LAVRENT'EV, A. S. A singularly perturbed spectral problem for a biharmonic operator with Neumann conditions. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 51, n. 11, p. 1467–1475, 1999. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/4748. Acesso em: 6 dec. 2025.