1. M. Fjeld, “ Stability and Control of Periodic Processes’; Rep. 71–52-W, Division of Automatic Control, The Technical University of Norway, Trondheim (April 1971).
2. G. Guardabassi, A. Locatelli, and S. Rïnaldi, “The Status of Periodic Optimization of Dynamical Systems”, Int. Rep. LCA 71–20, Istituto di Elettrotecnica ed Elettronica, Politecnico di Milano (Nov. 1971).
3. P. Hartman, “Ordinary Differential Equations”, John Wiley and Sons, Inc. (1964).
4. A. Halanay, “Differential Equations Academic Press (1966).
5. P.L. Falb and J.L. de Jong, “ Some Successive Approximation Methods in Control and Oscillation Theory”? Academic Press, (1969).