A Difference-Differential Scheme for the General Cauchy Problem Avner Friedman Indiana University Mathematics Journal 1962 text pdf 10.1512/iumj.1962.11.11047 10.1512/iumj.1962.11.11047 en Indiana Univ. Math. J. 11 (1962) 891 - 905 state-of-the-art mathematics http://iumj.org/access/