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 A086899 #21 Jul 02 2025 11:34:25
%S A086899 1,4,32,528,18596,1280468,180452552,49970930912,27618771417328,
%T A086899 30088644932329872
%N A086899 Number of real n X n invertible symmetric (0,1) matrices.
%H A086899 <a href="/index/Mat#binmat">Index entries for sequences related to binary matrices</a>
%F A086899 a(n) = A086900(n) + A118996(n) = 2^(n*(n+1)/2) - A086906(n). - _Max Alekseyev_, Jun 12 2025
%e A086899 For n = 2 the 4 matrices are 10/01, 01/10, 11/10, 01/11.
%t A086899 triamat[li_List] := (*see A086900*); Table[it=triamat/@IntegerDigits[Range[0, -1+2^(n(n+1)/2)], 2, n(n+1)/2]; Count[it, (q_)?MatrixQ/;(Det[q]=!=0)], {n, 5}]
%Y A086899 Cf. A086900, A086906, A118991, A118985, A118992, A118993, A118996.
%K A086899 nonn,more
%O A086899 1,2
%A A086899 _Wouter Meeussen_, Aug 23 2003
%E A086899 a(6) and a(7) from _Giovanni Resta_, May 08 2006
%E A086899 a(8)-a(10) from _Max Alekseyev_, Jun 17 2025