1. [B] D. R. Bell, The Malliavin calculus, Pitman Monographs and Surveys in Pure and Applied Mathematics, vol. 34, Longman Scientific & Technical, Harlow, 1987.
2. [BM] D. R. Bell and S.-E. A. Mohammed, Hypoelliptic parabolic operators with exponential degeneracies, C. R. Acad. Sci. Paris Sér. I Math. 317 (1993), no. 11, 1059–1064.
3. [FP] C. Fefferman and D. Phong, Subelliptic eigenvalue problems, Conference on harmonic analysis in honor of Antoni Zygmund, Vol. I, II (Chicago, Ill., 1981), Wadsworth Math. Ser., Wadsworth, Belmont, CA, 1983, pp. 590–606.
4. [H] L. Hörmander, Hypoelliptic second order differential equations, Acta Math. 119 (1967), 147–171.
5. [IW] N. Ikeda and S. Watanabe, Stochastic differential equations and diffusion processes, North-Holland Mathematical Library, vol. 24, North-Holland Publishing Co., Amsterdam, 1989.