1. J. Odhnoff, ?Operators generated by differential problems with eigenvalue parameter in equation and boundary condition,? Medd. Lunds Univ. Mat. Sem., 1?80 (1959).
2. A. N. Kozhevnikov, ?On the asymptotics of N(?) of a boundary problem with ? in the equation and boundary condition,? Metody Teorii Diff. Uravn. Ikh Prilozhen., Tematicheskii Sb. Nauchnykh Tr. Mosk. Aviatsion. Inst.,339, 81?87 (1975).
3. N. G. Askerov, S. G. Krein, and G. I. Laptev, ?On a class of non-self-adjoint boundary problems,? Dokl. Akad. Nauk SSSR,155, No. 3, 499?502 (1964).
4. I. C. Gohberg and M. G. Krein, Introduction to the Theory of Linear Nonselfadjoint Operators, Amer. Math. Soc. (1969).
5. L. Hörmander, ?Pseudodifferential operators and nonelliptic boundary problems,? Ann. Math.,83, 129?209 (1966).