Let $(\varphi_t)_{ t\geq 0}$ be a parabolic semigroup of analytic functions on $\mathbb{ D}$, with Koenigs function $h$ and Koenigs domain $\Omega = h(\mathbb{ D})$. We study the point spectrum $\sigma_p(\Delta\mid_{ H^p})$ of $\Delta$, the infinitesimal generator of the $C_0$-semigroup $(C_{ \varphi_t})_{ t\geq 0}$ of composition operators on $H^p$. This reduces to characterizing the frequencies of $\Omega$. That is, those $\lambda \in \mathbb{ C}$ such that $e^{ \lambda h} \in H^p$. We first derive containment relations for $\sigma_p(\Delta\mid_{ H^p})$ and provide sufficient conditions for its complete characterization. Our approach relies heavily on the geometric properties of $\Omega$ and on careful estimates of the harmonic measure of some boundary subsets of $\Omega$. Furthermore, assuming that $\Omega$ is convex, we also obtain necessary conditions for $\lambda$ to be a frequency of $\Omega$. Using these, we are able to completely describe $\sigma_p(\Delta\mid_{ H^p})$ in a broad range of situations e.g. when $\Omega$ contains an angular sector. We conclude with some consequences regarding the spectrum of the composition operators $(C_{ \varphi_t})_{ t\geq 0}$. These results extend a previous work of Betsakos on hyperbolic semigroups.
Categories: math.CV math.FA