1. Allen, R.G.D., 1959, Mathematical Economics, Chap. 8, Sects. 8.4, 8.5.
2. Allen, R.G.D., 1967, Macro-Economic Theory: A Mathematical Treatment, Chap. 19, Sects. 19.3, 19.4.
3. Asea, P.K. and P.J. Zak, 1999, Time-to-Build and Cycles.
4. Azbelev, N.V., V.P. Maksimov and L.F. Rakhmatullina, 2007, Introduction to the Theory of Functional Differential Equations: Methods and Applications.
5. Bellman, R. and K.L. Cooke, 1963, Differential-Difference Equations, Chaps. 3, 4, 13.