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On the sharpness of Jentzsch’s theorem generic properties. (English) Zbl 1236.30003

Let \(f(z)=\sum_{\nu\geq 0}a_{\nu}z^{\nu}\) be a holomorphic function in the open unit disk \(\mathbb{D}\) such that \(\limsup| a_{\nu}|^{1/{\nu}}=1.\) A point \(z_0\in\mathbb{C}\) is called a limit point of zeros of the partial sums \(S_n(f)(z)=\sum_{\nu=0}^na_{\nu}z^{\nu}\) if, for any \(\varepsilon>0,\) there exist infinitely many \(S_n(f)\) possessing a zero in \(\{z:\;| z-z_0|<\varepsilon\}.\) Let \(Z(S_n(f))\) be the set of all limit points of zeros of \(S_n(f)\). Jentzsch’s theorem ensures that the boundary of the disk of convergence \(\partial\mathbb{D}\) is a subset of \(Z(S_n(f))\). Later, W. Gehlen and W. Luh proved that there exists \(f\in H(\mathbb{D})\) such that for every closed \(C\subset\mathbb{C}\) with \(\partial\mathbb{D}\subset C\subset \mathbb{D}^c\) there exists a sequence \((n_k)\) of natural numbers such that \(Z(S_{n_k}(f))\cap \mathbb{D}^c=C\) [Arch. Math. 63, No. 1, 33–38 (1994; Zbl 0805.30002)]. The proof is constructive. In the same spirit of many results concerning universal series, the author shows in the paper under the review that this set of such functions is a \(G_\delta\) dense subset of \(H(\mathbb{D})\) (endowed with the topology of uniform convergence on compact subsets) using Baire’s theorem. Finally, the author combines this generic result with other classical universality properties to obtain new statements.

MSC:

30B10 Power series (including lacunary series) in one complex variable
30C15 Zeros of polynomials, rational functions, and other analytic functions of one complex variable (e.g., zeros of functions with bounded Dirichlet integral)
47A16 Cyclic vectors, hypercyclic and chaotic operators

Citations:

Zbl 0805.30002