1. E. B. Davies, ‘‘Wild spectral behaviour on
anharmonic oscillators,’’ Bull. London Math. Soc. 32,
432–438 (2000).
2. V. B. Lidskii, ‘‘Conditions for completeness of
a system of root subspaces for non-selfadjoint operators with
discrete spectra,’’ Am. Math. Soc. Transl. Ser. 34,
241–281 (1963).
3. V. B. Lidskii, ‘‘A non-self-adjoint operator of
Sturm-Liouville type with discrete spectrum,’’ Trans. Mosc. Math.
Soc. 9, 45–79 (1960).
4. E. B. Davies, ‘‘Pseudo-spectra, the harmonic
oscillator and complex resonances,’’ Proc. R. Soc. 455,
585–599 (1999).
5. Kh. K. Ishkin, ‘‘On the spectral instability of
the Sturm–Liouville operator with a complex potential,’’ Differ.
Equat. 45, 494–509 (2009).