1. M. V. Keldysh, “On the eigenfunctions and eigenvalues for some classes of nonself-adjoint equations,” Dokl. Akad. Nauk SSSR, 77:1) (1951), 11–14.
2. A. A. Shkalikov, “Perturbations of self-adjoint and normal operators with discrete spectrum,” Uspekhi Mat. Nauk, 71:5) (2016), 113–174; English transl.: Russian Math. Surveys, 71:5 (2016), 907–964.
3. V. B. Lidskii, “A nonself-adjoint operator of Sturm–Liouville type with a discrete spectrum,” Trudy Moskov. Mat. Obshch., 9 (1960), 45–79.
4. E. B. Davies, “Wild spectral behaviour of anharmonic oscillators,” Bull. Lond. Math. Soc., 32:4) (2000), 432–438.
5. Materials of the workshop ”Mathematical aspects with non-self-adjoint operators”. A list of open problems, http://aimath.org/pastworkshops/nonselfadjoint-problems.pdf.