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 A344676 #28 Jun 16 2021 04:08:24 %S A344676 1,2,6,48,120,1440,5040,2903040,203575680,41157849600,2414207980800 %N A344676 The number of n X n binary orthogonal matrices having an equal number of ones in each row. %C A344676 The inverse of an orthogonal matrix is its transpose. This implies the dot product of a row with itself must be 1. This further implies the number of ones in each row must be odd. Given that orthogonal matrices form a group, it must be the case the transpose is also an orthogonal matrix. This requires every column of a binary orthogonal matrix also have an odd number of ones. %C A344676 For 1 <= n <= 4 the counts are the same for the total number of binary orthogonal matrices (A003053). %e A344676 a(7) = 5040. There are 5040 7 X 7 binary orthogonal matrices where all rows have an equal number of ones. %Y A344676 Cf. A003053. %K A344676 nonn,hard,more %O A344676 1,2 %A A344676 _Nathan J. Russell_, May 26 2021 %E A344676 a(9)-a(10) from _Martin Ehrenstein_, Jun 13 2021 %E A344676 a(11) from _Martin Ehrenstein_, Jun 16 2021