Alpha-convex functions in complex Banach spaces

Abstract

Let $\mathbb{B}$ be the unit ball in a complex Banach space ${\mathbb X}$. We study biholomorphic mappings $F: \mathbb{B} \to {\mathbb X}$ of the form $F(z) = f(z)z$ for $z \in \mathbb{B}$, where $f: \mathbb{B} \to \mathbb{C}$ is a holomorphic function satisfying $f(0) = 1$. Our main contributions are as follows: Inclusion Results: We establish inclusion conditions for $\mathcal{M}_α(\mathbb{B})$, class of $α$-convex functions in $\mathbb{B}$ to belong to the classes of starlike mappings or quasi-convex mappings of type $B$ on $\mathbb{B}$ for specified ranges of $α$. Characterization: We prove that $F(z) = \frac{f(l_{u}(z))}{l_{u}(z)}z \in \mathcal{M}_α(\mathbb{B})$ if and only if $f \in \mathcal{M}_α(\mathbb{U})$, where $f \in \mathcal{S}(\mathbb{U})$, $u \in {\mathbb X}$ with $\|u\| = 1$, and $l_{u} \in T(u)$. Duality: We present a duality relationship between the classes $\mathcal{S}^*(\mathbb{U})$ and $\mathcal{M}_α(\mathbb{B})$ for $α\ge 0$. Hankel Determinant Bounds: We determine sharp upper bounds for the second Hankel determinant of mappings in $\mathcal{M}_α(\mathbb{B})$ when $α\ge 0$. The results established here directly extend the previous results to the broader setting of complex Banach spaces.