KARWOWSKI, W.; KOSHMANENKO, V. D. On the definition of singular bilinear forms and singular linear operators. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 45, n. 8, p. 1084–1089, 1993. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/5903. Acesso em: 5 dec. 2025.