Numerical ranges of cyclic shift matrices

Abstract

We study the numerical range of an $n\times n$ cyclic shift matrix, which can be viewed as the adjacency matrix of a directed cycle with $n$ weighted arcs. In particular, we consider the change in the numerical range if the weights are rearranged or perturbed. In addition to obtaining some general results on the problem, a permutation of the given weights is identified such that the corresponding matrix yields the largest numerical range (in terms of set inclusion), for $n\le 6$. We conjecture that the maximizing pattern extends to general $n\times n$ cylic shift matrices. For $n\le 5$, we also determine permutations such that the corresponding cyclic shift matrix yields the smallest numerical range.

25 pages