On the Convergence of an Approximation Method of M. J. Lighthill Wolfgang Wasow Indiana University Mathematics Journal 1955 text pdf 10.1512/iumj.1955.4.54029 10.1512/iumj.1955.4.54029 en Indiana Univ. Math. J. 4 (1955) 751 - 767 state-of-the-art mathematics http://iumj.org/access/