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Convex families of holomorphic mappings related to the convex mappings of the ball in \(\mathbb {C}^n\). (English) Zbl 1462.32019

Summary: Unlike the case in one dimension, there is still much to learn about the basic nature of the family \( \mathcal {K}(\mathbb{B})\) of normalized (\(f(0)=0\), \(Df(0)=I\), where \(Df\) is the Fréchet derivative of \(f\) and \(I\) is the identity operator on \( \mathbb{C}^n\)) biholomorphic mappings \(f\) of the Euclidean unit ball \( \mathbb{B} \subseteq \mathbb{C}^n\) onto convex domains in \( \mathbb{C}^n\) when \( n\geq 2\). We consider its closed convex hull \( \overline {\mathrm{co}}\, \mathcal {K}(\mathbb{B})\) in relation to the family \( \mathcal {R}(\mathbb{B})\) of normalized holomorphic mappings \( f\colon \mathbb{B} \rightarrow \mathbb{C}^n\) satisfying \( \operatorname {Re} \langle f(z),z \rangle > \Vert z\Vert^2/2\) for \( z\in \mathbb{B} \setminus \{0\}\), where \( \langle \cdot ,\cdot \rangle \) and \( \Vert\cdot \Vert\) are, respectively, the Hermitian inner product and Euclidean norm in \( \mathbb{C}^n\). In dimension \(n=1\), the sets are the same. Here, we identify some extreme points of \( \overline {\text{co}}\, \mathcal {K}(\mathbb{B})\) and use them to show that \( \overline {\text{co}}\, \mathcal {K}(\mathbb{B})\) is a proper subset of \( \mathcal {R}(\mathbb{B})\) when \( n\geq 2\). We also consider an extension operator related to \( \mathcal {R}(\mathbb{B})\) that helps illustrate where the known extreme points of \( \overline {\text{co}}\, \mathcal {K}(\mathbb{B})\) lie in \( \mathcal {R}(\mathbb{B})\) and make some observations on the related case of the unit polydisk.

MSC:

32H02 Holomorphic mappings, (holomorphic) embeddings and related questions in several complex variables
30C45 Special classes of univalent and multivalent functions of one complex variable (starlike, convex, bounded rotation, etc.)
46A55 Convex sets in topological linear spaces; Choquet theory
46E10 Topological linear spaces of continuous, differentiable or analytic functions
52A07 Convex sets in topological vector spaces (aspects of convex geometry)
Full Text: DOI

References:

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