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 A118990 #10 Jun 17 2025 22:05:34 %S A118990 0,4,32,512,15872,907008,104535552,22523623424,9599255461888, %T A118990 7747175087620096 %N A118990 Number of symmetric singular n X n (+1,-1) matrices over the reals. %F A118990 a(n) = 2^(n*(n+1)/2) - A118994(n) - A118997(n) = 2^(n*(n+1)/2) - A118992(n). - _Max Alekseyev_, May 08 2009 %Y A118990 Cf. A086906, A119010, A057982, A118992. %K A118990 nonn,more %O A118990 1,2 %A A118990 _Giovanni Resta_, May 08 2006 %E A118990 a(8)-a(10) from _Max Alekseyev_, Jun 17 2025