IL'KIV, V. S.; PTASHNIK, B. I. Problems for partial differential equations with nonlocal conditions. Metric approach to the problem of small denominators. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 58, n. 12, p. 1624–1650, 2006. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/3560. Acesso em: 6 dec. 2025.