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%I #10 May 06 2022 10:26:43
%S 1,2,3,12,39,388,8102
%N Number of inequivalent {-1,1} matrices of order n, up to permutation of rows and/or columns and multiplication of rows and/or columns by -1.
%C The equivalence operations described in the title are commonly used when discussing Hadamard matrices, for example (see A007299). See A353052 for the version of this sequence that also considers transposition as part of the equivalence relation.
%C Since the row and column multiplication operations can be used to force the first row and column to consist only of ones, 2^((n-1)^2) is an upper bound on this sequence. A lower bound is 2^((n-1)^2) / (n!)^2.
%Y Cf. A111368, A353052.
%K nonn,hard,more
%O 1,2
%A _Nathaniel Johnston_, May 05 2022