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 A056990 #14 Feb 16 2025 08:32:43 %S A056990 1,2,8,192,22272,11550720,25629327360,236229525504000, %T A056990 8858686914082897920,1331751782100764385607680 %N A056990 Number of nonsingular n X n (-1,1)-matrices. %H A056990 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/NonsingularMatrix.html">Nonsingular Matrix.</a> %F A056990 a(n) = 2^(2*n-1) * A055165(n-1) for n>=1. %Y A056990 Cf. A055165, A056989. %K A056990 nonn,nice %O A056990 0,2 %A A056990 _Eric W. Weisstein_ %E A056990 a(5) from Winston C. Yang (winston(AT)cs.wisc.edu), Aug 26 2000 %E A056990 a(6)-a(8) computed from A055165 by _David desJardins_, Apr 22 2001 %E A056990 a(9) from _Max Alekseyev_, computed from A055165, Jan 15 2007 %E A056990 a(0)=1 prepended by _Alois P. Heinz_, May 29 2024