Disintegration of positive isometric group representations on $\mathrm{L}^p$-spaces

Abstract

Let $G$ be a Polish locally compact group acting on a Polish space $X$ with a $G$-invariant probability measure $μ$. We factorize the integral with respect to $μ$ in terms of the integrals with respect to the ergodic measures on $X$, and show that $\mathrm{L}^p(X,μ)$ ($1\leq p<\infty$) is $G$-equivariantly isometrically lattice isomorphic to an $\mathrm{L}^p$-direct integral of the spaces $\mathrm{L}^{p}(X,λ)$, where $λ$ ranges over the ergodic measures on $X$. This yields a disintegration of the canonical representation of $G$ as isometric lattice automorphisms of $\mathrm{L}^p(X,μ)$ as an $\mathrm{L}^p$-direct integral of order indecomposable representations. If $(X^\prime,μ^\prime)$ is a probability space, and, for some $1\leq q<\infty$, $G$ acts in a strongly continuous manner on $\mathrm{L}^q(X^\prime,μ^\prime)$ as isometric lattice automorphisms that leave the constants fixed, then $G$ acts on $\mathrm{L}^{p}(X^{\prime},μ^{\prime})$ in a similar fashion for all $1\leq p<\infty$. Moreover, there exists an alternative model in which these representations originate from a continuous action of $G$ on a compact Hausdorff space. If $(X^\prime,μ^\prime)$ is separable, the representation of $G$ on $\mathrm{L}^p(X^\prime,μ^\prime)$ can then be disintegrated into order indecomposable representations. The notions of $\mathrm{L}^p$-direct integrals of Banach spaces and representations that are developed extend those in the literature.