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Rooks Problem


The rook is a Chess piece which may move any number of spaces either horizontally or vertically per move. The maximum number of nonattacking rooks which may be placed on an $n\times n$ Chessboard is $n$. This arrangement is achieved by placing the rooks along the diagonal (Madachy 1979). The total number of ways of placing $n$ nonattacking rooks on an $n\times n$ board is $n!$ (Madachy 1979, p. 47). The number of rotationally and reflectively inequivalent ways of placing $n$ nonattacking rooks on an $n\times n$ board are 1, 2, 7, 23, 115, 694, ... (Sloane's A000903; Dudeney 1970, p. 96; Madachy 1979, pp. 46-54).

The minimum number of rooks needed to occupy or attack all spaces on an $8\times 8$ Chessboard is 8 (Madachy 1979), arranged in the same orientation as above.

Consider an $n\times n$ chessboard with the restriction that, for every subset of $\{1, \ldots, n\}$, a rook may not be put in column $s+j$ (mod $n$) when on row $j$, where the rows are numbered 0, 1, ..., $n-1$. Vardi (1991) denotes the number of rook solutions so restricted as $\mathop{\rm rook}(s,n)$. $\mathop{\rm rook}(\{1\},n)$ is simply the number of Derangements on $n$ symbols, known as a Subfactorial. The first few values are 1, 2, 9, 44, 265, 1854, ... (Sloane's A000166). $\mathop{\rm rook}(\{1,2\},n)$ is a solution to the Married Couples Problem, sometimes known as Ménage Numbers. The first few Ménage Numbers are $-1$, 1, 0, 2, 13, 80, 579, ... (Sloane's A000179).

Although simple formulas are not known for general $\{1$, ..., $p\}$, Recurrence Relations can be used to compute $\mathop{\rm rook}(\{1, \dots, p\},n)$ in polynomial time for $p=3$, ..., 6 (Metropolis et al. 1969, Minc 1978, Vardi 1991).

See also Chess, Ménage Number, Rook Number, Rook Reciprocity Theorem


Dudeney, H. E. ``The Eight Rooks.'' §295 in Amusements in Mathematics. New York: Dover, p. 88, 1970.

Kraitchik, M. ``The Problem of the Rooks'' and ``Domination of the Chessboard.'' §10.2 and 10.4 in Mathematical Recreations. New York: W. W. Norton, pp. 240-247 and 255-256, 1942.

Madachy, J. S. Madachy's Mathematical Recreations. New York: Dover, pp. 36-37, 1979.

Metropolis, M.; Stein, M. L.; and Stein, P. R. ``Permanents of Cyclic (0, 1) Matrices.'' J. Combin. Th. 7, 291-321, 1969.

Minc, H. §3.1 in Permanents. Reading, MA: Addison-Wesley, 1978.

Riordan, J. Chs. 7-8 in An Introduction to Combinatorial Analysis. Princeton, NJ: Princeton University Press, 1978.

Sloane, N. J. A. Sequences A000903/M1761 A000166/M1937, and A000179/M2062 in ``An On-Line Version of the Encyclopedia of Integer Sequences.'' and extended entry in Sloane, N. J. A. and Plouffe, S. The Encyclopedia of Integer Sequences. San Diego: Academic Press, 1995.

Vardi, I. Computational Recreations in Mathematica. Reading, MA: Addison-Wesley, pp. 123-124, 1991.

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