文摘
Let \(A\in L({\mathbb {C}}^n)\) be a linear operator such that \(k_+(A)<2m(A)\), where \(k_+(A)\) is the upper exponential index of \(A\) and \(m(A)=\min \{\mathfrak {R}\langle A(z),z\rangle :\Vert z\Vert =1\}\). In this paper we are concerned with variations of \(A\)-normalized univalent subordination chains on the Euclidean unit ball \(\mathbb {B}^n\) in \(\mathbb {C}^n\). We also obtain growth results for the generating vector fields and the transition mappings associated with \(A\)-normalized univalent subordination chains \(f(z,t)\) that satisfy some regularity assumptions. Further, we give examples of normalized biholomorphic mappings on \(\mathbb {B}^n\) which are not extreme/support points for the compact family \(S_A^0(\mathbb {B}^n)\) of mappings with \(A\)-parametric representation on \(\mathbb {B}^n\). Next, we consider the (normalized) time-\(\log M\)-reachable family \(\tilde{\fancyscript{R}}_{\log M}(\mathrm{id}_{\mathbb {B}^n},{\fancyscript{N}}_A)\) generated by the Carathéodory mappings, where \(M\in (1,\infty )\) and \(k_+(A)<2m(A)\). We prove a result related to the support points \(f\) of the family \(\tilde{\fancyscript{R}}_{\log M}(\mathrm{id}_{\mathbb {B}^n},{\fancyscript{N}}_A)\) and the associated \(A\)-univalent subordination chains \(f(z,t)\) such that \(f=f(\cdot ,0)\). In particular, if \(f\in \tilde{\fancyscript{R}}_{\log M}(\mathrm{id}_{\mathbb {B}^n},{\fancyscript{N}}_A)\), then \(f\not \in (\mathrm{supp}\, S_A^0(\mathbb {B}^n)\cup \mathrm{ex}\, S_A^0(\mathbb {B}^n))\). In the last part of the paper, using some ideas based on a shearing process that has been introduced recently by F. Bracci, we obtain a sharp estimate for \(\Big |\frac{\partial ^2 f_1}{\partial z_2^2}(0)\Big |\), when \(f=(f_1,f_2)\in S_A^0(\mathbb {B}^2)\) and \(A\) is a diagonal matrix whose diagonal elements are \(\lambda \) and \(1, \lambda \in [1,2)\). This result allows us to construct an example of a bounded support mapping for the family \(S_A^0(\mathbb {B}^2)\), but which does not belong to any reachable family \(\tilde{\fancyscript{R}}_{\log M}(\mathrm{id}_{\mathbb {B}^2},{\fancyscript{N}}_A)\), for all \(M>1\). This example provides a basic difference between the theory of bounded univalent mappings on the unit disc \(U\) and that on the unit ball \(\mathbb {B}^n\) in \(\mathbb {C}^n, n\ge 2\). Finally, we construct an example of a support point \(F_A^M\) for the reachable family \(\tilde{\fancyscript{R}}_{\log M}(\mathrm{id}_{\mathbb {B}^2},{\fancyscript{N}}_A)\), but which is not a support point/extreme point for the family \(S_A^0(\mathbb {B}^2)\), and we conjecture that the mapping \(F_A^M\) is also an extreme point for the family \(\tilde{\fancyscript{R}}_{\log M}(\mathrm{id}_{\mathbb {B}^2},{\fancyscript{N}}_A)\).