A Unified Approach to Sharp Subordination Radii

Abstract

Let $φ$ and $ψ$ be univalent functions in the unit disk $\mathbb D$ satisfying $φ(0)=ψ(0)=1$ and $φ\not\precψ$, and let $\mathcal P_φ$ denote the class of analytic functions $p$ in $\mathbb D$ such that $p\precφ$. Under suitable analytic continuation, univalence, and boundary-behavior hypotheses on the branch of $φ^{-1}$ satisfying $φ^{-1}(1)=0$, we determine the sharp $\mathcal P_ψ$-radius of $\mathcal P_φ$ as \[ \mathcal R(φ,ψ) = \min_{|ζ|=1} \left| φ^{-1}\bigl(ψ(ζ)\bigr) \right|. \] For Ma--Minda functions satisfying these hypotheses, the same radius is sharp for the associated classes $\mathcal{ST}(φ)$ and $\mathcal{CV}(φ)$ with respect to $\mathcal{ST}(ψ)$ and $\mathcal{CV}(ψ)$, respectively. We apply the general result to $ φ_{\mathrm L}(z)=\sqrt{1+z}, φ_{\mathrm{Lune}}(z)=z+\sqrt{1+z^2}, φ_{\mathrm{Lim}}(z)=\left(1+{z}/{\sqrt2}\right)^2, $ for several choices of the Ma--Minda function $ψ$, and obtain parameter-dependent extensions for the corresponding generalized families. The resulting boundary minima are evaluated analytically.