English

Jensen's inequality for separately convex noncommutative functions

Operator Algebras 2021-02-08 v1 Functional Analysis

Abstract

Classically, Jensen's Inequality asserts that if XX is a compact convex set, and f:KRf:K\to \mathbb{R} is a convex function, then for any probability measure μ\mu on KK, that f(bar(μ))f  dμf(\text{bar}(\mu))\le \int f\;d\mu, where bar(μ)\text{bar}(\mu) is the barycenter of μ\mu. Recently, Davidson and Kennedy proved a noncommutative ("nc") version of Jensen's inequality that applies to nc convex functions, which take matrix values, with probability measures replaced by ucp maps. In the classical case, if ff is only a separately convex function, then ff still satisfies the Jensen inequality for any probability measure which is a product measure. We prove a noncommutative Jensen inequality for functions which are separately nc convex in each variable. The inequality holds for a large class of ucp maps which satisfy a noncommutative analogue of Fubini's theorem. This class of ucp maps includes any free product of ucp maps built from Boca's theorem, or any ucp map which is conditionally free in the free-probabilistic sense of M{\l}otkowski. As an application to free probability, we obtain some operator inequalities for conditionally free ucp maps applied to free semicircular families.

Keywords

Cite

@article{arxiv.2102.03021,
  title  = {Jensen's inequality for separately convex noncommutative functions},
  author = {Adam Humeniuk},
  journal= {arXiv preprint arXiv:2102.03021},
  year   = {2021}
}

Comments

24 pages

R2 v1 2026-06-23T22:51:49.285Z