Connections between rank and dimension for subspaces of bilinear forms

Abstract

Let $K$ be a field and let $V$ be a vector space of dimension $n$ over $K$. Let $M$ be a subspace of bilinear forms defined on $V\times V$. Let $r$ be the number of different non-zero ranks that occur among the elements of $M$. Our aim is to obtain an upper bound for $\dim M$ in terms of $r$ and $n$ under various hypotheses. As a sample of what we prove, we mention the following. Suppose that $m$ is the largest integer that occurs as the rank of an element of $M$. Then if $m\leq \lceil n/2\rceil$ and $|K|\geq m+1$, we have $\dim M\leq rn$. The case $r=1$ corresponds to a constant rank space and it is conjectured that $\dim M\leq n$ when $M$ is a constant rank $m$ space and $|K|\geq m+1$. We prove that the dimension bound for a constant rank $m$ space $M$ holds provided $|K|\geq m+1$ and either $K$ is finite or $K$ has characteristic different from 2 and $M$ consists of symmetric forms. In general, we show that if $M$ is a constant rank $m$ subspace and $|K|\geq m+1$, then $\dim M\leq \max\,(n,2m-1)$. We also provide more detailed results about constant rank subspaces over finite fields, especially subspaces of alternating or symmetric bilinear forms.