English

Logarithmic coefficients of the inverse of univalent functions

Complex Variables 2018-11-06 v1

Abstract

Let \es\es be the class of analytic and univalent functions in the unit disk z<1|z|<1, that have a series of the form f(z)=z+n=2anznf(z)=z+ \sum_{n=2}^{\infty}a_nz^n. Let FF be the inverse of the function f\esf\in\es with the series expansion %in a disk of radius at least 1/41/4 F(w)=f1(w)=w+n=2AnwnF(w)=f^{-1}(w)=w+ \sum_{n=2}^{\infty}A_nw^n for w<1/4|w|<1/4. The logarithmic inverse coefficients Γn\Gamma_n of FF are defined by the formula log(F(w)/w)=2n=1Γn(F)wn\log\left(F(w)/w\right)\,=\,2\sum_{n=1}^{\infty}\Gamma_n(F)w^n. % In this paper, we determine the logarithmic inverse coefficients bound of FF for the class In this paper, we first determine the sharp bound for the absolute value of Γn(F)\Gamma_n(F) when ff belongs to \es\es and for all n1n \geq 1. This result motivates us to carry forward similar problems for some of its important geometric subclasses. In some cases, we have managed to solve this question completely but in some other cases it is difficult to handle for n4n\geq 4. For example, in the case of convex functions ff, we show that the logarithmic inverse coefficients Γn(F)\Gamma_n(F) of FF satisfy the inequality Γn(F)12n\mboxforn1,2,3 |\Gamma_n(F)|\,\le \, \frac{1}{2n} \mbox{ for } n\geq 1,2,3 and the estimates are sharp for the function l(z)=z/(1z)l(z)=z/(1-z). Although this cannot be true for n10n\ge 10, it is not clear whether this inequality could still be true for 4n94\leq n\leq 9.

Keywords

Cite

@article{arxiv.1811.01208,
  title  = {Logarithmic coefficients of the inverse of univalent functions},
  author = {S. Ponnusamy and N. L. Sharma and K. -J. Wirths},
  journal= {arXiv preprint arXiv:1811.01208},
  year   = {2018}
}

Comments

17 pages; To appear in Results in Mathematics