Sharp Remez inequality

TL;DR

This paper establishes the exact bound for the maximum modulus of algebraic polynomials on the unit circle given a large subset where the polynomial is bounded, solving a long-standing problem in Remez inequalities.

Contribution

It provides the sharp Remez inequality for algebraic polynomials on the unit circle, including the precise extremal polynomial form, resolving a longstanding open problem.

Findings

Derived the exact supremum bound involving Chebyshev polynomials.

Characterized the extremal polynomials achieving equality.

Solved the long-standing problem on the sharp constant in Remez inequality.

Abstract

Let an algebraic polynomial $P_n(\zeta)$ of degree $n$ be such that $|P_n(\zeta)|\le 1$ for $\zeta\in E\subset\mathbb{T}$ and $|E|\ge 2\pi -s$. We prove the sharp Remez inequality $$ \sup_{\zeta\in\mathbb{T}}|P_n(\zeta)|\le \mathfrak{T}_{n}\left(\sec \frac{s} 4\right),$$ 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.