Generating functions for compositions with constrained even parts

TL;DR

This paper develops rational generating functions to count compositions of integers with controlled even parts exceeding a threshold, providing explicit formulas, recurrences, and statistical insights.

Contribution

It introduces a new rational generating function framework for compositions with constrained even parts, including explicit formulas and recurrence relations.

Findings

Derived explicit closed-form generating functions.

Established recurrence relations for counting compositions.

Analyzed positional and parity-based statistics of compositions.

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.