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 A051568 #19 May 23 2014 00:40:44 %S A051568 4,2,1,1,5,2,15,3 %N A051568 Let M(n) (A051755) be the maximal number of queens that can be placed on an n X n chessboard so that each queen attacks exactly two other queens; a(n) is the number of non-equivalent solutions. "Non-equivalent" means none of the a(n) solutions can be mapped onto any other solution by board rotations through 90, 180 or 270 degrees or mirror operations along the two diagonals or center lines. %D A051568 M. Gardner, The Last Recreations, Springer, 1997, p. 282. %D A051568 M. Gardner, The Colossal Book of Mathematics, 2001, p. 209. %H A051568 Ken Duisenberg, <a href="http://ken.duisenberg.com/potw/archive/arch00/000628.html">Doubly Attacking Queens, POTW 2000</a>. %Y A051568 Cf. A051567-A051571, A051754-A051759, A019654. %K A051568 nonn,nice,more %O A051568 3,1 %A A051568 _N. J. A. Sloane_, Dec 11 1999 %E A051568 More precise definition from _R. J. Mathar_, Mar 13 2006 %E A051568 Edited by _N. J. A. Sloane_, May 22 2014