Semicrossed products of the disc algebra

TL;DR

This paper studies the structure of semicrossed products of the disk algebra induced by finite Blaschke products, revealing their C*-envelopes and connections to solenoid systems and Morita equivalence.

Contribution

It characterizes the C*-envelopes of semicrossed products of the disk algebra under endomorphisms from finite Blaschke products, linking them to solenoid systems and Morita equivalence.

Findings

C*-envelope for non-constant Blaschke products is a crossed product of a solenoid system.

Semicrossed products embed into larger crossed products with explicit structure.

Constant Blaschke products lead to Morita equivalent crossed products.

Abstract

If $\alpha$ is the endomorphism of the disk algebra, $\AD$, induced by composition with a finite Blaschke product $b$, then the semicrossed product $\AD\times_{\alpha} \bZ^+$ imbeds canonically, completely isometrically into $\rC(\bT)\times_{\alpha} \bZ^+$. Hence in the case of a non-constant Blaschke product $b$, the C*-envelope has the form $ \rC(\S_{b})\times_{s} \bZ$, where $(\S_{b}, s)$ is the solenoid system for $(\bT, b)$. In the case where $b$ is a constant, then the C*-envelope of $\AD\times_{\alpha} \bZ^+$ is strongly Morita equivalent to a crossed product of the form $ \rC(\S_{e})\times_{s} \bZ$, where $e \colon \bT \times \bN \longrightarrow \bT \times \bN$ is a suitable map and $(\S_{e}, s)$ is the solenoid system for $(\bT \times \bN, \, e)$ .