%N 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 This problem is slightly different from A000769 or A219760. In the first example on an 8 x 8 board, are the queens c7, d5 and e3 (or queens a2, c5 and e8) 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 Vaclav Kotesovec, <a href="/A225623/a225623.pdf">Examples</a>
%Y Cf. A000170, A000769, A001499, A001501, A058528, A075754, A172544, A172541.
%A _Vaclav Kotesovec_, Aug 04 2013
%E Definition clarified by _Vaclav Kotesovec_, Dec 18 2014