A generalisation of uniform matroids

Abstract

A matroid is uniform if and only if it has no minor isomorphic to $U_{1,1}\oplus U_{0,1}$ and is paving if and only if it has no minor isomorphic to $U_{2,2}\oplus U_{0,1}$. This paper considers, more generally, when a matroid $M$ has no $U_{k,k}\oplus U_{0,\ell}$-minor for a fixed pair of positive integers $(k,\ell)$. Calling such a matroid $(k,\ell)$-uniform, it is shown that this is equivalent to the condition that every rank-$(r(M)-k)$ flat of $M$ has nullity less than $\ell$. Generalising a result of Rajpal, we prove that for any pair $(k,\ell)$ of positive integers and prime power $q$, only finitely many simple cosimple $GF(q)$-representable matroids are \kl-uniform. Consequently, if Rota's Conjecture holds, then for every prime power $q$, there exists a pair $(k_q,\ell_q)$ of positive integers such that every excluded minor of $GF(q)$-representability is $(k_q,\ell_q)$-uniform. We also determine all binary $(2,2)$-uniform matroids and show the maximally $3$-connected members to be $Z_5\backslash t, AG(4,2), AG(4,2)^*$ and a particular self-dual matroid $P_{10}$. Combined with results of Acketa and Rajpal, this completes the list of binary $(k,\ell)$-uniform matroids for which $k+\ell\leq 4$.