SAFARYAN, R. G. Two theorems on closeness of the set of Laplace-type transformations. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 45, n. 4, p. 582–584, 1993. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/5846. Acesso em: 5 dec. 2025.