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The related OEIS sequence is https://oeis.org/A000170. It only gives exact values for n up to and including 27. The value for n = 27 was calculated with the help of an FPGA. See: https://github.com/preusser/q27

Thanks to this I know that there is exactly 1 way of positioning 0 queens on 0x0 chessboard without any two of them attacking each other.