EROVENKO, V. A. Methods of perturbation theory in the qualitative essential spectral analysis of differential operators. Mathematical Modelling and Analysis, [S. l.], v. 3, n. 1, p. 86–92, 1998. DOI: 10.3846/13926292.1998.9637090. Disponível em: https://journals.vilniustech.lt/index.php/MMA/article/view/9991. Acesso em: 2 jun. 2025.