Geometric studies on the class ${\mathcal U}(\lambda)$

TL;DR

This paper simplifies the proof of bounds for functions in the family ${\\mathcal U}(\lambda)$, explores their properties under various transformations, and addresses a radius problem, contributing new subordination results and insights into their geometric behavior.

Contribution

The article provides a simpler proof for second coefficient bounds, establishes new subordination results, and analyzes the invariance of ${\mathcal U}(\lambda)$ under elementary transformations.

Findings

Simplified proof of second coefficient bounds.

${\mathcal U}(\lambda)$ is preserved under rotation, conjugation, dilation, and omitted value transformations.

The family is not preserved under the $n$-th root transformation for any $n\geq 2$.

Abstract

The article deals with the family ${\mathcal U}(\lambda)$ of all functions $f$ normalized and analytic in the unit disk such that $\big |\big (z/f(z)\big )^{2}f'(z)-1\big |<\lambda $ for some $0<\lambda \leq 1$. The family ${\mathcal U}(\lambda)$ has been studied extensively in the recent past and functions in this family are known to be univalent in $\ID$. However, the problem of determining sharp bounds for the second coefficients of functions in this family was solved recently in \cite{VY2013} by Vasudevarao and Yanagihara but the proof was complicated. In this article, we first present a simpler proof. We obtain a number of new subordination results for this family and their consequences. In addition, we show that the family ${\mathcal U}(\lambda )$ is preserved under a number of elementary transformations such as rotation, conjugation, dilation and omitted value transformations, but surprisingly this family is not preserved under the $n$-th root transformation for any $n\geq 2$. This is a basic here which helps to generate a number of new theorems and in particular provides a way for constructions of functions from the family ${\mathcal U}(\lambda)$. Finally, we deal with a radius problem.