SAPAGOVIENĖ, D. On calculation of Laplace’s operator eigenvalues. Lithuanian Mathematical Journal, [S. l.], v. 11, n. 4, p. 875–882, 1971. DOI: 10.15388/LMJ.1971.21041. Disponível em: https://www.journals.vu.lt/LMJ/article/view/21041. Acesso em: 14 may. 2025.