On the two-sphere problem in an absorbing medium. Annual of Sofia University St. Kliment Ohridski. Faculty of Mathematics and Informatics, [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: 14 dec. 2025.