Published online by Cambridge University Press: 16 January 2020
We show that an irreducible family  ${\mathcal{S}}$ of complex
${\mathcal{S}}$ of complex  $n\times n$ matrices satisfies Paz’s conjecture if it contains a rank-one matrix. We next investigate properties of families of rank-one matrices. If
$n\times n$ matrices satisfies Paz’s conjecture if it contains a rank-one matrix. We next investigate properties of families of rank-one matrices. If  ${\mathcal{R}}$ is a linearly independent, irreducible family of rank-one matrices then (i)
${\mathcal{R}}$ is a linearly independent, irreducible family of rank-one matrices then (i)  ${\mathcal{R}}$ has length at most
${\mathcal{R}}$ has length at most  $n$, (ii) if all pairwise products are nonzero,
$n$, (ii) if all pairwise products are nonzero,  ${\mathcal{R}}$ has length 1 or 2, (iii) if
${\mathcal{R}}$ has length 1 or 2, (iii) if  ${\mathcal{R}}$ consists of elementary matrices, its minimum spanning length
${\mathcal{R}}$ consists of elementary matrices, its minimum spanning length  $M$ is the smallest integer
$M$ is the smallest integer  $M$ such that every elementary matrix belongs to the set of words in
$M$ such that every elementary matrix belongs to the set of words in  ${\mathcal{R}}$ of length at most
${\mathcal{R}}$ of length at most  $M$. Finally, for any integer
$M$. Finally, for any integer  $k$ dividing
$k$ dividing  $n-1$, there is an irreducible family of elementary matrices with length
$n-1$, there is an irreducible family of elementary matrices with length  $k+1$.
$k+1$.