1. Brakhage, H., & P. Werner, Über das Dirichletsche Außenraumproblem für die Helmholtzsche Schwingungsgleichung. Archiv d. Math. 16, 325–329 (1965).
2. Bramble, J.H., Error estimates for difference methods in forced vibration problems. Tech. Note BN-388, Inst. Fluid Dyn. and Applied Math., U. Maryland, 1965.
3. Eidus, D.M., On the principle of limiting absorption. Math. Sbornik 57, 13–44 (1962). English translation: New York University, Courant Institute of Mathematical Sciences, Research Report No. EM-191, 1963.
4. Freudenthal, H., Über ein Beugungsproblem aus der elektromagnetischen Lichttheorie. Compositio Math. 6, 221–227 (1938).
5. Greenspan, D., Introductory Numerical Analysis of Elliptic Boundary Value Problems. New York: Harper and Row 1965.