Inner Functions on quotient domains related to the polydisc

• Published in: arXiv.org 2024-09
• Main Authors: Bhowmik, Mainak, Kumar, Poornendu
• Format: Article
• Language: English
• Subjects: Algebra; Analytic functions; Approximation; Interpolation; Mathematical analysis; Nodes; Operators (mathematics)
• ISSN: 2331-8422

Inner functions are the backbone of holomorphic function theory. This paper studies the inner functions on quotient domains of the open unit polydisc, \(\bD^d\), arising from the group action of finite pseudo-reflection groups. Such quotient domains are known to be biholomorphic to the proper image \(\theta(\bD^d)\) of \(\bD^d\) under certain polynomial map \(\theta: \bD^d \to \theta(\bD^d)\). The main contributions of this paper are as follows: 1) We show that the closed algebra generated by inner functions on \(\theta(\bD^d)\) forms a proper subalgebra of \(H^\infty(\theta(\bD^d))\), the algebra of bounded holomorphic functions on \(\theta(\bD^d)\). This in particular shows that Marshall's theorem, which states that the algbera generated by rational inner functions on the disc is the algebra of bounded analytic functions on \(\bD\), does not hold in these domains. En route, we also shed some light on the Shilov boundary corresponding to \(H^\infty(\theta(\bD^d))\). 2) The structure of rational inner functions on \(\theta(\bD^d)\) is found. 3) The set of all rational inner functions on \(\theta(\bD^d)\) is shown to be dense in the norm-unit ball of \(H^\infty(\theta(\bD^d))\) with respect to the uniform compact-open topology, thereby proving the Carathéodory approximation result. 4) As an application of the Carathéodory approximation theorem, we approximate holomorphic functions on \(\theta(\bD^d)\) that are continuous in the closure of \({\theta(\bD^d)}\) by convex combinations of rational inner functions in the \(L^2 \)-norm, thereby obtaining a version of the Fisher's theorem.