MOGILEVSKII, V. I. On exit space extensions of symmetric operators with applications to first order symmetric systems. Methods of Functional Analysis and Topology, [S. l.], v. 19, n. 3, p. 268–292, 2013. Disponível em: https://zen.imath.kiev.ua/index.php/mfat/article/view/552. Acesso em: 16 dec. 2025.