The Three-Dimensional Wave Equation in a Characteristic Quarter-Space Yanping ChenReuben Hersh Indiana University Mathematics Journal 1968 text pdf 10.1512/iumj.1968.17.17026 10.1512/iumj.1968.17.17026 en Indiana Univ. Math. J. 17 (1968) 449 - 459 state-of-the-art mathematics http://iumj.org/access/