Sylvester rank functions for amenable normal extensions

Abstract

We introduce a notion of amenable normal extension S of a unital ring R with a finite approximation system F, encompassing the amenable algebras over a field of Gromov and Elek, the twisted crossed product by an amenable group, and the tensor product with a field extension. It is shown that every Sylvester matrix rank function rk of R preserved by S has a canonical extension to a Sylvester matrix rank function rkF for S. In the case of twisted crossed product by an amenable group, and the tensor product with a field extension, it is also shown that rkF depends on rk continuously. When an amenable group has a twisted action on a unital C⁎-algebra preserving a tracial state, we also show that two natural Sylvester matrix rank functions on the algebraic twisted crossed product constructed out of the tracial state coincide.