English

Lower bounds for the complex polynomial Hardy--Littlewood inequality

Functional Analysis 2015-10-08 v1

Abstract

The Hardy--Littlewood inequality for complex homogeneous polynomials asserts that given positive integers m2m\geq2 and n1n\geq1, if PP is a complex homogeneous polynomial of degree mm on pn\ell_{p}^{n} with 2mp2m\leq p\leq\infty given by P(x1,,xn)=α=maαxαP(x_{1},\ldots,x_{n})=\sum_{|\alpha|=m}a_{\alpha }\mathbf{{x}^{\alpha}}, then there exists a constant CC,m,ppol1C_{\mathbb{C},m,p}^{\mathrm{pol}}\geq1 (which is does not depend on nn) such that (α=maα2mpmp+p2m)mp+p2m2mpCC,m,ppolP, \left( {\sum\limits_{\left\vert \alpha\right\vert =m}}\left\vert a_{\alpha }\right\vert ^{\frac{2mp}{mp+p-2m}}\right) ^{\frac{mp+p-2m}{2mp}}\leq C_{\mathbb{C},m,p}^{\mathrm{pol}}\left\Vert P\right\Vert , with P:=supzBpnP(z)\Vert P\Vert:=\sup_{z\in B_{\ell_{p}^{n}}}|P(z)|. In this short note, among other results, we provide nontrivial lower bounds for the constants CC,m,ppolC_{\mathbb{C},m,p}^{\mathrm{pol}}. For instance we prove that, for m2m\geq2 and 2mp<2m\leq p<\infty, CC,m,ppol2mp C_{\mathbb{C},m,p}^{\mathrm{pol}}\geq2^{\frac{m}{p}}% for mm even, and CC,m,ppol2m1p C_{\mathbb{C},m,p}^{\mathrm{pol}}\geq2^{\frac{m-1}{p}}% for mm odd. Estimates for the case p=p=\infty (this is the particular case of the complex polynomial Bohnenblust--Hille inequality) were recently obtained by D. Nu\~nez-Alarc\'on in 2013.

Keywords

Cite

@article{arxiv.1410.3037,
  title  = {Lower bounds for the complex polynomial Hardy--Littlewood inequality},
  author = {Gustavo Araujo and Daniel Pellegrino},
  journal= {arXiv preprint arXiv:1410.3037},
  year   = {2015}
}