KALITA, E. A. On nonlinear higher order elliptic systems positive in the first derivatives. Ukrains’kyi Matematychnyi Zhurnal, [S. l.], v. 45, n. 7, p. 942–947, 1993. Disponível em: https://umj.imath.kiev.ua/index.php/umj/article/view/5887. Acesso em: 5 dec. 2025.