KOSHMANENKO, V. D. Regularized approximations of singular perturbations from the $H_2$-class. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 52, n. 5, p. 626–637, 2000. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/4457. Acesso em: 5 dec. 2025.