1. M. Sh. Birman and M. Z. Solomyak, ?Asymptotics of the spectrum of pseudodifferential operators with anisotropic, homogeneous symbols,? Vestn. Leningr. Gos. Univ., No. 13, 13?21 (1977).
2. B. R. Vainberg and V. V. Grushin, ?On uniformly nonelliptic problems. I, II,? Mat. Sb.,72, No. 4, 602?636 (1967).; No. 1, 126?154 (1967).
3. R. Seeley, ?The resolvent of an elliptic boundary problem,? Am. J. Math.,91, No. 4, 889?920 (1969).
4. L. Hörmander, ?Pseudodifferential operators and nonelliptic boundary value problems,? in: Pseudodifferential Operators [Russian translation], Mir, Moscow (1967), pp. 166?296.
5. M. Sh. Birman and M. Z. Solomyak, ?On the asymptotics of the spectrum of variational problems with differential connections,? in: Proceedings of the Conference Dedicated to the 75th Birthday of Academician I. G. Petrovskii, Moscow State Univ. (1978).