1. L. Bers, The Dirichlet problem for a partial differential equation of mixed type, Bull. Amer. Math. Soc. Abstract 54-11-504.
2. On the continuation of a potential gas flow across the sonic line;Bers, Lipman;Tech. Notes Nat. Adv. Comm. Aeronaut.,1950
3. M. Cibrario, Sui teoremi di existenza e di unicita relativi ad alcune equazioni differenziali a derivate parziali, Rendiconti di R. Accademia dei Lincei (6) vol. 13 (1931).
4. G. Darboux, Leçons sur la théorie générale des surfaces, vol. 2, Paris, 1889.
5. F. I. Frankl, On the Cauchy problem for equations of mixed elliptic-hyperbolic type with initial data on the parabolic line, Bull. Acad. Sci. URSS. vol. 8 (1944).