English

Some non-commutative averaging theorems

Functional Analysis 2025-11-19 v1

Abstract

Given nNn\in\mathbb{N} any point on the closed unit disk D\overline{\mathbb{D}} can be written as the average of nn points on the unit circle S1\mathbb{S}^1. Here we discuss a non-commutative version of this result. We prove that for any Hilbert space H\mathcal{H} and a state ϕ:B(H)C\phi:B(\mathcal{H})\to\mathbb{C}, {ϕ(U):Uunitary}=D\{\phi(U): U\,\mathrm{ unitary}\}=\overline{\mathbb{D}}. We also show that if dim\dim H\mathcal{H} is finite, for any wDw\in\overline{\mathbb{D}} we can choose a unitary UU with atmost 33 distinct eigenvalues such that ϕ(U)=w\phi(U)=w. Lastly, we prove the divisibility property for any state ϕ\phi on B(H)B(\mathcal{H}) where H\mathcal{H} is infinite-dimensional, showing that {ϕ(P):P=P2=P}=[0,1]\{\phi(P) : P^*=P^2=P\}=[0,1].

Keywords

Cite

@article{arxiv.2511.14340,
  title  = {Some non-commutative averaging theorems},
  author = {Saptak Bhattacharya},
  journal= {arXiv preprint arXiv:2511.14340},
  year   = {2025}
}

Comments

10 pages, 0 figures

R2 v1 2026-07-01T07:42:57.845Z