Inequalities for the polar derivative of a polynomial with $S$-fold zeros at the origin

Authors

  • E. Khojastehnezhad Department of Mathematics‎, ‎University of Semnan‎, ‎Semnan‎, ‎Iran.
  • M. Bidkham Department of Mathematics‎, ‎University of Semnan‎, ‎Semnan‎, ‎Iran.
Abstract:

‎Let $p(z)$ be a polynomial of degree $n$ and for a complex number $alpha$‎, ‎let $D_{alpha}p(z)=np(z)+(alpha-z)p'(z)$ denote the polar derivative of the polynomial p(z) with respect to $alpha$‎. ‎Dewan et al proved‎ ‎that if $p(z)$ has all its zeros in $|z| leq k, (kleq‎ ‎1),$ with $s$-fold zeros at the origin then for every‎ ‎$alphainmathbb{C}$ with $|alpha|geq k$‎, ‎begin{align*}‎ ‎max_{|z|=1}|D_{alpha}p(z)|geq‎ ‎frac{(n+sk)(|alpha|-k)}{1+k}max_{|z|=1}|p(z)|‎. ‎end{align*} In this paper‎, ‎we obtain a refinement‎ ‎of above inequality‎. ‎Also as an application of our result‎, ‎we extend some inequalities for‎ ‎polar derivative of a polynomial of degree $n$ which‎ ‎does not vanish in $|z|< k$‎, ‎where $kgeq 1$‎, ‎except $s$-fold zeros at the origin‎. 

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Journal title

volume 43  issue 7

pages  2153- 2167

publication date 2017-12-30

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