A matroid extension result

TL;DR

This paper investigates conditions under which a matroid can be extended by a fixed element within a 3-separation, demonstrating that such extensions are possible if the matroid lacks Vamos-like structures.

Contribution

It proves that matroids can be extended by a fixed element in the guts of a 3-separation when no Vamos-like configuration exists.

Findings

Extensions are possible in the absence of Vamos-like structures.

The paper characterizes when fixed-element extensions are feasible.

It clarifies the limitations of extending matroids with complex separations.

Abstract

Adding elements to matroids can be fraught with difficulty. In the V\'amos matroid $V_8$, there are four independent sets $X_1,X_2, X_3,$ and $X_4$ such that $(X_1 \cup X_2,X_3 \cup X_4)$ is a $3$-separation while exactly three of the local connectivities $\sqcap(X_1,X_{3})$, $\sqcap(X_1,X_{4})$, $\sqcap(X_2,X_{3})$, and $\sqcap(X_2,X_{4})$ are one, with the fourth being zero. As is well known, there is no extension of $V_8$ by a non-loop element $p$ such that $X_j \cup p$ is a circuit for all $j$. This paper proves that a matroid can be extended by a fixed element in the guts of a $3$-separation provided no V\'amos-like structure is present.