Refined lattice path enumeration and combinatorial reciprocity

Abstract

It is well known that the set of $m$-Dyck paths with a fixed height and a fixed amount of valleys is counted by the Fu{\ss}-Narayana numbers. In this article, we consider the set of $m$-Dyck paths that start with at least $t$ north steps. We give exact formulas for the number of such paths with fixed height, fixed number of returns and (i) fixed number of valleys, (ii) fixed number of valleys with $x$-coordinate divisible by $m$ and (iii) fixed number of valleys with $x$-coordinate not divisible by $m$. The enumeration (ii) combinatorially realizes the $H$-triangle appearing in a recent article of Krattenthaler and the first author (Algebr. Comb. 5, 2022) in the context of certain parabolic noncrossing partitions. Through a transformation formula due to Chapoton, we give an explicit formula for the associated $F$-triangle. We realize this polynomial combinatorially by means of generalized Schr\"oder paths as well as flats in certain hyperplane arrangements. Along the way we exhibit two new combinatorial reciprocity results.