Extremal sizes of subspace partitions

Abstract

A subspace partition $\Pi$ of $V=V(n,q)$ is a collection of subspaces of $V$ such that each 1-dimensional subspace of $V$ is in exactly one subspace of $\Pi$. The size of $\Pi$ is the number of its subspaces. Let $\sigma_q(n,t)$ denote the minimum size of a subspace partition of $V$ in which the largest subspace has dimension $t$, and let $\rho_q(n,t)$ denote the maximum size of a subspace partition of $V$ in which the smallest subspace has dimension $t$. In this paper, we determine the values of $\sigma_q(n,t)$ and $\rho_q(n,t)$ for all positive integers $n$ and $t$. Furthermore, we prove that if $n\geq 2t$, then the minimum size of a maximal partial $t$-spread in $V(n+t-1,q)$ is $\sigma_q(n,t)$.