Sharp Remez inequality

Abstract

Let an algebraic polynomial $P_n(ζ)$ of degree $n$ be such that $|P_n(ζ)|\le 1$ for $ζ\in E\subset\mathbb{T}$ and $|E|\ge 2π-s$. We prove the sharp Remez inequality $$ \sup_{ζ\in\mathbb{T}}|P_n(ζ)|\le \mathfrak{T}_{n}\left(\sec \frac{s} 4\right),$$ where $\mathfrak{T}_{n}$ is the Chebyshev polynomial of degree $n$. The equality holds if and only if $$ P_n(e^{iz})=e^{i(nz/2+c_1)}\mathfrak{T}_n\left(\sec\frac s 4\cos \frac {z-c_0} 2\right), \quad c_0,c_1\in\mathbb{R}. $$ This gives the solution of the long-standing problem on the sharp constant in the Remez inequality for trigonometric polynomials.