1. Courant, R.,Partial Differential Equations, New York, 1962. Translated under the titleUravneniya schastnymi proizvodnymi, Moscow, 1964.
2. Hadamard, J.,Zadacha Koshi dlya lineinykh uravnenii s chastnymi proizvodnymi giperbolicheskogo tipa (The Cauchy Problem for Linear Partial Differential Equations of Hyperbolic Type), Moscow, 1978.
3. Reisz, M.,Acta Math., 1949, vol. 81, pp. 107–125.
4. Lundberg, L.,Comm. in Math. Phis., 1978, vol. 62, no. 2, pp. 107–118.
5. Tolen, J.,C. R. Acad. Sci. Paris, Ser. A-B, 1980, vol. 291, no. 1, pp. 49–52.