1. On hypoellipticity of second order equations;Beals;Comm. Partial Differential Equations,1976
2. Solvability and nonsolvability of second-order evolution equations;Colombini,2003
3. Geometric Measure Theory;Federer,1996
4. An example in the solvability theory of linear PDE's;Gilioli;Amer. J. Math.,1974
5. Some analytic bounds for subanalytic sets;Hardt,1983