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## How to place k bishops on an nxn chessboard

In how many different ways can k bishops be placed on an nxn chessboard such that no two bishops attack each other? Please try to respond with a formula and explanation.

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 A quick pointer in the right direction: you want "rook polynomials" (en.wikipedia.org/wiki/Rook_polynomial), calculated separately for black squares and white squares, and then recombined somehow. Also, this is not "combinatorial game theory" as that term is usually understood; I'll remove the tag. – Michael Lugo Feb 7 2011 at 18:27

Call this number $B_k(n)$. For fixed $k$ it is known that $B_k(n)$ has the form $P_k(n)+(-1)^nQ_k(n)$, where $P_k$ and $Q_k$ are polynomials. These polynomials have been computed for (at least) $k\leq 6$. We also have (unsurprisingly) the asymptotic formula $B_k(n)\sim n^{2k}/k!$. For further information see http://www.math.binghamton.edu/zaslav/Tpapers/bishops.slides.20100729.pdf.
Update. I learned from Tom Zaslavsky that an explicit formula for $B_k(n)$ as a triple sum was given by C. E. Arshon in 1936.