Higher order Melnikov functions for piecewise smooth differential systems with four zones. Gulf Journal of Mathematics, [S. l.], v. 22, n. 2, 2026. DOI: 10.56947/gjom.v22i2.4085. Disponível em: https://gjom.org/index.php/gjom/article/view/4085. Acesso em: 25 aug. 2026.