1. On the principle of invariant imbedding and propagation through inhomogeneous media;Bellman,1956
2. Invariant imbedding and neutron transport theory—II: Functional equations;Bellman;J. Math. Mech.,1958
3. Invariant imbedding and mathematical physics—I: Particle processes;Bellman;J. Math. Phys.,1960
4. Functional equations, wave propagation and invariant imbedding;Bellman;J. Math. Mech.,1959