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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. 2
4, 2, 1, 1, 5, 2, 15, 3 (list; graph; refs; listen; history; text; internal format)
OFFSET

3,1

REFERENCES

M. Gardner, The Last Recreations, Springer, 1997, p. 282.

M. Gardner, The Colossal Book of Mathematics, 2001, p. 209.

LINKS

Table of n, a(n) for n=3..10.

Ken Duisenberg, Doubly Attacking Queens, POTW 2000.

CROSSREFS

Cf. A051567-A051571, A051754-A051759, A019654.

Sequence in context: A288251 A051758 A024553 * A016509 A277646 A010313

Adjacent sequences:  A051565 A051566 A051567 * A051569 A051570 A051571

KEYWORD

nonn,nice,more

AUTHOR

N. J. A. Sloane, Dec 11 1999

EXTENSIONS

More precise definition from R. J. Mathar, Mar 13 2006

Edited by N. J. A. Sloane, May 22 2014

STATUS

approved

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Last modified December 6 19:22 EST 2019. Contains 329809 sequences. (Running on oeis4.)