Heaps of pieces for lattice paths

Abstract

We study heaps of pieces for lattice paths, which give a combinatorial visualization of lattice paths. We introduce two types of heaps: type $I$ and type $II$. A heap of type $I$ is characterized by peaks of a lattice path. We have a duality between a lattice path $\mu$ and its dual $\overline{\mu}$ on heaps of type $I$. A heap of type $II$ for $\mu$ is characterized by the skew shape between the lowest path and $\mu$. We give a determinant expression for the generating function of heaps for general lattice paths, and an explicit formula for rational $(1,k)$-Dyck paths by using the inversion lemma. We introduce and study heaps in $k+1$-dimensions which are bijective to heaps of type $II$ for $(1,k)$-Dyck paths. Further, we show a bijective correspondence between type $I$ and type $II$ in the case of rational $(1,k)$-Dyck paths. As another application of heaps, we give two explicit formulae for the generating function of heaps for symmetric Dyck paths in terms of statistics on Dyck paths and on symmetric Dyck paths respectively.