1. M.Sh. Birman, M.Z. Solomyak, Spectral asymptotics of pseudodifferential operators with anisotropic homogeheous symbols. I, Vestn. Leningr. Univ., Mat. Mekn. Astron. 13 (1977), 13–21; II, Vestn. Leningr. Univ., Mat. Mekn. Astron. 13 (1979), 5-10. (Russian).
2. M.Sh. Birman, M.Z. Solomyak, Estimates for the number of negative eigenvalues of the Schrödinger operator and its generalizations. AMS. Adv. Sov. Math. 7 (1991), 1–55.
3. M. Klaus On the point spectrum of Dirac operators. Helvetica Physica Acta 53 (1980), 453–462.
4. A. Laptev, Asymptotics of the negative discrete spectrum of a class of Schrödinger operators with large coupling constant. Proc. of AMS.
5. B. Thaller, The Dirac equation. Texts and Monographs in Physics, Springer-Verlag, 1992.