
Let \(G \subset\mathbb C\) be a domain and \(\zeta\in \partial G\). A function \(\varphi\) is called \(T_\zeta\)-universal on \(G\) if it is holomorphic on \(G\) and satisfies the following property: For all compact sets \(K\) with connected complement and all complex valued functions \(f\) on \(K\) there exist sequences \(\{a_n\}\) and \(\{b_n\}\) with \(a_nz + b_n\in G\) \(\forall z\in K\) and \(\forall n \in\mathbb N,\) \(a_nz + b_n\to\zeta\) \(\forall z\in K\), \(n\to\infty\), \(\varphi(a_nz + b_n)\to f(z)\) uniformly on \(K\), \(n\to\infty\). It is a natural question whether a holomorphic function \(\varphi\) can exhibit universal properties in the sense that it is \(T_\zeta\)-universal on \(G\) for all \(\zeta\) of a prescribed subset \(E\subset\partial G\) and such that \(\varphi\) is not \(T_\zeta\)-universal for all \(\zeta\) on the complementary boundary part \(\partial G \setminus E\). If a closed set \( E\subset \partial \mathbb D\) is prescribed then it is proved that there exists a function \(\varphi\) which is \(T_\zeta\)-universal for all \(\zeta\in E\) and is not \(T_\zeta\)-universal for all \(\zeta\in F :=\partial \mathbb D\setminus E\). In addition the function \(\varphi\) can be chosen such that \(\varphi\in C^\infty(F)\) and has a lacunary power series expansion \(\varphi(z) =\sum_{n=0}^\infty a_nz^n\quad\text{with}\quad a_n = 0\quad\text{if}\quad n\notin Q,\) where \(Q\) is a subsequence of \(\mathbb N_0\) with upper density \(\overline d(Q) = 1\). The construction of such a function follows by an inductive process, where a lemma on lacunary polynomial approximation and the existence of special \(T_\zeta\)-universal functions on starlike domains are the essential tools.
Mathematics(all), Numerical Analysis, gap power series, boundary behaviour of holomorphic functions, universal functions, lacunary power series expansion, Applied Mathematics, lacunary polynomial approximation, starlike domains, Power series (including lacunary series) in one complex variable, Approximation by polynomials, restricted universal functions, Analysis
Mathematics(all), Numerical Analysis, gap power series, boundary behaviour of holomorphic functions, universal functions, lacunary power series expansion, Applied Mathematics, lacunary polynomial approximation, starlike domains, Power series (including lacunary series) in one complex variable, Approximation by polynomials, restricted universal functions, Analysis
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