Arndt and De Morgan Integer Compositions

TL;DR

This paper provides a new combinatorial proof connecting Fibonacci numbers to specific integer compositions with parity constraints, generalizing descent conditions and establishing recurrence relations.

Contribution

It introduces a novel combinatorial proof linking Fibonacci numbers to compositions with parity restrictions and generalizes descent conditions to broader composition families.

Findings

Verified Arndt's observation using compositions with odd parts.

Established recurrence relations for compositions with mixed parity parts.

Connected new results to compositions studied by Andrews and Viennot.

Abstract

In 2013, Joerg Arndt recorded that the Fibonacci numbers count integer compositions where the first part is greater than the second, the third part is greater than the fourth, etc. We provide a new combinatorial proof that verifies his observation using compositions with only odd parts as studied by De Morgan. We generalize the descent condition to establish families of recurrence relations related to two types of compositions: those made of any odd part and certain even parts, and those made of any even part and certain odd parts. These generalizations connect to compositions studied by Andrews and Viennot. New tools used in the combinatorial proofs include two permutations of compositions and a statistic based on the signed pairwise difference between parts.