Hamming Polynomial of a Demimatroid

TL;DR

This paper introduces the Hamming polynomial for demimatroids, generalizes it via the Tutte polynomial, and explores its applications to simplicial complexes and combinatroids, extending coding theory concepts beyond matroids.

Contribution

It defines the Hamming polynomial for demimatroids, proves its relation to Betti numbers, and extends coding theory invariants to combinatroids, broadening the scope of combinatorial structures in coding theory.

Findings

Hamming polynomial is a specialization of the Tutte polynomial.

Betti numbers determine the Hamming polynomial of a demimatroid.

Many coding theory invariants extend to combinatroids.

Abstract

Following Britz, Johnsen, Mayhew and Shiromoto, we consider demi\-ma\-troids as a(nother) natural generalization of matroids. As they have shown, demi\-ma\-troids are the appropriate combinatorial objects for studying Wei's duality. Our results here apport further evidence about the trueness of that observation. We define the Hamming polynomial of a demimatroid $M$, denoted by $W(x,y,t)$, as a generalization of the extended Hamming weight enumerator of a matroid. The polynomial $W(x,y,t)$ is a specialization of the Tutte polynomial of $M$, and actually is equivalent to it. Guided by work of Johnsen, Roksvold and Verdure for matroids, we prove that Betti numbers of a demimatroid and its elongations determine the Hamming polynomial. Our results may be applied to simplicial complexes since in a canonical way they can be viewed as demimatroids. Furthermore, following work of Brylawski and Gordon, we show how demimatroids may be generalized one step further, to combinatroids. A combinatroid, or Brylawski structure, is an integer valued function $\rho$, defined over the power set of a finite ground set, satisfying the only condition $\rho(\emptyset)=0$. Even in this extreme generality, we will show that many concepts and invariants in coding theory can be carried on directly to combinatroids, say, Tutte polynomial, characteristic polynomial, MacWilliams identity, extended Hamming polynomial, and the $r$-th generalized Hamming polynomial; this last one, at least conjecturelly, guided by the work of Jurrius and Pellikaan for linear codes. All this largely extends the notions of deletion, contraction, duality and codes to non-matroidal structures.