On the Principle of Invariant Imbedding and Neutron Transport Theory. I-One-Dimensional Case Richard BellmanRobert KalabaG. Wing Indiana University Mathematics Journal 1958 text pdf 10.1512/iumj.1958.7.57013 10.1512/iumj.1958.7.57013 en Indiana Univ. Math. J. 7 (1958) 149 - 162 state-of-the-art mathematics http://iumj.org/access/