A lower bound for the norm of the minimal residual polynomial

Abstract

Let $S$ be a compact infinite set in the complex plane with $0\notin S$, and let $R_n$ be the minimal residual polynomial on $S$, i.e., the minimal polynomial of degree at most $n$ on $S$ with respect to the supremum norm provided that $R_n(0)=1$. For the norm $L_n(S)$ of the minimal residual polynomial, the limit $\kappa (S):={\lim }_{n\to \infty }\sqrt[n]{L_n(S)}$ exists. In addition to the well-known and widely referenced inequality $L_n(S)\ge \kappa (S{)}^n$, we derive the sharper inequality $L_n(S)\ge 2\kappa (S{)}^n/(1+\kappa (S{)}^{2n})$ in the case that $S$ is the union of a finite number of real intervals. As a consequence, we obtain a slight refinement of the Bernstein--Walsh Lemma.

References in corpus (1)

• A lower bound for the minimum deviation of the Chebyshev polynomial on a compact real set