English

Norm attaining vectors and Hilbert points

Functional Analysis 2024-07-01 v2

Abstract

Let HH be a Hilbert space that can be embedded as a dense subspace of a Banach space XX such that the norm of the embedding is equal to 11. We consider the following statements for a nonzero vector φ\varphi in HH: (A) φX=φH\|\varphi\|_X = \|\varphi\|_H. (H) φ+fXφX\|\varphi+f\|_X \geq \|\varphi\|_X for every ff in HH such that f,φ=0\langle f, \varphi \rangle =0. We use duality arguments to establish that (A)     \implies (H), before turning our attention to the special case when the Hilbert space in question is the Hardy space H2(Td)H^2(\mathbb{T}^d) and the Banach space is either the Hardy space H1(Td)H^1(\mathbb{T}^d) or the weak product space H2(Td)H2(Td)H^2(\mathbb{T}^d) \odot H^2(\mathbb{T}^d). If d=1d=1, then the two Banach spaces are equal and it is known that (H)     \implies (A). If d2d\geq2, then the Banach spaces do not coincide and a case study of the polynomials φα(z)=z12+αz1z2+z22\varphi_\alpha(z) = z_1^2 + \alpha z_1 z_2 + z_2^2 for α0\alpha\geq0 illustrates that the statements (A) and (H) for the two Banach spaces describe four distinct sets of functions.

Keywords

Cite

@article{arxiv.2310.20346,
  title  = {Norm attaining vectors and Hilbert points},
  author = {Konstantinos Bampouras and Ole Fredrik Brevig},
  journal= {arXiv preprint arXiv:2310.20346},
  year   = {2024}
}

Comments

This paper has been has been accepted for publication in Studia Math

R2 v1 2026-06-28T13:07:14.322Z