TRETTER, Ch.; MENNICKEN, R.; LANGER, H. A self-adjoint linear pencil $Q-\lambda P$ of ordinary differential operators. Methods of Functional Analysis and Topology, [S. l.], v. 2, n. 1, p. 38–54, 1996. Disponível em: https://zen.imath.kiev.ua/index.php/mfat/article/view/9. Acesso em: 19 dec. 2025.