Invariant Imbedding and Neutron Transport Theory II-Functional Equations Richard BellmanRobert KalabaG. Wing Indiana University Mathematics Journal 1958 text pdf 10.1512/iumj.1958.7.57041 10.1512/iumj.1958.7.57041 en Indiana Univ. Math. J. 7 (1958) 741 - 756 state-of-the-art mathematics http://iumj.org/access/