M. S. Ashbaugh and L. Hermi, On Harrell-Stubbe type inequalities for the discrete spectrum of a self-adjoint operator. arXiv:0712.4396v1 [math, p.42, 2007.

E. Barletta and S. Dragomir, On the spectrum of a strictly pseudoconvex cr Manifold, Abhandlungen aus dem Mathematischen Seminar der Universit??t Hamburg, vol.13, issue.12, pp.33-46, 1997.
DOI : 10.1007/BF02940818

E. Barletta, The Lichnerowicz theorem on CR manifolds, Tsukuba J. Math, vol.31, issue.1, pp.77-97, 2007.

E. Barletta and S. Dragomir, Sublaplacians on CR manifolds. Bull, Math. Soc. Sci. Math. Roumanie (N.S.), vol.52, issue.1001, pp.3-32, 2009.

E. Barletta, S. Dragomir, and H. Urakawa, Pseudoharmonic maps from nondegenerate CR manifolds to Riemannian manifolds, Indiana University Mathematics Journal, vol.50, issue.2, pp.719-746, 2001.
DOI : 10.1512/iumj.2001.50.1931

URL : http://doi.org/10.1512/iumj.2001.50.1931

E. Barletta, S. Dragomir, and H. Urakawa, Yang???Mills fields on CR manifolds, Journal of Mathematical Physics, vol.47, issue.8, pp.83504-83545, 2006.
DOI : 10.1063/1.2222082

S. Chang and H. Chiu, On the CR analogue of Obata???s theorem in a pseudohermitian 3-manifold, Mathematische Annalen, vol.14, issue.3, pp.33-51, 2009.
DOI : 10.1007/s00208-009-0339-3

D. Chen and Q. Cheng, Extrinsic estimates for eigenvalues of the Laplace operator, Journal of the Mathematical Society of Japan, vol.60, issue.2, pp.325-339, 2008.
DOI : 10.2969/jmsj/06020325

Q. Cheng and H. Yang, Estimates on Eigenvalues of Laplacian, Mathematische Annalen, vol.7, issue.2, pp.445-460, 2005.
DOI : 10.1007/s00208-004-0589-z

Q. Cheng and H. Yang, Bounds on eigenvalues of Dirichlet Laplacian, Mathematische Annalen, vol.7, issue.1, pp.159-175, 2007.
DOI : 10.1007/s00208-006-0030-x

D. Danielli, N. Garofalo, and D. M. Nhieu, Sub-Riemannian calculus on hypersurfaces in Carnot groups, Advances in Mathematics, vol.215, issue.1, pp.292-378, 2007.
DOI : 10.1016/j.aim.2007.04.004

S. Dragomir and G. Tomassini, Differential geometry and analysis on CR manifolds, Progress in Mathematics. Birkhäuser Boston Inc, vol.246, 2006.

J. Eells and L. Lemaire, Another Report on Harmonic Maps, Bulletin of the London Mathematical Society, vol.20, issue.5, pp.385-524, 1988.
DOI : 10.1112/blms/20.5.385

A. Soufi and S. Ilias, Une in??galit?? du type ???Reilly??? pour les sous-vari??t??s de l'espace hyperbolique, Commentarii Mathematici Helvetici, vol.67, issue.1, pp.167-181, 1992.
DOI : 10.1007/BF02566494

A. El-soufi, M. Evans, I. Harrell, and S. Ilias, Universal inequalities for the eigenvalues of Laplace and Schr??dinger operators on submanifolds, Transactions of the American Mathematical Society, vol.361, issue.05, pp.2337-2350, 2009.
DOI : 10.1090/S0002-9947-08-04780-6

A. Greenleaf, The first eigenvalue of a sublaplacian on a pseudohermitian manifold, Communications in Partial Differential Equations, vol.13, issue.2, pp.191-217, 1985.
DOI : 10.1080/03605308508820376

M. Evans and I. Harrell, Some geometric bounds on eigenvalue gaps, Comm. Partial Differential Equations, vol.18, issue.12, pp.179-198, 1993.

M. Evans and I. Harrell, Commutators, eigenvalue gaps, and mean curvature in the theory of Schrödinger operators, Comm. Partial Differential Equations, vol.32, issue.1-3, pp.401-413, 2007.

M. Evans, P. L. Harrell, and . Michel, Commutator bounds for eigenvalues, with applications to spectral geometry, Comm. Partial Differential Equations, vol.19, pp.11-122037, 1994.

M. Evans, J. Harrell, and . Stubbe, On trace identities and universal eigenvalue estimates for some partial differential operators, Trans. Amer. Math. Soc, vol.349, issue.5, pp.1797-1809, 1997.

M. Evans, J. Harrell, and . Stubbe, Universal bounds and semiclassical estimates for eigenvalues of abstract Schrödinger operators, SIAM J. Math. Anal, vol.42, issue.5, pp.2261-2274, 2010.

S. Li and H. Luk, The sharp lower bound for the first positive eigenvalue of a sub-Laplacian on a pseudo-Hermitian manifold, Proc. Amer, pp.789-798, 2004.

A. Menikoff and J. Sjöstrand, On the eigenvalues of a class of hypoelliptic operators, Mathematische Annalen, vol.1, issue.1, pp.55-85, 1978.
DOI : 10.1007/BF01421593

P. Niu and H. Zhang, Payne???Polya???Weinberger type inequalities for eigenvalues of nonelliptic operators, Pacific Journal of Mathematics, vol.208, issue.2, pp.325-345, 2003.
DOI : 10.2140/pjm.2003.208.325

L. E. Payne, G. Pólya, and H. F. Weinberger, On the Ratio of Consecutive Eigenvalues, Journal of Mathematics and Physics, vol.35, issue.1-4, pp.289-298, 1956.
DOI : 10.1002/sapm1956351289

X. C. Peng, W. Y. , and C. , Spectra of subelliptic operators on S 3, J. Math. (Wuhan), vol.29, issue.3, pp.297-299, 2009.

S. Raphaël and . Ponge, Heisenberg calculus and spectral theory of hypoelliptic operators on Heisenberg manifolds, Mem. Amer. Math. Soc, vol.194, issue.134, 2008.

H. C. Yang, An estimate of the difference between consecutive eigenvalues, 1995.