Lattice paths and branched continued fractions: An infinite sequence of generalizations of the Stieltjes--Rogers and Thron--Rogers polynomials, with coefficientwise Hankel-total positivity

TL;DR

This paper introduces a new family of polynomials generalizing classical sequences, proves their Hankel-total positivity, and applies these results to various combinatorial and hypergeometric series, revealing deep structural properties.

Contribution

It defines an infinite sequence of generalized polynomials via branched continued fractions and proves their coefficientwise Hankel-total positivity, extending classical results and connecting combinatorics with hypergeometric functions.

Findings

Proved Hankel-total positivity for generalized polynomials.

Derived branched continued fractions for hypergeometric ratios.

Connected combinatorial structures with hypergeometric series.

Abstract

We define an infinite sequence of generalizations, parametrized by an integer $m \ge 1$, of the Stieltjes--Rogers and Thron--Rogers polynomials; they arise as the power-series expansions of some branched continued fractions, and as the generating polynomials for $m$-Dyck and $m$-Schr\"oder paths with height-dependent weights. We prove that all of these sequences of polynomials are coefficientwise Hankel-totally positive, jointly in all the (infinitely many) indeterminates. We then apply this theory to prove the coefficientwise Hankel-total positivity for combinatorially interesting sequences of polynomials. Enumeration of unlabeled ordered trees and forests gives rise to multivariate Fuss--Narayana polynomials and Fuss--Narayana symmetric functions. Enumeration of increasing (labeled) ordered trees and forests gives rise to multivariate Eulerian polynomials and Eulerian symmetric functions, which include the univariate $m$th-order Eulerian polynomials as specializations. We also find branched continued fractions for ratios of contiguous hypergeometric series ${}_r \! F_s$ for arbitrary $r$ and $s$, which generalize Gauss' continued fraction for ratios of contiguous ${}_2 \! F_1$; and for $s=0$ we prove the coefficientwise Hankel-total positivity. Finally, we extend the branched continued fractions to ratios of contiguous basic hypergeometric series ${}_r \! \phi_s$.