A Pseudo-Algebra of Observables for the Dirac Equation. Heinz O. Cordes — 1983 Manuscripta mathematica
On the two-fold symbol chain of a C*-algebra of singular integral operators on a polycylinder. Heinz O. Cordes — 1986 Revista Matemática Iberoamericana
The C*-Algebra of a Singular Elliptic Problem on a Noncompact Riemannian Manifold. Heinz O. Cordes; Robert C. McOwen — 1977 Mathematische Zeitschrift