### On the isomorphism question for complete Pick multiplier algebras

#### by Orr Shalit

In this post I want to tell you about our new preprint, “On the isomorphism question for complete Pick multiplier algebras“, which Matt Kerr, John McCarthy and myself just uploaded to the arXiv. Very broadly speaking, the motif of this paper is the connection between algebra and geometry; to be a little bit more precise, it is the connection between complex geometry and nonself-adjoint operator algebras.

Here is an excerpt from the introduction to our papaer:

The general theme of this note is the following question.

(Q) Given two specific function algebras and associated with two analytic varieties and , if there is a biholomorphic equivalence , does composition with induce an isomorphism from to ?

This type of question — and its converse, whether isomorphism of the algebras implies biholomorphic equivalence of the varieties — can be asked in many contexts, e.g. in algebraic geometry it is well known that two affine algebraic varieties are isomorphic if and only the algebras of polynomial functions on the varieties are isomorphic. Coming back to complex analysis, a theorem of L. Bers says that two domains and in the plane are conformally equivalent if and only if the corresponding algebras of holomorphic functions and are isomorphic. Note that in the setting of Bers’s theorem, (Q) is trivial and it is the converse that is the interesting part. In our setting, (Q) turns out to be much harder than its converse.

A quick path to explain what is the setting in which we work in the paper is as follows. Fix an integer . Let be the Hilbert space of analytic functions on the unit ball for which

(We are using here the standard multi-index notation). Now let be the algebra of functions

This algebra is the multiplier algebra of the Hilbert function space . Given a variety in the ball, one may construct the algebra

You may ask: *Of all algebras in the world, why these algebras?* That is a long story to be told another day — now I wish just to describe the result. But a short answer for now is that all algebras which may arise as multiplier algebras of an irreducible complete Pick kernel arise as one of our s.

In previous works (references may be found in the paper), it was shown that the answer to (Q) is *yes* in the case that is a homogeneous variety, and in the case that is (sufficiently regular) proper image of planar domain of finite connectivity. In our paper we show that the answer to (Q) is *yes* in the case where is a (sufficiently regular) proper image of a finite Riemann surface, and also in the case where and are both disjoint unions of varieties, say , , where each is the image of under an automorphism of the ball (which may be a different for every ). There are other results of a more technical nature which I don’t want to discuss here.

Understanding the isomorphism problem for these algebras (multiplier algebras of complete Pick kernels) has been an interest of mine for the past few years, and it is still one of my research objectives. It is a big challenge to obtain some positive answers for wider classes of varieties. I conjecture that the answer to (Q) is positive if and are algebraic varieties. I have no idea how to prove this.

[…] are based) I will surely like to discuss it. But on this blog I already discussed it previously, in this post. Perhaps now it will be easier to understand (and more […]

[…] I take this opportunity also to put up a link to this a new paper By Michael Hartz, Ken Davidson and myself, “Multipliers of embedded discs”. In this paper we continue our journey to understand the algebraic structure of complete Pick algebras in terms of the varieties on which they naturally live, I explained this problem in this older post. […]

[…] deals with what I call “the isomorphism problem for complete Pick algebras”. (see this previous post for a short overview what this is). Recall that every (irreducible) complete Pick multiplier […]