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http://mathoverflow.net/questions/80942/how-to-find-the-sum is in my opinion a poorly written question, was cross-posted on MO minutes after M.SE, is filled with irrelevant details (defining mod p, for example), has weak motivation ('to analyze some properties of the prime numbers') and the user certainly has a poor history. It has at present 3 votes to close (with no real reasons given - Yemon pointed out the cross-posting). But I can't outright say why this could not fit on MO. I don't think we want MO to be a place where people post their most recent infinite sum and ask for a solution. Was there some discussion on this recently? The question about a certain trigonometric sum comes to mind, and that survived. But this has a different feel.
I think cross-posting is a decent but not great reason to close. Once a question is cross-posted, anyone here who knows how to answer the question is made aware that there is a good place to post it.
I must admit that I personally feel more lenient toward well-written or interesting questions, and that is probably because I would be interested in seeing the answers.
@Will - that much is obvious.
@Scott - "more lenient toward..." yes. And I don't see how the answer to this question will interested many people.
And the question is now closed. Answerers to M.SE!