1. Burnat, M.: The method of Riemann invariants for multidimensional nonelliptic systems. Bull. Acad. Pol. Sci., Sér. Sci. Tech. 17, 11 (1969)
2. Burnat, M.: Hyperbolic double waves. Bull. Acad. Pol. Sci., Sér. Sci. Tech. 16, 1 (1969)
3. Burnat, M., Grundland, A.M.: Conical solutions of the autonomous partial differential systems. Rep. Math. Phys. 30(1), 71–80 (1991)
4. Cartan, E.: Les systèmes différentielles en involution. In: Sur la Structure des Groupes Infinis de Transformations. Gauthiers-Villars, Paris (1953)
5. Friedrich, K.O.: Nonlinear hyperbolic differential equations for functions of two independent variables. Am. J. Math. 70, 555–589 (1948)