Inequalities and geometry of hyperbolic-type metrics, radius problems and norm estimates

TL;DR

This paper investigates inequalities among various hyperbolic-type metrics, their geometric implications, and properties of univalent functions, revealing restrictions on domains and establishing isometries and coefficient conditions.

Contribution

It provides new insights into metric inequalities, domain restrictions, isometry characterizations, and coefficient conditions for subclasses of univalent functions.

Findings

Most metric inequalities cannot occur in certain domains.

Characterization of isometries of specific metrics in complex domains.

Necessary and sufficient conditions for univalent function subclasses.

Abstract

We consider certain inequalities among the Apollonian metric, the Apollonian inner metric, the $j$ metric and the quasihyperbolic metric. We verify that whether these inequalities can occur in simply connected planar domains and in proper subdomains of $\mathbb{R}^n~(n\ge 2)$. We have seen from our verification that most of the cases cannot occur. This means that there are many restrictions on domains in which these inequalities can occur. We also consider two metrics $j$ and $d$, and investigate whether a plane domain $D\varsubsetneq\mathbb{C}$, for which there exists a constant $c>0$ with $j(z,w) \le c\, d(z,w)$ for all $z,w \in D$, is a uniform domain. In particular, we study the case when $d$ is the $\lambda$-Apollonian metric. We also investigate the question, whether simply connected quasi-isotropic domains are John disks and conversely. Isometries of the quasihyperbolic metric, the Ferrand metric and the K--P metric are also obtained in several specific domains in the complex plane. In addition to the above, some problems on univalent functions theory are also solved. We denote by $\mathcal{S}$, the class of normalized univalent analytic functions defined in the unit disk. We consider some geometrically motivated subclasses, say $\mathcal{F}$, of $\mathcal{S}$. We obtain the largest disk $|z|<r$ for which $\frac{1}{r}f(rz)\in \mathcal{F}$ whenever $f\in \mathcal{S}$. We also obtain necessary and sufficient coefficient conditions for $f$ to be in $\mathcal{F}$. Finally, we present the pre-Schwarzian norm estimates of functions from $\mathcal{F}$ and that of certain convolution or integral transforms of functions from $\mathcal{F}$.