Amenability of the component group of the unitary group

Let AA be a nuclear CC^*-algebra. Its unitary group is denoted by U(A){\rm U}(A), and U0(A){\rm U}_0(A) is the connected component of the identity. The quotient U(A)/U0(A){\rm U}(A)/{\rm U}_0(A) is viewed as a discrete group.

Component-group amenability conjecture. The discrete group

U(A)/U0(A){\rm U}(A)/{\rm U}_0(A)

is amenable.

This would follow from the preceding equivalence conjecture and is known in many natural classification-theoretic examples where the quotient is isomorphic to the abelian group K1(A)K_1(A). The general assertion remains open, and the source notes that the quotient can nevertheless be non-abelian in natural examples.

Sources & referencesView supporting material

Primary source

Vadim Alekseev, Max Schmidt and Andreas Thom, “Amenability for unitary groups of C*-algebras”, arXiv:2305.13181 (2024).

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