[1] Christ M. Hypoellipticity in the infinitely degenerate regime//McNeal J D. Complex Analysis and Geometry. Ohio State Univ Math Res Inst Publ 9. Berlin: de Gruyter, 2001: 59–84

[2] Koike M. A note on hypoellipticity for degenerate elliptic operates. Publ RIMS Kyoto Univ, 1991, 27: 995–1000

[3] Kohn J J. Hypoellipticity at points of infinite type//Grinberg E L, et al. Analysis, Geometry, Number Theory: the Mathematics of Leon Ehrenpreis (Philadelphia, 1998). 251. Contemp Math, 2000: 393–398

[4] Morimoto Y, Morika T. The positivity of Schr¨odinger operators and the hypoellipticity of second order degenerate elliptic operators. Bull Sc Math, 1997, 121: 507-547

[5] Morimoto Y, Morika T. Hypoellipticity for elliptic operators with infinite degeneracy//Chen Hua, Rodino L, eds. Partial Differential Equations and Their Applications. River Edge, NJ: World Sci Publishing, 1999: 240–259

[6] Morimoto Y, Xu C J. Logarithmic sobolev inequality and semi-linear Dirichlet problems for infinitely degenerate elliptic operators. Ast´erisque, 2003, 234: 245–264

[7] Xu C J. Semilinear subelliptic equations and Sobolev inequality for vector fields satisfying H¨ormander’s condition. Chinese J Contemp Math, 1994, 15: 183–193

[8] Struwe M. Variational Methods: Applications to Nonlinear Partial Differential Equations and Hamitonian Systems. Second ed. Springer-Verlag, 2000

[9] Chen H, Li K. The existence and regularity of multiple solutions for a class of infinitely degenerate elliptic equations. Math Nach, 2009, 282(3): 368–385

[10] Chen H, Li W X, Xu C J. Gevrey regularity for solution of the spatially homogeneous Landau equation. Acta Math Sci, 2009, 29B(3): 673–686