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2. S. Gellerstedt, “Sur un problème aux limites pour une équation linéaire aux dérivées partielles du second ordre de tipe mixte,” PhD Thesis (Uppsala, 1935).
3. A. V. Bitsadze, Equations of the Mixed Type (Izd. Akad. Nauk SSSR, Moscow, 1959; Pergamon Press, Oxford, 1964).
4. K. I. Babenko, “On the theory of equations of mixed type,” Extended Abstract of the Doctoral Dissertation in Physics and Mathematics (Akad. Nauk SSSR, Moscow, 1952) [in Russian].
5. I. L. Karol’, “On a boundary problem for an equation of mixed elliptic-hyperbolic type,” Dokl. Akad. Nauk SSSR 88 (2), 197–200 (1953).