BURSKII, V. P.; Shtepina Т. V. On the Spectrum of an Equivariant Extension of the Laplace Operator in a Ball. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 52, n. 11, p. 1473–1483, 2000. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/4554. Acesso em: 5 dec. 2025.