Logarithmic Coefficients and a Coefficient Conjecture for Univalent Functions

Abstract

Let ${\mathcal U}(\lambda)$ denote the family of analytic functions $f(z)$, $f(0)=0=f'(0)-1$, in the unit disk $\ID$, which satisfy the condition $\big |\big (z/f(z)\big )^{2}f'(z)-1\big |<\lambda $ for some $0<\lambda \leq 1$. The logarithmic coefficients $\gamma_n$ of $f$ are defined by the formula $\log(f(z)/z)=2\sum_{n=1}^\infty \gamma_nz^n$. In a recent paper, the present authors proposed a conjecture that if $f\in {\mathcal U}(\lambda)$ for some $0<\lambda \leq 1$, then $|a_n|\leq \sum_{k=0}^{n-1}\lambda ^k$ for $n\geq 2$ and provided a new proof for the case $n=2$. One of the aims of this article is to present a proof of this conjecture for $n=3, 4$ and an elegant proof of the inequality for $n=2$, with equality for $f(z)=z/[(1+z)(1+\lambda z)]$. In addition, the authors prove the following sharp inequality for $f\in{\mathcal U}(\lambda)$: $$\sum_{n=1}^{\infty}|\gamma_{n}|^{2} \leq \frac{1}{4}\left(\frac{\pi^{2}}{6}+2{\rm Li\,}_{2}(\lambda)+{\rm Li\,}_{2}(\lambda^{2})\right), $$ where ${\rm Li}_2$ denotes the dilogarithm function. Furthermore, the authors prove two such new inequalities satisfied by the corresponding logarithmic coefficients of some other subfamilies of $\mathcal S$.