Notes on automorphisms of ultrapowers of II1 factors
Abstract
In functional analysis, approximative properties of an object become precise in its ultrapower. We discuss this idea and its consequences for automorphisms of II1 factors. Here are some sample results: (1) an automorphism is approximately inner if and only if its ultrapower is aleph0-locally inner; (2) the ultrapower of an outer automorphism is always outer; (3) for unital *-homomorphisms from a separable nuclear C*-algebra into an ultrapower of a II1 factor, equality of the induced traces implies unitary equivalence. All statements are proved using operator algebraic techniques, but in the last section of the paper we indicate how the underlying principle is related to theorems of Henson's positive bounded logic.
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