Functional Equations, Wave Propagation and Invariant Imbedding Richard BellmanRobert Kalaba Indiana University Mathematics Journal 1959 text pdf 10.1512/iumj.1959.8.58045 10.1512/iumj.1959.8.58045 en Indiana Univ. Math. J. 8 (1959) 683 - 704 state-of-the-art mathematics http://iumj.org/access/