When should a~polynomial's root nearest to a~real number be real itself?

A. Dubickas

Algebra i analiz, Tome 25 (2013) no. 6, pp. 37-49.

Résumé

The conditions are studied under which the root of an integer polynomial nearest to a given real number $y$ is real. It is proved that if a polynomial $P\in\mathbb Z[x]$ of degree $d\geq2$ satisfies $|P(y)|\ll1/M(P)^{2d-3}$ for some real number $y$, where the implied constant depends on $d$ only, then the root of $P$ nearest to $y$ must be real. It is also shown that the exponent $2d-3$ is best possible for $d=2,3$ and that it cannot be replaced by a number smaller than $(2d-3)d/(2d-2)$ for each $d\geq4$.

Détail

Keywords: polynomial root separation, real roots, Mahler's measure, discriminant.

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