Paper detail

Enumerations of humps and peaks in $(k,a)$-paths and $(n,m)$-Dyck paths via bijective proofs

Recently Mansour and Shattuck studied $(k,a)$-paths and gave formulas that relate the total number of humps (peaks) in all $(k,a)$-paths to the number of super $(k,a)$-paths. These results generalize earlier results of Regev on Dyck paths and Motzkin paths. Their proofs are based on generating functions and they asked for bijective proofs for their results. In this paper we first give bijective proofs of Mansour and Shattuck's results, then we extend our study to $(n,m)$-Dyck paths. We give a bijection that relates the total number of peaks in all $(n,m)$-Dyck paths to certain free $(n,m)$-paths when $n$ and $m$ are coprime. From this bijection we get the number of $(n,m)$-Dyck paths with exactly $j$ peaks, which is a generalization of the well-known result that the number Dyck paths of order $n$ with exactly $j$ peaks is the Narayana number $\frac{1}{k}{n-1\choose k-1}{n\choose k-1}$.