OAI.KirchbergRordam.character_criterion

The theorem states that, for a nontrivial separable C*-algebra A (in the base universe Type) and a free ultrafilter ω on the natural numbers, meaning an ultrafilter that refines the cofinite filter, the norm central-sequence algebra of A has no characters if and only if A is isomorphic to its minimal (spatial) C*-tensor product with the Jiang–Su algebra. The norm ultrapower of A is the quotient of the C*-algebra of bounded sequences in A by the ideal of sequences whose norms tend to zero along ω; the central algebra is the closed star-subalgebra of this ultrapower consisting of elements commuting with the diagonal copy of A, the images of constant sequences. The central algebra has no characters means that every star-homomorphism from it to ℂ (a linear, multiplicative, star-preserving map, not required to preserve the unit) is the zero map. The Jiang–Su algebra is the inductive limit C*-algebra of the standard prime-dimension-drop model system. The right-hand side asserts the existence of a star-algebra isomorphism over ℂ from A onto this tensor product. The proof is admitted in the source.