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I would appreciate some pointers on this problem about finding the number of ways to choose 3 squares from an $n\times n$ grid such that no two are on the same diagonal.

This is http://oeis.org/A172123, where an explicit formula is given, but I am not sure how to derive that formula. Unfortunately the reference by Karl Fabel is in German and I cannot read German.

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    The correct reference is to [A172124](http://oeis.org/A172124); your link is for two bishops.2012-06-05

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