GEORGIEV, G. Perturbations in a champagne bottle. Ann. Sofia Univ. Fac. Math. and Inf., [S. l.], v. 89, p. 141–157, 1997. Disponível em: https://ftl5.uni-sofia.bg/index.php/fmi/article/view/363. Acesso em: 7 apr. 2025.