English

Sharp Remez inequality

Classical Analysis and ODEs 2018-10-23 v2

Abstract

Let an algebraic polynomial Pn(ζ)P_n(\zeta) of degree nn be such that Pn(ζ)1|P_n(\zeta)|\le 1 for ζET\zeta\in E\subset\mathbb{T} and E2πs|E|\ge 2\pi -s. We prove the sharp Remez inequality supζTPn(ζ)Tn(secs4), \sup_{\zeta\in\mathbb{T}}|P_n(\zeta)|\le \mathfrak{T}_{n}\left(\sec \frac{s} 4\right), where Tn\mathfrak{T}_{n} is the Chebyshev polynomial of degree nn. The equality holds if and only if Pn(eiz)=ei(nz/2+c1)Tn(secs4coszc02),c0,c1R. 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.

Keywords

Cite

@article{arxiv.1809.09726,
  title  = {Sharp Remez inequality},
  author = {S. Tikhonov and P. Yuditskii},
  journal= {arXiv preprint arXiv:1809.09726},
  year   = {2018}
}

Comments

12 pages, 1 figure; reference added