A051906 Number of ways of placing n nonattacking queens on an n X n toroidal chessboard.

1, 0, 0, 0, 10, 0, 28, 0, 0, 0, 88, 0, 4524, 0, 0, 0, 140692, 0, 820496, 0, 0, 0, 128850048, 0, 1957725000, 0, 0, 0, 605917055356, 0, 13404947681712, 0, 0, 0

Offset

1,5

Comments

The sequence has been computed up to n = 23 by Rivin, Vardi & Zimmermann, see p. 637 of their paper from 1994. Further terms were calculated by the submitter, Dec 17 1999 and Jan 11 2001.

a(n) is divisible by n.

Only terms indexed by odd numbers coprime to 3 are nonzero, therefore A007705(n) = a(2n+1) is the main entry. - M. F. Hasler, Jul 01 2019

From Eduard I. Vatutin, Nov 27 2023: (Start)

For n <= 11 all solutions can be found using a knight moving with some displacements dx and dy starting from some cell with coordinates (x,y): (x,y) -> (x+dx,y+dy) -> (x+2*dx,y+2*dy) -> ... -> (x,y) (all operations modulo n). For n >= 13 some solutions are same, but not all (see examples).

All solutions of n-queens problem on toroidal chessboard are also solutions of n-queens problem on classical chessboard; the converse is not true, so a(n) <= A000170(n).

(End)

Links

Table of n, a(n) for n=1..34.

M. R. Engelhardt, A group-based search for solutions of the n-queens problem, Discr. Math., 307 (2007), 2535-2551.

Vaclav Kotesovec, Non-attacking chess pieces, 6ed, 2013, p. 62-63.

Kevin Pratt, Closed-Form Expressions for the n-Queens Problem and Related Problems, arXiv:1609.09585 [cs.DM], 2016.

I. Rivin, I. Vardi and P. Zimmermann, The n-queens problem, Amer. Math. Monthly, 101 (1994), 629-639.

Formula

a(n) = A071607((n-1)/2) * n for odd n. - Eduard I. Vatutin, Nov 27 2023, corrected Apr 10 2024

Example

From Eduard I. Vatutin, Nov 27 2023: (Start)
n=5 (all 10 solutions are shown below):
+-----------+ +-----------+ +-----------+ +-----------+ +-----------+
| Q . . . . | | Q . . . . | | . Q . . . | | . Q . . . | | . . Q . . |
| . . Q . . | | . . . Q . | | . . . Q . | | . . . . Q | | Q . . . . |
| . . . . Q | | . Q . . . | | Q . . . . | | . . Q . . | | . . . Q . |
| . Q . . . | | . . . . Q | | . . Q . . | | Q . . . . | | . Q . . . |
| . . . Q . | | . . Q . . | | . . . . Q | | . . . Q . | | . . . . Q |
+-----------+ +-----------+ +-----------+ +-----------+ +-----------+
+-----------+ +-----------+ +-----------+ +-----------+ +-----------+
| . . Q . . | | . . . Q . | | . . . Q . | | . . . . Q | | . . . . Q |
| . . . . Q | | Q . . . . | | . Q . . . | | . Q . . . | | . . Q . . |
| . Q . . . | | . . Q . . | | . . . . Q | | . . . Q . | | Q . . . . |
| . . . Q . | | . . . . Q | | . . Q . . | | Q . . . . | | . . . Q . |
| Q . . . . | | . Q . . . | | Q . . . . | | . . Q . . | | . Q . . . |
+-----------+ +-----------+ +-----------+ +-----------+ +-----------+
n=7:
+---------------+
| Q . . . . . . |
| . . . Q . . . |
| . . . . . . Q |
| . . Q . . . . |
| . . . . . Q . |
| . Q . . . . . |
| . . . . Q . . |
+---------------+
n=11:
+-----------------------+
| Q . . . . . . . . . . |
| . . Q . . . . . . . . |
| . . . . Q . . . . . . |
| . . . . . . Q . . . . |
| . . . . . . . . Q . . |
| . . . . . . . . . . Q |
| . Q . . . . . . . . . |
| . . . Q . . . . . . . |
| . . . . . Q . . . . . |
| . . . . . . . Q . . . |
| . . . . . . . . . Q . |
+-----------------------+
n=13 (first example can be found using a knight moving from cell (1,1) with dx=1 and dy=2, second example can't be obtained in the same way):
+---------------------------+ +---------------------------+
| Q . . . . . . . . . . . . | | Q . . . . . . . . . . . . |
| . . Q . . . . . . . . . . | | . . Q . . . . . . . . . . |
| . . . . Q . . . . . . . . | | . . . . Q . . . . . . . . |
| . . . . . . Q . . . . . . | | . . . . . . Q . . . . . . |
| . . . . . . . . Q . . . . | | . . . . . . . . . . . Q . |
| . . . . . . . . . . Q . . | | . . . . . . . . . Q . . . |
| . . . . . . . . . . . . Q | | . . . . . . . . . . . . Q |
| . Q . . . . . . . . . . . | | . . . . . Q . . . . . . . |
| . . . Q . . . . . . . . . | | . . . Q . . . . . . . . . |
| . . . . . Q . . . . . . . | | . Q . . . . . . . . . . . |
| . . . . . . . Q . . . . . | | . . . . . . . Q . . . . . |
| . . . . . . . . . Q . . . | | . . . . . . . . . . Q . . |
| . . . . . . . . . . . Q . | | . . . . . . . . Q . . . . |
+---------------------------+ +---------------------------+
(End)

Crossrefs

See A007705, which is the main entry for this sequence.

Cf. A000170, A071607.

Sequence in context: A107781 A107772 A087474 * A171809 A167302 A302576

Adjacent sequences: A051903 A051904 A051905 * A051907 A051908 A051909

Keyword

nonn,nice,hard,more

Author

Matthias Engelhardt, Dec 17 1999

Extensions

Term a(31) added from A007705 by Vaclav Kotesovec, Aug 25 2012

Status

approved