Brown–Douglas–Fillmore theory, Encyclopedia of Mathematics, web
L.G. Brown, R.G. Douglas, P.A. Fillmore, Unitary equivalence modulo the compact operators and extensions of C$^\ast$-algebras, Proc. Conf. Operator Theory (Dalhousie Univ., Halifax, N.S., 1973) , Lecture Notes in Mathematics 345, Springer (1973) pp. 58–128, MR380478
L.G. Brown, R.G. Douglas, P.A. Fillmore, Extensions of C$^\ast$-algebras and K-homology, Ann. of Math. (2) 105: 2 (1977) pp. 265–324, MR458196, doi
L. G. Brown, R. G. Douglas, P. A. Fillmore, Extensions of C$^\ast$-algebras, operators with compact self-commutators, and K-homology, Bull. Amer. Math. Soc. 79:973-978, 1973, MR346540, doi
J. Kaminker, C. Schochet, K-theory and Steenrod homology: applications to the Brown–Douglas–Fillmore theory of operator algebras, Trans. Amer. Math. Soc. 227 (1977) pp. 63–107, MR431136