The spectrum of a Harmonic Oscillator Operator Perturbed by Point   Interactions
  We consider the operator $ L = - (d/dx)^2 + x^2 y + w(x) y , y \in L^2(\mathbb{R}) $, where $ w(x) = s [ \delta(x - b) - \delta(x + b)], b \neq 0,$ real, $s \in \mathbb{C}$. This operator has a discrete spectrum: eventually the eigenvalues are simple and $\lambda_n = (2n + 1) + s^2 (\kappa(n) / n) + \rho(n)$, where $ \kappa(n) = \frac{1}{2\pi} [(-1)^{n + 1} \sin ( 2 b \sqrt{2n} ) - \frac{1}{2} \sin ( 4 b \sqrt{2n} ) ]$ and $ |\rho(n) | \leq C (\log n) / (n^{3/2})$ If $s = i \gamma$, $\gamma$ real, the number $T(\gamma)$ of non-real eigenvalues is finite, and $T(\gamma) \leq [ C (1 + | \gamma |) \log (e + | \gamma |)]^2.$ The analogue of the above equations is given in the case of any two-point interaction perturbation $w(x) = c_+ \delta(x - b) + c_- \delta(x + b), c_+, c_- \in \mathbb{C}. $ 
Boris Mityagin
math.SP math-ph math.MP