:
Where would someone likely go to look for an answer to this question? Differential equations are very useful in physics, but most DE questions probably live in our maths sites. We don't want to have the same question in two places, or worse, questions randomly spread between the two sites.
Finally, remember that if you can answer a question here, you can also do so on the maths sites!
In short:
- migrate pure maths questions to the appropriate level mathematics site
- criterion to define "pure maths": if answering the question requires physics knowledge, then keep the question; if answering the question does not require any physics knowledge, migrate away.