M. S. Agranovich and A. S. Markus, On spectral proporties of elliptic pseudo-differentiel operators far from selfadjoint ones, Zeitschrift für Analysis unt ihre Anwendungen, Bd, vol.8, issue.3, pp.237-260, 1989.

M. S. Baouendi and C. , Nonanalytic-hypoellipticity for some degenerate elliptic operators, Bulletin of the American Mathematical Society, vol.78, issue.3, pp.483-486, 1972.
DOI : 10.1090/S0002-9904-1972-12955-0

R. Beals, A general calculus of pseudo-differential operators, Duke Math, J, pp.1-42, 1977.

M. Birman and M. Solomjak, Spectral Theory of self-adjoint operators in Hilbert space, 1986.
DOI : 10.1007/978-94-009-4586-9

A. Bouzouina and D. Robert, Bibliographie Uniform semiclassical estimates for the propagation of quantum observables, Duke Math, J, vol.111, issue.2, pp.223-252, 2002.

S. Chanillo, B. Helffer, and A. Laptev, Nonlinear eigenvalues and analytic hypoellipticity, Journal of Functional Analysis, vol.209, issue.2, pp.425-443, 2004.
DOI : 10.1016/S0022-1236(03)00105-8

A. Fridman and M. Shinbrot, Nonlinear eigenvalue problems, Acta Mathematica, vol.121, issue.0, pp.77-128, 1968.
DOI : 10.1007/BF02391910

B. Helffer, Remarques sur des résultats de G. Métivier sur la non-hypo-analyticité, pp.1978-1979

B. Helffer and D. Robert, Semiclassical Analysis of nonlinear eigenvalue problem and non analytic hypoellipticity, 2003.

B. Helffer, D. Robert, and X. P. Wang, Semiclassical analysis of a nonlinear eigenvalue problem and nonanalytic hypoellipticity, St. Petersburg Mathematical Journal, vol.16, issue.01, pp.285-296, 2005.
DOI : 10.1090/S1061-0022-04-00851-9

I. Gohberg and M. G. , Krein : Introduction à la théorie des opérateurs linéaires non autoadjoints, 1972.

M. G. Krein and H. Langer, On some mathematical principles in the linear theory of damped oscillations of continua I, Integral Equations and operators Theory, pp.364-399, 1978.

G. Lebeau, Equation des Ondes Amorties, Séminaire Équations aux partielles (Polytechnique), pp.1-14, 1993.
DOI : 10.1007/978-94-017-0693-3_4

A. S. Markus, Introduction to the spectral theory of polynomial operator pencils Translation of mathematical monographs

D. Robert, Non linear eigenvalue problems, Matemàtica Contemporanea, vol.26, pp.109-127, 2004.
URL : https://hal.archives-ouvertes.fr/hal-00003472

D. Robert, Non linear eigen value problems with small parameter, Integral Equations and Operators Theory, pp.231-240, 1984.

R. T. Seeley, Complex power of an elliptic operator, Singular integrals, Proc. Symposia Pure Math, pp.288-307, 1967.