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 A333166 #4 Mar 14 2020 18:54:16
%S A333166 1,2,3,12,118,9638,10622074,135037240786,18621890255342234,
%T A333166 28688490385422625653266,511030957184968000138445253202
%N A333166 Number of n-regular graphs on 2n unlabeled vertices with half-edges.
%C A333166 A half-edge is like a loop except it only adds 1 to the degree of its vertex.
%C A333166 a(n) is the number of non-isomorphic 2n X 2n symmetric matrices with entries in {+1, -1} and all rows and columns summing to zero where isomorphism is up to simultaneous permutation of rows and columns. The case where rows and columns can be permuted independently is covered by A333165.
%F A333166 a(n) = A333161(2*n, n).
%F A333166 Cf. A333164, A333165.
%e A333166 The a(1) = 1 matrix is:
%e A333166 [+ -]
%e A333166 [- +]
%e A333166 .
%e A333166 The a(2) = 2 matrices are:
%e A333166 [+ + - -] [- - + +] [+ + - -]
%e A333166 [+ + - -] [- - + +] [+ - + -]
%e A333166 [- - + +] [+ + - -] [- + - +]
%e A333166 [- - + +] [+ + - -] [- - + +]
%Y A333166 Central coefficients of A333161.
%K A333166 nonn,more
%O A333166 0,2
%A A333166 _Andrew Howroyd_, Mar 12 2020