Cyclic polynomials in anisotropic Dirichlet~spaces

TL;DR

This paper characterizes which polynomials are cyclic in anisotropic Dirichlet spaces on the bidisk, based on the sum and minimum of the parameters  and , and their zero sets.

Contribution

It provides a complete characterization of cyclic polynomials in anisotropic Dirichlet spaces depending on parameters and zero set conditions.

Findings

Polynomials are cyclic if + ; 1.

Cyclicity depends on the number of zeros in the two-torus .

Zero-free polynomials in  are cyclic when , > 1.

Abstract

Consider the Dirichlet-type space on the bidisk consisting of holomorphic functions $f(z_1,z_2):=\sum_{k,l\geq 0}a_{kl}z_1^kz_2^l$ such that $\sum_{k,l\geq 0}(k+1)^{\alpha_1} (l+1)^{\alpha_2}|a_{kl}|^2 <\infty.$ Here the parameters $\alpha_1,\alpha_2$ are arbitrary real numbers. We characterize the polynomials that are cyclic for the shift operators on this space. More precisely, we show that, given an irreducible polynomial $p(z_1,z_2)$ depending on both $z_1$ and $z_2$ and having no zeros in the bidisk: if $\alpha_1+\alpha_2\leq 1$, then $p$ is cyclic; if $\alpha_1+\alpha_2>1$ and $\min\{\alpha_1,\alpha_2\}\leq 1$, then $p$ is cyclic if and only if it has finitely many zeros in the two-torus $\mathbb T^2$; if $\min\{\alpha_1,\alpha_2\}>1$, then $p$ is cyclic if and only if it has no zeros in $\mathbb T^2$.