Scattering Theory for the Wave Equation with a Short Range Perturbation R. Phillips Indiana University Mathematics Journal 1982 text pdf 10.1512/iumj.1982.31.31045 10.1512/iumj.1982.31.31045 en Indiana Univ. Math. J. 31 (1982) 609 - 639 state-of-the-art mathematics http://iumj.org/access/