1. On the number of positive integers ≤𝑥 and free of prime factors >𝑦;de Bruijn, N. G.;Nederl. Acad. Wetensch. Proc. Ser. A.,1951
2. On the computational solution of differential-difference equations;Bellman, Richard;J. Math. Anal. Appl.,1961
3. R. Bellman, & B. Kotkin, On the Computational Solution of a Class of Nonlinear Differential-Difference Equations, The RAND Corporation, Paper P-2233, February 1961.