KUZHEL', S. A. Spectral analysis of doubly $J$-nonexpanding operators. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 45, n. 3, p. 384–388, 1993. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/5820. Acesso em: 5 dec. 2025.