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%I #23 Mar 30 2020 09:13:36
%S 1,4,9,18,58,348,1862,10188,57600,376692,2640422,19469324,151978440,
%T 1258451524,10963084588,100087600184
%N Number of distinct ways to place queens (even fewer than n) on an n X n chessboard so that no queen is attacking another and that it is not possible to add another queen.
%C In other words, number of maximal independent vertex sets (and minimal vertex covers) in the n X n queen graph. - _Eric W. Weisstein_, Jun 20 2017
%H S. W. Golomb and L. D. Baumert, <a href="http://dx.doi.org/10.1145/321296.321300">Backtrack Programming</a>, Journal of the ACM, 4 (2001), 516-524.
%H Stefan Kral, <a href="/A146303/a146303.cpp.txt">C++11 code using OpenMP</a>
%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/MaximalIndependentVertexSet.html">Maximal Independent Vertex Set</a>
%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/MinimalVertexCover.html">Minimal Vertex Cover</a>
%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/QueenGraph.html">Queen Graph</a>
%e The a(2) = 4 solutions are to place a single queen in each of the squares of the chessboard. For n=3, there is a single one-queen solution (placing the queen in b2) and eight two-queen solutions, but no three-queen solution (see A000170).
%Y Cf. A000170, A146304.
%K hard,nonn,more
%O 1,2
%A _Paolo Bonzini_, Oct 29 2008
%E a(12)-a(16) from _Stefan Kral_, Aug 10 2016
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