Govil N. K., Rahman Q. I and Schmeisser G., On the derivative of a polynomial, Illinois Math. Jour. 23, 319-329(1979).

Marden M, Geometry of Polynomials, Mathematical Surveys Number 3, Amer. Math.. Soc. Providence, RI, (1966).

Mohammad Q. G., On the Zeros of a Polynomial, Amer. Math. Monthly, 72, 631-633 (1966). http://dx.doi.org/10.2307/2313853

Titchmarsh E.C., Theory of Functions, Oxford University Press, London, (1949).