A Matroid Extension Result

Abstract

Adding elements to matroids can be fraught with difficulty. In the Vámos matroid $V_8$, there are four pairs $X_1,X_2, X_3,$ and $X_4$ that partition $E(V_8)$ 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 nonloop 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 Vámos-like structure is present.