Localization of eigenvalues of Doubly Cyclic Matrices

Abstract

Fix positive numbers $\alpha$ and $\beta$. For the family of doubly cyclic matrices of the form $diag(a_1, a_2, ... ,a_n) - diag(b_1, b_2, ... ,b_n) \Sigma_*$, where $\Sigma_*$ is a permutation matrix for the $n$-cycle $1 \to 2$, $2 \to 3$, ... ,$n-1 \to n$, $n \to 1$ [cycle notation (1, 2, ... , n-1, n)], and with fixed geometric mean $\alpha$ for the $a_k$'s and $\beta$ for the $b_k$'s, the maximum number of eigenvalues in the left half-plane is attained by $diag(\alpha, \alpha, ... , \alpha) - diag(\beta, \beta, ... , \beta) \Sigma_*$. This confirms a conjecture of C. Johnson, Z. Price, and I. Spitkovsky.' Moreover, the complete range of possibilities for the number of eigenvalues in the left half-plane is demonstrated: if $\alpha < \beta$, then any odd number between 1 and the maximum, inclusive, is attainable, and these are the only possibiliites.