# Phase Diagrams of Meromorphic Functions

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## Abstract

The paper demonstrates the use of phase diagrams as tools for visualizing and exploring meromorphic functions. With any such function \(f:D\ \rightarrow\ \hat{\rm C}\) we associate two mappings
with an appropriate definition at zeros and poles. Color-coding the points of \({\rm T}\cup\lbrace{0,\infty}\rbrace\) converts the function

$$P_{f}:D\rightarrow{\rm T}\cup \lbrace {0,\infty}\rbrace,z\mapsto {f(z)\over|f(z)|},\qquad V_{f}:D\rightarrow {\rm C},z\mapsto - {f(z){\overline f^\prime(z)}\over |f(z)|^{2}+|f^{\prime}(z)|^{2}},$$

*P*_{ f }to an image which visualizes the function*f*directly on its domain. Endowing this*phase plot*with the orbits of the vector field*V*_{ f }yields the*phase diagram*of*f*.We describe the local normal forms of phase diagrams, study properties of their orbits, and investigate the basins of attraction of zeros. Special attention is paid to the interplay between zeros, poles and critical points. In particular we derive formulas which relate the numbers of these points in a Jordan domain *G* to the winding numbers of *P* _{ f } and *V* _{ f } along the boundary of *G*. A short proof of Walsh’s theorem on the critical points of Blaschke products serves as an illustration.

## Keywords

meromorphic function phase plot phase diagram basin of attraction visualization of complex functions Gauss-Lucas theorem Walsh theorem## 2000 MSC

30D30 30A99## Preview

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## References

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*Elias Wegert*Google Scholar

## Copyright information

© Heldermann Verlag 2010