The higher rank numerical range of nonnegative matrices
  In this article the well known "Perron-Frobenius theory" is investigated involving the higher rank numerical range $\Lambda_{k}(A)$ of an irreducible and entrywise nonnegative matrix $A$ and extending the notion of elements of maximum modulus in $\Lambda_{k}(A)$. Further, an application of this theory to the $\Lambda_{k}(L(\lambda))$ of a Perron polynomial $L(\lambda)$ is elaborated via its companion matrix $C_{L}$. 
Aikaterini Aretaki, John Maroulas
math.RA