Best approximations for the weighted combination of the Cauchy--Szegö kernel and its derivative in the mean

Abstract

In this paper, we study an extremal problem concerning best approximation in the Hardy space $H^1$ on the unit disk $\mathbb D$. Specifically, we consider weighted combinations of the Cauchy-Szeg\"o kernel and its derivative, parametrized by an inner function $\varphi$ and a complex number $\lambda$, and provide explicit formula of the best approximation $e_{\varphi,z}(\lambda)$ by the subspace $H^1_0$. We also describe the extremal functions associated with this approximation.