MIKHAILETS, V. A.; MOLYBOGA, V. M. Singularly perturbed periodic and semiperiodic differential operators. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 59, n. 6, p. 785–797, 2007. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/3345. Acesso em: 6 dec. 2025.