PACHPATTE, B. G. . On nonlinear finite difference inequalities in two independent variables. Tamkang Journal of Mathematics, [S. l.], v. 33, n. 1, p. 57-66, 2002. DOI: 10.5556/j.tkjm.33.2002.305. Disponível em: https://journals.math.tku.edu.tw/index.php/TKJM/article/view/305. Acesso em: 12 may. 2021.