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 A225623 #29 Jan 09 2022 09:38:11 %S A225623 0,1,2,11,92,1097,19448,477136,14244856,537809179,24194010708, %T A225623 1317062528249 %N A225623 Number of ways to arrange 2n queens on an n X n chessboard, with no more than 2 queens in each row, column or diagonal. %C A225623 This problem is slightly different from A000769 or A219760. In the first example on an 8 x 8 board, the queens c7, d5 and e3 (or queens a2, c5 and e8) are in a line, but such case is allowed. The elementary step can be only [0,1], [1,0] or [1,1], not for example [1,2] or [2,3]. %H A225623 Vaclav Kotesovec, <a href="/A225623/a225623.pdf">Examples</a> %Y A225623 Cf. A000170, A000769, A001499, A001501, A058528, A075754, A172544, A172541. %K A225623 nonn,more,hard %O A225623 1,3 %A A225623 _Vaclav Kotesovec_, Aug 04 2013 %E A225623 Definition clarified by _Vaclav Kotesovec_, Dec 18 2014 %E A225623 a(10)-a(12) from _Martin Ehrenstein_, Jan 09 2022