This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A352099 #31 Dec 29 2024 05:01:49
%S A352099 1,2,3,12,39,388,8102,656108,199727714,224693292768,893966897828288,
%T A352099 12397352268917562436,598097093939369977901540,
%U A352099 100707091308314174859433507948,59497535893138933753768955970555554,124081719421265185713331815803874814236572,919072633264334061873768956083917736204779032768
%N A352099 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 A352099 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 A352099 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.
%H A352099 Eugene Nonko, <a href="/A352099/b352099.txt">Table of n, a(n) for n = 1..45</a>
%H A352099 Project Euler, <a href="https://projecteuler.net/problem=626">Problem 626: Counting Binary Matrices</a>.
%Y A352099 Cf. A007299, A111368, A353052.
%K A352099 nonn
%O A352099 1,2
%A A352099 _Nathaniel Johnston_, May 05 2022
%E A352099 a(8) onwards from _Eugene Nonko_, Nov 30 2024