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

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.