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 A333164 #9 Apr 08 2020 17:13:57
%S A333164 1,2,18,1760,1944530,26615510712,4762109992158288,
%T A333164 11528251571501588791296,386860001875783390762182911250,
%U A333164 183238648400953515891207813311894423000,1242327573587456839123512395835593196519781229768,121917941188016997420391711911475819481799846311888884541952
%N A333164 Number of 2n X 2n symmetric matrices with entries in {+1, -1} and all rows and columns summing to zero.
%C A333164 a(n) is the number of 2n X 2n symmetric binary matrices with n ones in every row and column.
%F A333164 a(n) = A333157(2*n, n).
%F A333164 a(n) = A213793(2*n). - _Andrew Howroyd_, Apr 08 2020
%e A333164 The a(1) = 2 matrices are:
%e A333164 [ 1 -1] [-1 1]
%e A333164 [-1 1] [ 1 -1]
%Y A333164 Central coefficients of A333157.
%Y A333164 Cf. A213793, A333165, A333166.
%K A333164 nonn
%O A333164 0,2
%A A333164 _Andrew Howroyd_, Mar 11 2020