1. Bitsadze, A. V.: Equations of Mixed Type. Oxford-London-New York-Paris: Pergamon Press. 1964.
2. Frankl, F.: On the problem of Chaplygin for mixed sub-and supersonic flows. Bull. Acad. Sci. USSR8, 195?224 (1945).
3. Haack, W., undW. Wendland: �ber partielle und Pfaffsche Differentialgleichungen. Basel and Stuttgart: Birkh�user. 1969.
4. Mikhlin, S. G.: Linear Equations of Mathematical Physics. Holt, Reinehart and Winston, Inc. 1967.
5. Morawetz, C. S.: A uniqueness theorem for Frankl's problem. Comm. Pure Appl. Math.7, 697?703 (1954).