Global Solutions for Certain Systems of Quasi-Linear Hyperbolic Equations J. JohnsonJ. Smoller Indiana University Mathematics Journal 1968 text pdf 10.1512/iumj.1968.17.17034 10.1512/iumj.1968.17.17034 en Indiana Univ. Math. J. 17 (1968) 561 - 576 state-of-the-art mathematics http://iumj.org/access/