Controlled approximation for some classes of holomorphic functions

This paper reports on constructive approximation methods for three classes of holomorphic functions on the unit disk which are closely connected to each other: the class of starlike and spirallike functions, the class of semigroup generators, and the class of functions with positive real part. It is more-or-less known that starlike or spirallike functions can be defined as solutions of singular differential equations which, in general, are not stable under the motion of interior singular points to the boundary. At the same time, one can establish a perturbation formula which continuously transforms a starlike (or spirallike) function with respect to a boundary point to a starlike (or spirallike) function with respect to an interior point. This formula is based on an appropriate approximation method of holomorphic generators which determine the above-mentioned differential equations. In turn, the well-known Berkson-Porta parametric representation of holomorphic generators leads us to study an approximation-interpolation problem for the class of functions with positive real part. While this problem is of independent interest, the solution we present here is again based on the Berkson-Porta formula. Finally, we apply our results to solve a natural perturbation problem for one-parameter semigroups of holomorphic self-mappings, as well as the eigenvalue problem for the semigroup of composition operators.