1 2 Journal of Integer Sequences, Vol. 13 (2010), 3 Article 10.6.7 47 6 23 11 Generalized j-Factorial Functions, Polynomials, and Applications Maxie D. Schmidt University of Illinois, Urbana-Champaign Urbana, IL 61801 USA
[email protected] Abstract The paper generalizes the traditional single factorial function to integer-valued mul- tiple factorial (j-factorial) forms. The generalized factorial functions are defined recur- sively as triangles of coefficients corresponding to the polynomial expansions of a subset of degenerate falling factorial functions. The resulting coefficient triangles are similar to the classical sets of Stirling numbers and satisfy many analogous finite-difference and enumerative properties as the well-known combinatorial triangles. The generalized triangles are also considered in terms of their relation to elementary symmetric poly- nomials and the resulting symmetric polynomial index transformations. The definition of the Stirling convolution polynomial sequence is generalized in order to enumerate the parametrized sets of j-factorial polynomials and to derive extended properties of the j-factorial function expansions. The generalized j-factorial polynomial sequences considered lead to applications expressing key forms of the j-factorial functions in terms of arbitrary partitions of the j-factorial function expansion triangle indices, including several identities related to the polynomial expansions of binomial coefficients. Additional applications include the formulation of closed-form identities and generating functions for the Stirling numbers of the first kind and r-order harmonic number sequences, as well as an extension of Stirling’s approximation for the single factorial function to approximate the more general j-factorial function forms. 1 Notational Conventions Donald E.