GRETSKII, O. S. $ℂ$-differentiability of mappings of Hilbert spaces. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 47, n. 3, p. 424–428, 1995. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/5436. Acesso em: 14 apr. 2025.