Generic tensor products are not maximal

Abstract

We construct tensor products of uniform matroids which are strictly freer than their generic linear tensor products. In particular, the generic tensor product of two copies of $U_{7,9}$ is not maximal. More generally, for every $a,b\ge2$, this holds for $U_{7,a+7}$ and $U_{7,b+7}$. The construction disproves the uniform tensor-product formulation of Mason's conjecture. Its dual is a relaxation of an explicit circuit-hyperplane of a generic birigidity matroid. By the author's representability theorem, the resulting matroids are non-representable over every field. We also obtain full-rank counterexamples to the Jackson--Tanigawa hyperconnectivity conjecture in every dimension $d\ge2$.