1. This work has been accomplished during the semester following the EMBO course “Formal analysis of Genetic Regulation”. Université Libre de Bruxelles — September 6–17, 1977.
2. Thomas (R), Boolean formalization of genetic control circuits, J. Theor. Biol (1973) 42, 563–585
3. Florinne (J), La synthèse des machines logiques et son automatisation — Presses Académiques Européennes, Dunod, 1964
4. Nicolis (G), Prigogine (I), Self organization in non equilibrium systems, Wiley, 1977
5. In this section, we follow the approach by Allen Pred presented in his book : The spatial dynamics of U.S. Urban-Industrial growth, 1600–1914 — Interpretive and Theoretical Essays, M.I.T. Press, 1966