Classical rook theory was developed in the 1940's by Riordan and Kaplansky as a
framework for studying permutations with restricted position. In the game of chess,
rooks are permitted to move horizontally or vertically across the board to attack an
opposing piece. Thus, one can envision a permutation as a placement of n rooks on
an n x n chessboard such that no two rooks have the ability to attack one another
(i.e. no two rooks are in the same row or column). The notions of rook numbers and
hit numbers were formulated, with the k-th hit number counting the number of ways
that n non-attacking rooks can be placed on an n x n chessboard such that k of those
rooks lie on a certain predened subset of the squares on that board. A fundamental
result of rook theory shows that these hit numbers can be calculated in terms of
the more easily calculated rook numbers, where the k-th rook number is dened to
be the number of ways of placing k non-attacking rooks on some predened subset
of squares on a chessboard. One can likely already see the potential applications
of this theory to permutations with restricted position. For instance, if we take
the predened subset to be the squares along the diagonal from bottom left to top
right on an n x n chessboard, the 0-th hit number corresponds to the number of
permutations in Sn with no xed points (i.e. the number of derangements in Sn).
Similarly, any problem of permutations with restricted position can be formulated
in terms of rook numbers and hit numbers