The Cauchy dual subnormality problem in two variables

Abstract

The Cauchy dual subnormality problem asks whether the Cauchy dual of a $2$-isometry is subnormal. This problem has attracted considerable attention due to its connections with moment theory, operator theory, and complex function theory. In particular, it has been closely related to operator-theoretic models on function spaces such as de Branges--Rovnyak spaces and Dirichlet-type spaces. In this paper, we investigate the Cauchy dual subnormality problem in the multivariable setting of toral $2$-isometries. We show that if $\mathbf{T}=(T_1,T_2)$ is an analytic left-inverse commuting pair of toral $2$-isometries satisfying the joint kernel condition, then it is unitarily equivalent to an operator-valued weighted shift with invertible weights $\{W_I^{(j)}:j=1,2,I\in {\mathbb{Z}}_{+}^2\},$ where the initial weights $W_{0,0}^{(1)}$ and $W_{0,0}^{(2)}$ are positive operators. Moreover, if these initial weights commute, then the Cauchy dual ${\mathbf{T}}^{\prime }:=(T_1^{\prime },T_2^{\prime })$ is subnormal. We also construct an example in which the initial weights do not commute, and the corresponding Cauchy dual tuple ${\mathbf{T}}^{\prime }=(T_1^{\prime },T_2^{\prime })$ fails to be subnormal. Finally, we establish a complete set of unitary invariants for pure toral $2$-isometries.