Operator theory on generalized Hartogs triangles

TL;DR

This paper explores the operator theory on generalized Hartogs triangles, analyzing associated Hilbert spaces, kernels, and multiplication operators, revealing their convexity properties and spectral behavior, and establishing an analog of von Neumann's inequality.

Contribution

It introduces a new class of generalized Hartogs triangles, studies their associated Hilbert spaces and operators, and establishes foundational operator-theoretic results including an analog of von Neumann's inequality.

Findings

The domains are never polynomially convex but always holomorphically convex.

The multiplication operators are never rationally cyclic.

The joint kernel dimension is constant except at a boundary singularity, where it becomes infinite.

Abstract

We consider the family $\mathcal P$ of $n$-tuples $P$ consisting of polynomials $P_1, \ldots, P_n$ with nonnegative coefficients which satisfy $\partial_i P_j(0) = \delta_{i, j},$ $i, j=1, \ldots, n.$ With any such $P,$ we associate a Reinhardt domain $\triangle^{\!n}_{_P}$ that we will call the generalized Hartogs triangle. We are particularly interested in the choices $P_a = (P_{1, a}, \ldots, P_{n, a}),$ $a \geq 0,$ where $P_{j, a}(z) = z_j + a \prod_{k=1}^n z_k,~ j=1, \ldots, n.$ The generalized Hartogs triangle associated with $P_a$ is given by \begin{equation} \triangle^{\!n}_a = \Big\{z \in \mathbb C \times \mathbb C^{n-1}_* : |z_j|^2 < |z_{j+1}|^2(1-a|z_1|^2), ~j=1, \ldots, n-1, |z_n|^2 + a|z_1|^2 < 1\Big\}. \end{equation} The domain $\triangle^{\!n}_{_P},$ $n \geq 2$ is never polynomially convex. However, $\triangle^{\!n}_{_P}$ is always holomorphically convex. With any $P \in \mathcal P$ and $m \in \mathbb N^n,$ we associate a positive semi-definite kernel $\mathscr K_{_{P, m}}$ on $\triangle^{\!n}_{_P}.$ This combined with the Moore's theorem yields a reproducing kernel Hilbert space $\mathscr H^2_m(\triangle^{\!n}_{_P})$ of holomorphic functions on $\triangle^{\!n}_{_P}.$ We study the space $\mathscr H^2_m(\triangle^{\!n}_{_P})$ and the multiplication $n$-tuple $\mathscr M_z$ acting on $\mathscr H^2_m(\triangle^{\!n}_{_P}).$ It turns out that $\mathscr M_z$ is never rationally cyclic. Although the dimension of the joint kernel of $\mathscr M^*_z-\lambda$ is constant of value $1$ for every $\lambda \in \triangle^{\!n}_{_P}$, it has jump discontinuity at the serious singularity $0$ of the boundary of $\triangle^{\!n}_{_P}$ with value equal to $\infty.$ We capitalize on the notion of joint subnormality to define a Hardy space on $\triangle^{\!n}_{_0}.$ This in turn gives an analog of the von Neumann's inequality for $\triangle^{\!n}_{_0}.$