A generalization of the Schmidt lemma to the case of an n- or d-normal operator in a Banach space. Neliniini Kolyvannya, [S. l.], v. 12, n. 4, p. 443–450, 2009. Disponível em: https://twinhead.imath.kiev.ua/index.php/nosc/article/view/537. Acesso em: 21 jan. 2026.