EROVENKO, B. A. Essential spectra of non-collective ordinary differential operators with complex coefficients. Mathematical Modelling and Analysis, [S. l.], v. 1, n. 1, p. 21–24, 1996. DOI: 10.3846/13926292.1996.9637043. Disponível em: https://journals.vilniustech.lt/index.php/MMA/article/view/10030. Acesso em: 13 jul. 2025.