Combinatorics of the zeta map on rational Dyck paths

Abstract

An $(a,b)$-Dyck path $P$ is a lattice path from $(0,0)$ to $(b,a)$ that stays above the line $y=\frac{a}{b}x$. The zeta map is a curious rule that maps the set of $(a,b)$-Dyck paths into itself; it is conjecturally bijective, and we provide progress towards proof of bijectivity in this paper, by showing that knowing zeta of $P$ and zeta of $¶$ conjugate is enough to recover $P$. Our method begets an area-preserving involution $χ$ on the set of $(a,b)$-Dyck paths when $ζ$ is a bijection, as well as a new method for calculating $ζ^{-1}$ on classical Dyck paths. For certain nice $(a,b)$-Dyck paths we give an explicit formula for $ζ^{-1}$ and $χ$ and for additional $(a,b)$-Dyck paths we discuss how to compute $ζ^{-1}$ and $χ$ inductively. We also explore Armstrong's skew length statistic and present two new combinatorial methods for calculating the zeta map involving lasers and interval intersections. We provide a combinatorial statistic $δ$ that can be used to recursively compute $ζ^{-1}$ and show that $δ$ is computable from $ζ(P)$ in the Fuss-Catalan case.