IBRAHIM, S. E.-S. On the product of self-adjoint Sturm-Liouville differential operators in direct sum spaces. Tamkang Journal of Mathematics, [S. l.], v. 37, n. 1, p. 77–92, 2006. DOI: 10.5556/j.tkjm.37.2006.181. Disponível em: https://journals.math.tku.edu.tw/index.php/TKJM/article/view/181. Acesso em: 1 may. 2024.