On the structure of characteristic surfaces related with partial differential equations of first and higher orders. Pt 2. Neliniini Kolyvannya, [S. l.], v. 8, n. 4, p. 529–543, 2005. Disponível em: https://twinhead.imath.kiev.ua/index.php/nosc/article/view/371. Acesso em: 22 jan. 2026.