On the Occurrence of "Vacuum States" for 2 $\times$ 2 Quasilinear Hyperbolic Conservation Laws. Journal of Partial Differential Equations, [S. l.], v. 8, n. 1, p. 64–72, 1995. Disponível em: https://global-sci.com/index.php/jpde/article/view/14847. Acesso em: 5 dec. 2025.