Let ϕ be a faithful normal semi-finite weight on a von Neumann algebraM. For each normal semi-finite weight ϕ onM, invariant under the modular automorphism group Σ of ϕ, there is a unique self-adjoint positive operator h, affiliated with the sub-algebra of fixed-points for Σ, such that ϕ=ϕ(h·). Conversely, each such h determines a Σ-invariant normal semi-finite weight. An easy application of this non-commutative Radon-Nikodym theorem yields the result thatM is semi-finite if and only if Σ consists of inner automorphisms.
Partially supported by NSF Grant # 28976 X.
Partially supported by NSF Grant # GP-28737
This revised version was published online in November 2006 with corrections to the Cover Date.
"The Radon-Nikodym theorem for von neumann algebras." Acta Math. 130 53 - 87, 1973. https://doi.org/10.1007/BF02392262