Winding number criterion for the origin to belong to the numerical range of a matrix on a loop of matrices

Abstract

Let $A:[0,1]\to GL(n,\mathbb{C})$ be continuous with $A(0)=A(1)$, thus the winding number of $\det A$ is well-defined. If the winding number is not divisible by $n$, then the origin belongs to the numerical range of $A(\ddot{I}•)$ for some $\ddot{I}•\in [0,1]$.

19 pages, 2 figures