“…Originally Levi's condition (7 = 1 was introduced as Condition C in Case I by E. E. Levi [23], A. Lax [20], M. Yamaguti [35], S. Mizohata-Y. Ohya [26] and J. Chazarain [2]. They proved that the Cauchy problem (17) is correctly posed in Q and S)' if Conditions A and B hold and if every real characteristic element is non-singular and of irregularity 1.…”