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 A189840 #6 Sep 12 2015 11:00:25 %S A189840 1,2,6,24,108,544,3264,23040,171072,1409664,12916224,131217408, %T A189840 1428028032,16709309440,210367491840,2847184825728 %N A189840 Number of ways to place n nonattacking composite pieces rook + rider[4,4] on an n X n chessboard. %C A189840 a(n) is also number of permutations p of 1,2,...,n satisfying |p(j+4k)-p(j)|<>4k for all j>=1, k>=1, j+4k<=n %H A189840 V. Kotesovec, <a href="https://oeis.org/wiki/User:Vaclav_Kotesovec">Number of ways of placing non-attacking queens, kings, bishops and knights</a> (in English and Czech) %H A189840 Wikipedia, <a href="http://en.wikipedia.org/wiki/Fairy_chess_piece">Fairy chess piece</a> %Y A189840 Cf. A189255, A000170, A189838, A189839 %K A189840 nonn,hard %O A189840 1,2 %A A189840 _Vaclav Kotesovec_, Apr 29 2011