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Journal of Mathematical Analysis and Applications
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Journal of Mathematical Analysis and Applications
Article . 2000
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Article . 2000
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Extreme Points and Support Points of Subordination Families

Extreme points and support points of subordination families
Authors: Hallenbeck, David J; Hallenbeck, K.T;

Extreme Points and Support Points of Subordination Families

Abstract

The family \(A\) of analytic functions \(f\) of the unit disk \(\Delta\), equipped with the topology of uniform convergence in compact subsets forms a locally convex linear space. Let \(B_0:=\{\omega\in A |\omega(0)=0, |\omega|0\}=\{p\in A |p\prec {1+z \over 1-z}\} =s({1+z \over 1-z}) . \] The Krein-Milman theorem guarantees that a subordination family has \textit{extreme points} \(\text{ E} s(F)\neq \emptyset\), and these reconstruct \(s(F)\), i.~e. \(\overline{\text{ co}} \text{ E} s(F)=\overline{\text{ co}} s(F)\), \(\overline{\text{ co}} M\) denoting the closed convex hull of \(M\). The Herglotz representation formula shows via Choquet's theorem that \[ \text{ E} P=\{{1+xz \over 1-xz} ||x|=1\}. \] It is easy to see that in subordination families for every extreme point \(f\in \text{ E} s(F)\) (or \(f\in \text{ E} \overline{\text{ co}} s(F)\)), and \(f\prec g\), it follows that also \(g\in \text{ E} s(F)\) (or \(g\in \text{ E} \overline{\text{ co}} s(F)\), respectively) [W. Koepf: Extrempunkte und Stützpunkte in Familien nicht-verschwindender analytischer Funktionen; \(PhD\) thesis, Free University Berlin (1984)]. Hence the extreme points are large with respect to \(\prec\). A similar statement holds for the \textit{support points} of a linear functional \(L\). In the case of \(P\), the set of extreme points is smallest possible and consists only of the maximal elements w.~r.~t.\ \(\prec\). The authors show under which conditions for \(F\) this situation occurs. In contrast, for \(B_0\) one has \[ \text{ E} B_0= \{f\in B_0 |\int_0^{2\pi} \log (1-|f(e^{it})|) dt=-\infty\} \] Hence in this case, the set of extreme points is rather large. The article informs about the situation for general \(F\) which lies between the two extremes. Furthermore the set of support points of subordination families is considered. Finally, extreme point results for functions subordinate to convex, starlike and close-to-convex functions are given. Similar results for functions normalized by \(f(0)=1\) and \(f\neq 0\) can be found in [Complex Variables, Theory Appl. 8, 153-171 (1987; Zbl 0622.30002)].

Keywords

compact family, convex functions, starlike functions, Applied Mathematics, close-to-convex functions, subordination, extreme point, Krein-Milman theorem, support point, Special classes of univalent and multivalent functions of one complex variable (starlike, convex, bounded rotation, etc.), univalent functions, Maximum principle, Schwarz's lemma, Lindelöf principle, analogues and generalizations; subordination, Choquet theorem, closed convex hull, continuous linear functional, Herglotz representation formula, Radon-Nikodým, Kreĭn-Milman and related properties, Analysis

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selected citations
These citations are derived from selected sources.
This is an alternative to the "Influence" indicator, which also reflects the overall/total impact of an article in the research community at large, based on the underlying citation network (diachronically).
BIP!Citations provided by BIP!
popularity
This indicator reflects the "current" impact/attention (the "hype") of an article in the research community at large, based on the underlying citation network.
BIP!Popularity provided by BIP!
influence
This indicator reflects the overall/total impact of an article in the research community at large, based on the underlying citation network (diachronically).
BIP!Influence provided by BIP!
impulse
This indicator reflects the initial momentum of an article directly after its publication, based on the underlying citation network.
BIP!Impulse provided by BIP!
2
Average
Average
Average
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