The Karpelevi\v{c} Region Revisited

Abstract

We consider the Karpelevi\v{c} region $\Theta_n \subset \mathbb{C}$ consisting of all eigenvalues of all stochastic matrices of order $n$. We provide an alternative characterisation of $\Theta_n$ that sharpens the original description given by Karpelevi\v{c}. In particular, for each $\theta \in [0, 2\pi),$ we identify the point on the boundary of $\Theta_n$ with argument $\theta.$ We further prove that if $n \in \mathbb{N}$ with $n \ge 2,$ and $t \in \Theta_n,$ then $t$ is a subdominant eigenvalue of some stochastic matrix of order $n$.