GORBACHUK, V. I.; GORBACHUK, M. L. On the representation of the vacuum mean of field operators in a space with an indefinite metrics. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 18, n. 6, p. 108–111, 1966. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/8464. Acesso em: 6 dec. 2025.