Stable and real-zero polynomials in two variables

TL;DR

This paper develops determinantal representations for bivariate polynomials with stability properties, linking polynomial stability to matrix factorizations and providing explicit constructions for real-zero polynomials.

Contribution

It introduces new determinantal representations for stable bivariate polynomials and characterizes when the associated contraction matrix is unitary, extending polynomial stability theory.

Findings

Constructed determinantal representations for stable bivariate polynomials.

Characterized when the contraction matrix can be chosen unitary.

Provided explicit matrix constructions for real-zero polynomials.

Abstract

For every bivariate polynomial $p(z_1, z_2)$ of bidegree $(n_1, n_2)$, with $p(0,0)=1$, which has no zeros in the open unit bidisk, we construct a determinantal representation of the form $$p(z_1,z_2)=\det (I - K Z),$$ where $Z$ is an $(n_1+n_2)\times(n_1+n_2)$ diagonal matrix with coordinate variables $z_1$, $z_2$ on the diagonal and $K$ is a contraction. We show that $K$ may be chosen to be unitary if and only if $p$ is a (unimodular) constant multiple of its reverse. Furthermore, for every bivariate real-zero polynomial $p(x_1, x_2),$ with $p(0,0)=1$, we provide a construction to build a representation of the form $$p(x_1,x_2)=\det (I+x_1A_1+x_2A_2),$$ where $A_1$ and $A_2$ are Hermitian matrices of size equal to the degree of $p$. A key component of both constructions is a stable factorization of a positive semidefinite matrix-valued polynomial in one variable, either on the circle (trigonometric polynomial) or on the real line (algebraic polynomial).