If there is an answer to a question that I have asked on MathOverflow or MathStackExchange which helps me to prove a result appropriate for a published paper, what should I do:

**(1)** Cite the MO or MSE question as a bibliography item.

**(2)** Mention the username of the one who answered my question in the acknowledgements or where my question is cited.

**(3)** Email the one who answered my question, and ask him/her what he/she prefers, including adding him/her as a second author of the paper; the problem is that it is not always possible to find his/her email address, especially when the username is not the exact name of the user (as in my case..).

I guess that an answer to my current question depends on how significant the answer on MO or MSE is for the proof of the result in my paper; well, the answer in itself is quite 'trivial' either because a graduate student can understand it or because it relies on a known result in another field of mathematics (after being familiar with the known result, my question becomes easy).

Thank you very much.

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