Analysis of a Problem of Neutron Transport in a Changing Medium G. Wing Indiana University Mathematics Journal 1962 text pdf 10.1512/iumj.1962.11.11002 10.1512/iumj.1962.11.11002 en Indiana Univ. Math. J. 11 (1962) 21 - 34 state-of-the-art mathematics http://iumj.org/access/