GRUSHKA, Ya. I. Direct approximation problems for semi-groups in Hilbert space. Methods of Functional Analysis and Topology, [S. l.], v. 5, n. 2, p. 12–21, 1999. Disponível em: https://zen.imath.kiev.ua/index.php/mfat/article/view/85. Acesso em: 18 dec. 2025.