1. R. Courant and D. Hilbert, Methods of Mathematical Physics, Vol. II (John Wiley and Sons, New York, 1962).
2. V. Ja. Ivrii and V. M. Petkov, “Necessary Conditions for the Correctness of the Cauchy Problem for Non-Strictly Hyperbolic Equations,” Uspehi Mat. Nauk 29(5), 3–70 (1974).
3. V. P. Maslov and M. V. Fedoryuk, Semiclassical Approximation for the Equations of Quantum Mechanics (Nauka, Moscow, 1976; D. Reidel Publishing Co., Dordrecht-Boston, Mass., 1981).
4. V. V. Kucherenko and A. V. Krivko, “On an Existence Theorem for Hyperbolic Systems with a Multiplicity Change Point of Order Three or Greater,” Mat. Zametki 85(1), 139–143 (2009) [translation in Math. Notes 85 (1–2), 128–132 (2009), “Existence Theorem for Hyperbolic Systems with Multiplicity Change Point of at Most Third Order”].
5. A. V. Krivko and V. V. Kucherenko, “On Real Hyperbolic Systems with Characteristics of Variable Multiplicity,” Dokl. Akad. Nauk 412(4), 451–455 (2007) [Doklady Math. 75 (1), 83–86 (2007)].