Automorphisms of subalgebras of bounded analytic functions

Abstract

Let $H^\infty$ denote the algebra of all bounded analytic functions on the unit disk. It is well-known that every (algebra) automorphism of $H^\infty$ is a composition operator induced by disc automorphism. Maurya et al., (J. Math. Anal. Appl. 530 : Paper No: 127698, 2024) proved that every automorphism of the subalgebras $\{f\in H^\infty : f(0) = 0\}$ or $\{f\in H^\infty : f'(0) = 0\}$ is a composition operator induced by a rotation. In this article, we give very simple proof of their results. As an interesting generalization, for any $\psi\in H^\infty$, we show that every automorphism of $\psi H^\infty$ must be a composition operator and characterize all such composition operators. Using this characterization, we find all automorphism of $\psi H^\infty$ for few choices of $\psi$ with various nature depending on its zeros.