1. Avrett, E. H. and Loeser, R.: 1966,Smithsonian Astrophys. Obs., Spec. Rep. No. 201.
2. Bellman, R.: 1957,Proc. Am. Math. Soc. 8, 435.
3. Bellman, R. and Ueno, S.: 1972, Department of Electrical Engineering, University of Southern California, TR No. 72-19; 1973,J. Math. Analys. Appl. 44, 264.
4. Hummer, D. G. and Rybicki, G.: 1967, in B. Alder, S. Fernbach, and M. Rotenberg (eds.),Methods in Computational Physics, pp. 53?127.
5. Kagiwada, H. H. and Kalaba, R. E.: 1970,J. Assoc. Comp. Machinery 17, 412.