IBRAHIM, S. E.-S. On the boundary conditions for products of Sturm-Liouville differential operators. Tamkang Journal of Mathematics, [S. l.], v. 32, n. 3, p. 187–199, 2001. DOI: 10.5556/j.tkjm.32.2001.374. Disponível em: https://journals.math.tku.edu.tw/index.php/TKJM/article/view/374. Acesso em: 19 apr. 2024.