AKHMED-ZAKI, D. K. Systems of first order partial differential equations with singular lines. Journal of Mathematics, Mechanics and Computer Science, [S. l.], v. 71, n. 4, p. 3–7, 2011. Disponível em: https://bm.kaznu.kz/index.php/kaznu/article/view/213. Acesso em: 22 dec. 2024.