Hilbert function spaces and multiplier algebras of analytic discs

TL;DR

This thesis investigates the isomorphism problem for multiplier algebras of analytic discs and explores the embedding dimension of complete Pick spaces, providing new insights into their structure and classification.

Contribution

It establishes conditions under which multiplier algebra isomorphisms imply geometric equivalences of analytic discs and relates embedding dimension to kernel properties in weighted Hardy spaces.

Findings

Isomorphism of multiplier algebras preserves boundary self-crossings of analytic discs.

Only specific maps induce algebra isomorphisms between multiplier algebras.

Certain weighted Hardy spaces have infinite embedding dimension.

Abstract

The thesis is devoted to two related problems. 1. The isomorphism problem for analytic discs: Suppose $V$ is the unit disc $\mathbb{D}$ embedded in the $d$-dimensional unit ball $\mathbb{B}_d$ and attached to the unit sphere. Consider the space $\mathcal{H}_V$, the restriction of the Drury-Arveson space to the variety $V$, and its multiplier algebra $\mathcal{M}_V = \operatorname{Mult}(\mathcal{H}_V)$. The isomorphism problem is the following: Is $V_1 \cong V_2$ equivalent to $\mathcal{M}_{V_1} \cong \mathcal{M}_{V_2}$? A theorem of Alpay, Putinar and Vinnikov states that for $V$ without self-crossings on the boundary $\mathcal{M}_V$ is the space of bounded analytic functions on $V$. We consider what happens when there are self-crossings on the boundary and prove that if $\mathcal{M}_{V_1} \cong \mathcal{M}_{V_2}$ algebraically, then $V_1$ and $V_2$ must have the same self-crossings up to a unit disc automorphism. We prove that an isomorphism between $\mathcal{M}_{V_1}$ and $\mathcal{M}_{V_2}$ can only be given by a composition with a map from $V_1$ to $V_2$. In the case of a single self-crossing we show that there are only two possible candidates for this map and find these candidates. 2. The embedding dimension for complete Pick spaces: A Theorem of Agler and McCarthy states that any complete Pick space can be realized as $\mathcal{H}_V$, for some $V$ in $\mathbb{B}_d$, where $d$ can be infinite. The smallest such $d$ is called the embedding dimension. Given a complete Pick space can we find its embedding dimension? Can we at least determine if it is finite or infinite? We look into this problem for rotation-invariant spaces on the unit disc $\mathbb{D}$. We prove a general result which explicitly relates the embedding dimension with the kernel of the space. This allows us to prove that the embedding dimension for certain weighted Hardy-type spaces is infinite.