Matroids with at least two regular elements

TL;DR

This paper characterizes the structure of non-regular matroids with at least two regular elements, showing they can be constructed from known matroids and regular components, advancing understanding of matroid regularity properties.

Contribution

It provides a complete structural classification of non-regular matroids with multiple regular elements, including their construction from fundamental matroids and regular components.

Findings

Most such matroids are built from $F_7$, $S_8$, and regular matroids via 3-sums.

The classification includes four small exceptional matroids.

The results progress toward solving Seymour's problem on 3-connected non-regular matroids with regular elements.

Abstract

For a matroid $M$, an element $e$ such that both $M\backslash e$ and $M/e$ are regular is called a regular element of $M$. We determine completely the structure of non-regular matroids with at least two regular elements. Besides four small size matroids, all 3-connected matroids in the class can be pieced together from $F_7$ or $S_8$ and a regular matroid using 3-sums. This result takes a step toward solving a problem posed by Paul Seymour: Find all 3-connected non-regular matroids with at least one regular element [5, 14.8.8].