OSPANOV, K. N. Solvability conditions for the first order elliptic systems on the plane. Eurasian Mathematical Journal, [S. l.], v. 3, n. 1, p. 139–142, 2012. Disponível em: https://emj.enu.kz/index.php/main/article/view/784. Acesso em: 31 oct. 2025.