Perturbations of non self-adjoint Sturm-Liouville problems, with applications to harmonic oscillators

We study the behavior of the limit of the spectrum of a non self-adjoint Sturm-Liouville operator with analytic potential as the semi-classical parameter $h\to 0$. We get a good description of the spectrum and limit spectrum near $\infty$. We also study the action of one special perturbation of the operator (adding a Heaviside function), and prove that the limit spectrum is very unstable. As an illustration we describe the limit spectrum as $h\to 0$ for $P^h=-h^2\Delta+i x^2$ and the effect of this perturbation.