1. A. Yu. Anikin, S. Yu. Dobrokhotov, and V. E. Nazaikinskii, “Simple asymptotics for a generalized wave equation with degenerating velocity and their applications in the linear long wave run-up problem,” Mat. Notes 104(4), 471–488 (2018) [transl. from Mat. Zametki 104 (4), 483–504 (2018)].
2. V. I. Arnol’d, “Characteristic class entering in quantization conditions,” Funct. Anal. Appl. 1(1), 1–13 (1967) [transl. from Funkts. Anal. Prilozh. 1 (1), 1–14 (1967)].
3. V. I. Arnold, Mathematical Methods of Classical Mechanics (Nauka, Moscow, 1989). Engl. transl. of the 1st ed.: V. I. Arnold, Mathematical Methods of Classical Mechanics (Springer, New York, 1978), Grad. Texts Math. 60.
4. V. I. Arnold, A. N. Varchenko, and S. M. Gusein-Zade, Singularities of Differentiable Maps: The Classification of Critical Points, Caustics and Wave Fronts (Nauka, Moscow, 1982). Engl. transl.: V. I. Arnold, S. M. Gusein-Zade, and A. N. Varchenko, Singularities of Differentiable Maps, Vol. 1: The Classification of Critical Points, Caustics and Wave Fronts (Birkhäuser, Boston, 1985), Monogr. Math. 82.
5. M. S. Birman and M. Z. Solomjak, Spectral Theory of Self-Adjoint Operators in Hilbert Space (Leningr. Gos. Univ., Leningrad, 1980; Kluwer, Dordrecht, 1987).