Unique expansion matroids and union minimal matroids

TL;DR

This paper introduces unique expansion matroids and union minimal matroids, exploring their properties, relationships, and extensions, thereby advancing the theoretical understanding of matroid structures and their axioms.

Contribution

It defines and characterizes unique expansion and union minimal matroids, establishing their properties, relationships, and extensions within matroid theory.

Findings

Unique expansion matroids are characterized by forming base families that are partitions.

All unique expansion matroids are also union minimal matroids.

Both unique expansion matroids and their duals are shown to be unique exchange matroids.

Abstract

The expansion axiom of matroids requires only the existence of some kind of independent sets, not the uniqueness of them. This causes that the base families of some matroids can be reduced while the unions of the base families of these matroids remain unchanged. In this paper, we define unique expansion matroids in which the expansion axiom has some extent uniqueness; we define union minimal matroids in which the base families have some extent minimality. Some properties of them and the relationship between them are studied. First, we propose the concepts of secondary base and forming base family. Secondly, we propose the concept of unique expansion matroid, and prove that a matroid is a unique expansion matroid if and only if its forming base family is a partition. Thirdly, we propose the concept of union minimal matroid, and prove that unique expansion matroids are union minimal matroids. Finally, we extend the concept of unique expansion matroid to unique exchange matroid and prove that both unique expansion matroids and their dual matroids are unique exchange matroids.