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 A189843 #6 Sep 12 2015 11:00:25 %S A189843 1,2,5,18,71,356,2097,14212,105821,887576,8093601,81310936,876456695, %T A189843 10257217440,127631146697,1705775408656 %N A189843 Number of ways to place n nonattacking composite pieces rook + semi-rider[2,2] on an n X n chessboard. %C A189843 a(n) is also number of permutations p of 1,2,...,n satisfying p(j+2k)-p(j)<>2k for all j>=1, k>=1, j+2k<=n %C A189843 about semi-pieces see semi-bishop (A187235) and semi-queen (A099152) %H A189843 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 A189843 Wikipedia, <a href="http://en.wikipedia.org/wiki/Fairy_chess_piece">Fairy chess piece</a> %Y A189843 Cf. A189281, A099152, A189838, A187235 %K A189843 nonn,hard %O A189843 1,2 %A A189843 _Vaclav Kotesovec_, Apr 29 2011