What is the policy on crossposting to/from MathOverflow or other StackExchange sites?
Crossposting from MathOverflowMathOverflow and other StackExchange sites is perfectly fine, as long as they aren't done in parallel. That is, if you post a question on one site, you should only post to the other site after you have not received a satisfactory answer for some time, and you should provide a link in each post (or in the comments on it) to the other one. As a courtesy, if you post your question here after trying MathOverflow or another StackExchange site, please try to integrate in your question the answers you received on MathOverflow/the other site (even if they did not answer everything).