Existence and uniqueness of the $L^1$-Karcher mean

TL;DR

This paper extends the Karcher mean to $L^1$-measures on positive operators, proving existence, uniqueness, and convergence properties using a nonlinear ODE framework despite complex metric space challenges.

Contribution

It introduces a novel $L^1$-extension of the Karcher mean, develops a nonlinear ODE theory for operator entropy, and proves convergence and product formula results.

Findings

Existence and uniqueness of the $L^1$-Karcher mean established.

Proved the norm convergence of power means to the Karcher mean.

Derived a Trotter-Kato product formula for nonlinear semigroups.

Abstract

We extend the domain of the Karcher mean $\Lambda$ of positive operators on a Hilbert space to $L^1$-Borel probability measures on the cone of positive operators equipped with the Thompson part metric. We establish existence and uniqueness of $\Lambda$ as the solution of the Karcher equation and develop a nonlinear ODE theory for the relative operator entropy in the spirit of Crandall-Liggett, such that the solutions of the Karcher equation are stationary solutions of the ODE, and all generated solution curves enjoy the exponential contraction estimate. This is possible despite the facts that the Thompson metric is non-Euclidean, non-differentiable, non-commutative as a metric space as well as non-separable, and the positive cone is non-locally compact as a manifold. As further applications of the ODE approach, we prove the norm convergence conjecture of the power means of positive operators to the Karcher mean, and a Trotter-Kato product formula for the nonlinear semigroups explicitly expressed by compositions of two-variable geometric means. This can be regarded as a nonlinear continuous-time version of the law of large numbers.