1. J. Hadamard, “Sur les problemes aux derivees partielles et leur signification physique,” Princeton Univ. Bull., 13, 49–52 (1902).
2. A. V. Bitsadze, Equations of Mixed Type [in Russian], Akademiya Nauk SSSR, Moscow (1959).
3. A. M. Nakhushev, Problems with Shift for Partial Differential Equations [in Russian], Nauka, Moscow (2006).
4. D. G. Bourgin and R. Duffin, “The Dirichlet problem for the vibrating string equation,” Bull. Amer. Math. Soc., 45, 851–858 (1939).
5. D. W. Fox and C. Pucci, “The Dirichlet problem for the wave equation,” Ann. Math. Pura Appl., 46, 155–182 (1958).