1. [1] W. Arveson, “Notes on extensions of -algebras”, Duke Math. J. 44:2 (1977), 329–355.
2. [2] B. Blackadar, -theory for operator algebras, 2nd ed., Mathematical Sciences Research Institute Publications 5, Cambridge University Press, New York, 1998.
3. [3] L. G. Brown, R. G. Douglas, and P. A. Fillmore, “Unitary equivalence modulo the compact operators and extensions of -algebras”, pp. 58–128 in Proceedings of a Conference on Operator Theory (Halifax, NS, 1973), Lecture Notes in Math. 345, 1973.
4. [4] L. G. Brown, R. G. Douglas, and P. A. Fillmore, “Extensions of -algebras and -homology”, Ann. of Math. (2) 105:2 (1977), 265–324.
5. [5] R. G. Douglas, -algebra extensions and -homology, Annals of Mathematics Studies 95, Princeton University Press, 1980.