RADZIEVSKII, G. V. On the best approximations and rate of convergence of decompositions in the root vectors of an operator. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 49, n. 6, p. 754–773, 1997. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/5060. Acesso em: 6 dec. 2025.