If I post a question that I invented myself on Mathematics SE, I receive comments like "did you do homework?" and, eventually, "why do you ask if you know?". Fair enough. It seems Mathematics SE is about asking questions one does *not* know answer for.

On Puzzling SE, it is almost the other way around: one is supposed to know the answer and one can challenge others to find answers.

What is MathOverflow's policy? Can I ask questions I invented myself on MathOverflow, perhaps to challenge others, but, also to learn that what I invented is known and **not** new or original?

Thanks for letting me know. I have a few questions I *think* are original and I would be interested to learn from experts if these are.

canask questions to which one knows the answer, there's even a way to tell the software that you are going to answer your own question. The intention there is that your question and answer get posted simultaneously. What's considered unacceptable there is posting a question without letting on that you know the answer. $\endgroup$state in the questionthat you know the answer. $\endgroup$