1. R. Courant and D. Hilbert, Methods of Mathematical Physics (Interscience, New York, 1953), Vol. 1, Ch. 7, § 5.
2. V. I. Arnol’d, “Distribution of Ovals of the Real Plane Algebraic Curves, Involutions of Four-Dimensional Smooth Manifolds, and the Arithmetic of Integer-Valued Quadratic Forms,” Funkts. Anal. Prilozh. 5(3), 1–9 (1971) [Funct. Anal. Appl. 5, 169–176 (1971)].
3. V. I. Arnol’d, “Modes and Quasimodes,” Funkts. Anal. Prilozh. 6(2), 12–20 (1972) [Funct. Anal. Appl. 6, 94–101 (1972)].
4. V. I. Arnol’d and E. I. Korkina, “The Growth of a Magnetic Field in a Three-Dimensional Steady Incompressible Flow,” Vestn. Mosk. Univ., Ser. 1: Mat., Mekh., No. 3, 43–46 (1983) [Moscow Univ. Math. Bull. 38 (3), 50–54 (1983)].
5. V. I. Arnold, “Remarks on Eigenvalues and Eigenvectors of Hermitian Matrices, Berry Phase, Adiabatic Connections and Quantum Hall Effect,” Sel. Math., New Ser. 1(1), 1–19 (1995).