DIAS, J. P.; FIGUEIRA, M. On the Existence of Weak Solutions for a Semilinear Singular Hyperbolic Systems. Revista Matemática Complutense, v. 4, n. 1, p. 137, 1 ene.1991 https://doi.org/10.5209/rev_REMA.1991.v4.n1.18009.