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Undeleted, reopened, reclosed (as duplicate) Yesterday I posted this this question. When I checked it last night, it had the two sound-looking answers you see. I woke up today to find it had been not just closed but completely deleted. This seems to me completely preposterous. It's pretty clearly not a homework question, but a reasonably deep integral ...


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Deleted, Undeleted Proving $\sum_{k = 1}^{n} (-1)^{k}{n \choose k} {nk \choose m} = 0$. Question: Show that $\sum_{k = 1}^{n} (-1)^{k}{n \choose k} {nk \choose m} = 0$ for $1\le m\le n-1$. I think this was a nice combinatorics question, with two well-written answers that offer interesting perspectives, in addition to an answer of my own. I see why it was ...


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Reopened Please reopen https://math.stackexchange.com/q/4074358. I have also voted for closing the question as a duplicate but retracted the vote after the question was edited. It is clear from the content of the edited question that its issue was a search for an error in the OP solution (and not for a correct way of solving the problem). In this sense it ...


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