Hypergeometric Mixed-Type Multiple Orthogonal Polynomials

Abstract

Two hypergeometric families of mixed-type multiple orthogonal forms are constructed on the positive real line for rank-one $q\times p$ matrices of weights, with arbitrary $p$ and $q$: Jacobi-like and Laguerre-like systems. In the admissible near-diagonal range, both mixed forms are obtained explicitly. Their power-vector components are single terminating generalized hypergeometric polynomials, whereas their hypergeometric-vector components are finite sums of such polynomials. In the Jacobi-like case these sums reduce to finite combinations of terminating Kampé de Fériet polynomials evaluated at $(x,1)$; in the mixed beta--Euler Laguerre-like case they are governed by a finite triangular connection system. Gamma-quotient Mellin formulas, Meijer $G$-representations, and Rodrigues-type formulas are obtained for the complete forms. On the step-line, the associated biorthogonal sequences satisfy dual recurrences with $p$ subdiagonals and $q$ superdiagonals, and every band coefficient is given explicitly by finite Gamma--Pochhammer expressions. Christoffel transformations and Gauss--Borel factorization yield bidiagonal factorizations of the recurrence matrices. The lower factors have closed Pochhammer formulas, while the upper factors are expressed through Christoffel tau-determinants or cross-ratios of shifted moment minors. Both chains close explicitly in the mixed Piñeiro specialization. Confluent limits connect the Jacobi-like systems with the Laguerre-like and Hermite-like systems.