On unitary representability of topological groups

TL;DR

The paper proves that certain topological groups derived from $ ext{L}_ ext{infinity}$-Banach spaces are unitarily representable, meaning they can be embedded into the unitary group of a Hilbert space, with implications for operator algebras.

Contribution

It establishes the unitarily representability of additive groups of $ ext{L}_ ext{infinity}$-Banach spaces with specific topologies, extending to preduals of commutative von Neumann algebras.

Findings

Additive groups of $ ext{L}_ ext{infinity}$-Banach spaces are unitarily representable.

Preduals of commutative von Neumann algebras are unitarily representable.

Noncommutative von Neumann algebras are not unitarily representable.

Abstract

We prove that the additive group $(E^\ast,\tau_k(E))$ of an $\mathscr{L}_\infty$-Banach space $E$, with the topology $\tau_k(E)$ of uniform convergence on compact subsets of $E$, is topologically isomorphic to a subgroup of the unitary group of some Hilbert space (is \emph{unitarily representable}). This is the same as proving that the topological group $(E^\ast,\tau_k(E))$ is uniformly homeomorphic to a subset of $\ell_2^\kappa$ for some $\kappa$. As an immediate consequence, preduals of commutative von Neumann algebras or duals of commutative $C^\ast$-algebras are unitarily representable in the topology of uniform convergence on compact subsets. The unitary representability of free locally convex spaces (and thus of free Abelian topological groups) on compact spaces, follows as well. The above facts cannot be extended to noncommutative von Neumann algebras or general Schwartz spaces.