MAKHNEI, O.V.; TATSII, R.M. Expansion by eigenvectors in case of simple eigenvalues of singular differential operator. Carpathian Mathematical Publications, [S. l.], v. 3, n. 1, p. 94–105, 2011. Disponível em: https://journals.pnu.edu.ua/index.php/cmp/article/view/3959. Acesso em: 24 may. 2024.