The Non-Isentropic Perturbation of an Arbitrary Simple Wave Roy Gundersen Indiana University Mathematics Journal 1960 text pdf 10.1512/iumj.1960.9.59008 10.1512/iumj.1960.9.59008 en Indiana Univ. Math. J. 9 (1960) 141 - 145 state-of-the-art mathematics http://iumj.org/access/