Integer Parameter Shifts in One-Variable Restrictions of Lauricella's $F_C$

Abstract

We construct rational functions of the coefficient index for one-variable restrictions of Lauricella's function $F_C$ at arguments related by cube roots of unity, with arbitrary integer differences among the lower parameters. We express the product coefficients in each residue class as these rational functions times the coefficients of Henrici's known triple product. We obtain finite sums of ordinary ${}_8F_7$ functions whose lower parameters have positive real part, with each summand multiplied by a nonnegative integer power of the variable, and give an explicit formula when the first of three equal lower parameters is increased by one, together with bounds for the polynomial degrees and number of summands. We interpret the expressions for the three residue classes by taking limits of the entire summands when upper and lower parameters approach singular values together. We prove that the Taylor series has radius of convergence exactly $1/4$ whenever all original lower and upper parameters avoid nonpositive integers, without assuming integer differences among the lower parameters.