Generating functions for compositions with constrained even parts

Abstract

We study compositions of a positive integer $n$ in which the occurrence of even parts larger than a fixed threshold $k$ is controlled. More precisely, for each composition $m=(m_1,\dots,m_r)$ we consider the number of even parts strictly larger than $k$, and we introduce a two-variable generating function that encodes this statistic. We show that this generating function is rational and obtain explicit closed forms, depending on the parity of $k$. As a consequence, we derive exact counting formulas and linear recurrence relations for the number of compositions of $n$ with a prescribed number of even parts greater than $k$. We also obtain explicit formulas for related refined quantities, such as the number of compositions with an even or odd number of such parts, the total number of their occurrences among all compositions of $n$, and positional statistics describing how late the first such part appears in a composition. This combinatorial problem is motivated by questions arising from combinatorial expansions related to zeta functions of algebraic curves over finite fields, although the results of this paper are entirely combinatorial.