
A Ferrers board \((h_1,\dots, h_n)\) is a board which takes the lower \(h_i\) cells of the \(i\)th column of the \(n\times N\) chessboard where \(0\leq h_1\leq\cdots\leq h_n\). The theory of non-taking rooks on Ferrers boards was first developed by \textit{D. Foata} and \textit{M. P. Schützenberger} [Comb. Theory Appl., Colloq. Math. Soc. János Bolyai 4, 413-436 (1970; Zbl 0217.01802)]. \textit{J. R. Goldman} et al. [Proc. Am. Math. Soc. 52, 485-492 (1975; Zbl 0312.05002)] showed that their rook polynomial of a Ferrers board completely factors into linear polynomials over the nonnegative integers. In the present paper the authors introduce the \(i\)-creation rook placement rule where \(i\) new rows, on which rooks may subsequently be placed, are drawn above and to the right in the row where a rook is placed (\(i= 0\) is classic rook placement). The \(i\)-rook number \(r^{(i)}_k(B)\) of a Ferrers board \(B\) is the number of \(i\)-creation rook placements of \(k\) non-taking rooks on \(B\) where \(r^{(i)}_0(B)= 1\). The \(i\)-rook polynomial of \(B\) is \(r^{(i)}(B, x)= \sum^n_{k=0} r^{(i)}_k(B) x^{(n- k,i-1)}\) where \(x^{(n,m)}= x(x+ m)\cdots (x+ (n-1)m)\) and \(x^{(0,m)}\equiv 1\). The authors show that \(r^{(i)}(B, x)= \prod^n_{j=1} (x+ h_j+ (j-1)(i-1))\) for a Ferrers board \(B= (h_1,\dots, h_n)\). The \(m\)-jump board \(J_{n,m}\) is the Ferrers board \((0,m,2m,\dots, (n-1)m)\). From the factorization theorem, \(r^{(1)}(J_{n,1}, x)= x^{(n,1)}\) so that \(r^{(1)}_k(J_{n, 1})\) is the unsigned Stirling number of the first kind \({\mathfrak s}(n,n-k)\); two bijective proofs are also given (the superscript (i) in Theorem 3.1 should be (1)). Two bijective proofs are given that \(r^{(2)}_k(J_{n,1})\) is the number of \(k\)-edge matchings in the complete graph \(K_{n+k-1}\). Consequently, \(\sum^{n+1}_{k=0} r^{(2)}_k(J_{n+1,1}) x^k\) is the Bessel polynomial of degree \(n\). The Abel board \(A_n\) is the \(n\)-column Ferrers board \((0,n,\dots, n)\). From the factorization theorem, \(r^{(1)}(A_n, x)= x(x+ n)^{n-1}\) (a special Abel polynomial) so that \(r^{(1)}_k(A_n)\) is the number of labelled forests on \(n\) vertices composed of \(n-k\) rooted trees; a bijective proof is also given (the sum for \(r^{(1)}(A_n, x)\) in the paper should run from \(0\) to \(n-1\)). Finally, a weighted generalization of the classic rook polynomial is given and a factorization theorem and reciprocity theorem proven for them. The paper concludes by giving \(q\)-analogs of several results.
forests, binomial type, absolute Stirling numbers, Abel polynomials, multiset permutations, Exact enumeration problems, generating functions, rook placement, rook polynomial, Theoretical Computer Science, rook theory, factorization theorem, Computational Theory and Mathematics, \(q\)-calculus and related topics, Bessel polynomials, Discrete Mathematics and Combinatorics, Ferrers board
forests, binomial type, absolute Stirling numbers, Abel polynomials, multiset permutations, Exact enumeration problems, generating functions, rook placement, rook polynomial, Theoretical Computer Science, rook theory, factorization theorem, Computational Theory and Mathematics, \(q\)-calculus and related topics, Bessel polynomials, Discrete Mathematics and Combinatorics, Ferrers board
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