Parking functions and tree inversions revisited

Kreweras proved that the reversed sum enumerator for parking functions of length n   is equal to the inversion enumerator for labeled trees on n+1n+1 vertices. Recently, Perkinson, Yang, and Yu gave a bijective proof of this equality that moreover generalizes to graphical parking functions. Using a depth-first search variant of Dhar's burning algorithm they proved that the reversed sum enumerator for G-parking functions equals the κ-number enumerator for spanning trees of G. The κ-number is a kind of generalized tree inversion number originally defined by Gessel. We extend the work of Perkinson–Yang–Yu to what are referred to as “generalized parking functions” in the literature, but which we prefer to call vector parking functions   because they depend on a choice of vector x∈Nnx∈Nn. Specifically, we give an expression for the reversed sum enumerator for x-parking functions in terms of inversions in rooted plane trees with respect to certain admissible vertex orders. Along the way we clarify the relationship between graphical and vector parking functions.