Borisov I D., Zezyulin D.A., Znojil M., “Bifurcations of Thresholds in Essential Spectra of Elliptic Operators Under Localized Non-Hermitian Perturbations”, Stud. Appl. Math., 146:4 (2021), 834–880
Borisov I D., Zezyulin D.A., “Bifurcations of Essential Spectra Generated By a Small Non-Hermitian Hole. i. Meromorphic Continuations”, Russ. J. Math. Phys., 28:4 (2021), 416–433
Д. И. Борисов, М. Знойил, “О собственных значениях $\mathscr{P\!T}$-симметричного оператора в тонком слое”, Матем. сб., 208:2 (2017), 3–30 ; D. I. Borisov, M. Znojil, “On eigenvalues of a $\mathscr{P\!T}$-symmetric operator in a thin layer”, Sb. Math., 208:2 (2017), 173–199