On the polar derivative of a polynomial

Abstract

Let $P(z)$ be a polynomial of degree $n$ having no zero in $|z|<k$ where $k\ge 1,$ then for every real or complex number $\alpha$ with $|\alpha |\ge 1$ it is known \begin{equation*} \underset{|z|=1}{\max}|D_αP(z)|\leq n\left(\dfrac{|α|+k}{1+k}\right)\underset{|z|=1}{\max}|P(z)|, \end{equation*} where $D_{\alpha }P(z)=nP(z)+(\alpha -z)P^{\prime }(z)$ denote the polar derivative of the polynomial $P(z)$ of degree $n$ with respect to a point $\alpha \in \mathbb{C}.$ In this paper, by a simple method, a refinement of above inequality and other related results are obtained.

5 pages