KOVACH, Y. I. Theorem of the «fork» in Cauchy’s problem for nonlinear partial differential equations of higher orders. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 18, n. 5, p. 23–35, 1966. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/8438. Acesso em: 6 dec. 2025.