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 A086900 #22 Jul 02 2025 11:34:28
%S A086900 1,1,5,338,14186,526876,52658844,28076946520,18518751047608,
%T A086900 13637385623943256
%N A086900 Number of real n X n symmetric (0,1) matrices with positive determinant.
%H A086900 <a href="/index/Mat#binmat">Index entries for sequences related to binary matrices</a>
%F A086900 a(n) = A086899(n) - A118996(n) = 2^(n*(n+1)/2) - A086906(n) - A118996(n). - _Max Alekseyev_, Jun 12 2025
%e A086900 For n = 2 the only example is the identity matrix.
%t A086900 triamat[li_List] := Block[{len=Sqrt[8Length[li]+1]/2-1/2}, If[IntegerQ[len], Part[li, # ]& /@ Table[If[j>i, j(j-1)/2+i, i(i-1)/2+j], {i, len}, {j, len}], li]]; 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 A086900 Cf. A086899, A086906, A118994, A118995, A118996, A119002, A119008.
%K A086900 nonn,more
%O A086900 1,3
%A A086900 _Wouter Meeussen_, Aug 23 2003
%E A086900 a(6)-a(7) from _Giovanni Resta_, May 08 2006
%E A086900 a(8)-a(10) from _Max Alekseyev_, Jun 17 2025