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 A143891 #7 Nov 25 2018 12:29:53 %S A143891 1,2,1,5,2,15,7,68,23,370,104,2649,571,25209,4176,313503,39827, %T A143891 5212881,500254,114569731,8348616 %N A143891 Number of ways of placing kings with no more than 4 mutual attacks on an n X n chessboard symmetric about the diagonal and under 90-degree rotation. %K A143891 nonn,more,hard %O A143891 0,2 %A A143891 _R. H. Hardin_, Sep 04 2008