1. Cauchy problems with characteristic initial hypersurface;Baouendi;Comm. Pure Appl. Math.,1973
2. On Differential equations having non-analytic solutions;Bronšteǐn;Uspehi Mat. Nauk,1975
3. On the theory of general partial differential operators;Hörmander;Acta Math.,1955
4. Non-uniqueness for the Cauchy problem;Hörmander,1975
5. Irregularity of characteristic elements and construction of null-solutions;Komatsu;J. Fac. Sci. Univ. Tokyo Sect. IA,1976