KOLEV, M. On the two-sphere problem in an absorbing medium. Ann. Sofia Univ. Fac. Math. and Inf., [S. l.], v. 86, n. 2, p. 31–39, 1994. Disponível em: https://annual.uni-sofia.bg/index.php/fmi/article/view/505. Acesso em: 18 apr. 2025.