English

Birkhoff-James classification of norm's properties

Functional Analysis 2024-02-22 v1

Abstract

For an arbitrary normed space X\mathcal X over a field F{R,C}\mathbb F \in \{ \mathbb R, \mathbb C \}, we define the directed graph Γ(X)\Gamma(\mathcal X) induced by Birkhoff-James orthogonality on the projective space P(X)\mathbb P(\mathcal X), and also its nonprojective counterpart Γ0(X)\Gamma_0(\mathcal X). We show that, in finite-dimensional normed spaces, Γ(X)\Gamma(\mathcal X) carries all the information about the dimension, smooth points, and norm's maximal faces. It also allows to determine whether the norm is a supremum norm or not, and thus classifies finite-dimensional abelian CC^\ast-algebras among other normed spaces. We further establish the necessary and sufficient conditions under which the graph Γ0(R)\Gamma_0(\mathcal{R}) of a (real or complex) Radon plane R\mathcal{R} is isomorphic to the graph Γ0(F2,2)\Gamma_0(\mathbb F^2, \|\cdot\|_2) of the two-dimensional Hilbert space and construct examples of such nonsmooth Radon planes.

Keywords

Cite

@article{arxiv.2402.13416,
  title  = {Birkhoff-James classification of norm's properties},
  author = {Alexander Guterman and Bojan Kuzma and Sushil Singla and Svetlana Zhilina},
  journal= {arXiv preprint arXiv:2402.13416},
  year   = {2024}
}

Comments

Accepted for publications in AOT in The Special Issue Dedicated to Professor Chi-Kwong Li

R2 v1 2026-06-28T14:55:11.252Z