Isometric Composition Operators on $BMOA$ for $0<p<\infty$

Abstract

We characterize the analytic self-maps of the unit disk inducing isometric composition operators on \(\BMOA\) with respect to the Möbius-invariant \(H^p\) norm for \(p\ge1\) and the corresponding quasi-norm for \(0<p<1\). In particular, this gives a complete answer to Laitila&#39;s question on the characterization of isometric composition operators for the Möbius-invariant \(H^p\) norms, while also covering the quasi-Banach range. As a consequence, the isometric property of \(C_φ\) is independent of the exponent \(p\in(0,\infty)\).