On the parity of the number of $(a,b,m)$-copartitions of $n$

Abstract

We continue the study of the $(a,b,m)$-copartition function ${\mathrm{cp}}_{a,b,m}(n)$, which arose as a combinatorial generalization of Andrews' partitions with even parts below odd parts. The generating function of ${\mathrm{cp}}_{a,b,m}(n)$ has a nice representation as an infinite product. In this paper, we focus on the parity of ${\mathrm{cp}}_{a,b,m}(n)$. As with the ordinary partition function, it is difficult to show positive density of either even or odd values of ${\mathrm{cp}}_{a,b,m}(n)$ for arbitrary $a,b$, and $m$. However, we find specific cases of $a,b,m$ such that ${\mathrm{cp}}_{a,b,m}(n)$ is even with density 1. Additionally, we show that the sequence $\{{\mathrm{cp}}_{a,m-a,m}(n){\}}_{n=0}^{\infty }$ takes both even and odd values infinitely often.