1. Non-analytic hypoellipticity for some degenerate elliptic operators;Baouendi;Bull. Amer. Math. Soc.,1972
2. Uniform hypoelliptic Green's functions;Beals;J. Math. Pures Appl.,1998
3. R. Beals, B. Gaveau, and, P. C. Greiner, On Green's functions for subelliptic operators, in, Proc. Cuernavaca Conference, in press.
4. Exact fundamental solutions for a class of degenerate elliptic equations;Beals;Comm. Partial Differential Equations,1999
5. Optimal non-isotropic Gevrey exponents for sums of squares of vector fields;Bove;Comm. Partial Differential Equations,1997