Equivariant extensions of *-algebras

Journal article, 2010

A bivariant functor is defined on a category of *-algebras and a category of operator ideals, both with actions of a second countable group G, into the category of abelian monoids. The elements of the bivariant functor will be G-equivariant extensions of a *-algebra by an operator ideal under a suitable equivalence relation. The functor is related with the ordinary Ext-functor for C^*-algebras defined by Brown-Douglas-Fillmore. Invertibility in this monoid is studied and characterized in terms of Toeplitz operators with abstract symbol.