Inequalities for an operator on the space of polynomials

Abstract

Let $mathcal{P}_n$ be the class of all complex polynomials of degree at most $n.$ Recently Rather et. al.[ On the zeros of certain composite polynomials and an operator preserving inequalities, Ramanujan J., 54(2021) 605–612. url{https://doi.org/10.1007/s11139-020-00261-2}] introduced an operator $N : mathcal{P}_nrightarrow mathcal{P}_n$ defined by $N[P](z):=sum_{j=0}^{k}lambda_jleft(frac{nz}{2}right)^jfrac{P^{(j)}(z)}{j!}, ~ k leq n$ where $lambda_jinmathbb{C}$, $j=0,1,2,ldots,k$ are such that all the zeros of $phi(z) = sum_{j=0}^{k} binom{n}{j}lambda_j z^j$ lie in the half plane $|z| leq left| z - frac{n}{2}right|$ and established certain sharp Bernstein-type polynomial inequalities. In this paper, we prove some more general results concerning the operator $N : mathcal{P}_n rightarrow mathcal{P}_n$ preserving inequalities between polynomials. Our results not only contain several well known results as special cases but also yield certain new interesting results as special cases.