Composition operators on Hardy-Sobolev spaces with bounded reproducing kernels

Abstract

For any real $\beta$ let $H^2_\beta$ be the Hardy-Sobolev space on the unit disc $\mathbb{D}$. $H^2_\beta$ is a reproducing kernel Hilbert space and its reproducing kernel is bounded when $\beta>1/2$. In this paper, we characterize that for a non-constant analytic function $\varphi:\mathbb{D}\to\mathbb{D}$, when the composition operator $C_{\varphi }$ on $H^{2}_{\beta }$ is Fredholm. For $1/2<\beta<1$, we also prove that $C_{\varphi }$ has dense range in $H_{\beta }^{2}$ if and only if the polynomials are dense in a certain Dirichlet space of the domain $\varphi(\mathbb{D})$. It follows that if the range of $C_{\varphi }$ is dense in $H_{\beta }^{2}$, then $\varphi $ is a weak-star generator of $H^{\infty}$, although the conclusion is false for the classical Dirichlet space $\mathfrak{D}$. Moreover, we study the relation between the density of the rang of $C_{\varphi }$ and the cyclic vector of the multiplier $M_{\varphi}^{\beta}.$