2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/215877We investigate certain matrices composed of mixed, second-order moments of unitaries. The unitaries are taken from C*-algebras with moments taken with respect to traces, or, alternatively, from matrix algebras with the usual trace. These sets are of interest in light of a theorem of E. Kirchberg about Connes' embedding problem.12 pages. In the revision (to version 2) we add some citations to earlier results on extreme correlation matrices that include some results we useOperator Algebras46L10, 15A48text