Discontinuous Initial Value Problems for Symmetric Hyperbolic Linear Differential Equations Robert Lewis Indiana University Mathematics Journal 1958 text pdf 10.1512/iumj.1958.7.57034 10.1512/iumj.1958.7.57034 en Indiana Univ. Math. J. 7 (1958) 571 - 592 state-of-the-art mathematics http://iumj.org/access/