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%I #16 Oct 01 2021 19:07:05
%S 1,2,4,31,307,2,9,755
%N Consider the problem of placing A051756(n) queens on an n X n board so that each queen attacks precisely 3 others. Sequence gives number of solutions up to square symmetry.
%C A051569 (taken from the Martin Gardner references) is another version of this sequence. - _N. J. A. Sloane_, May 22 2014
%D M. Gardner, The Last Recreations, Springer, 1997, p. 282.
%D M. Gardner, The Colossal Book of Mathematics, 2001, p. 209.
%H Erich Friedman, <a href="/A051759/a051759.gif">Illustration of initial terms</a>
%H Sean A. Irvine, <a href="https://github.com/archmageirvine/joeis/blob/master/src/irvine/oeis/a051/A051759.java">Java program</a> (github)
%Y Cf. A051754-A051758, A051567-A051571, A019654.
%K nonn,nice,more
%O 2,2
%A Robert Trent (trentrd(AT)hotmail.com), Aug 23 2000
%E a(8)-a(9) from _Sean A. Irvine_, Oct 01 2021