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The city-agents are able to communicate with the agents representing the other cities. The hypothesis is that the interactions between cities are the driving force in the evolution and that they determine the future of each city as well as the evolution of the macroscopic properties of the system of cities. These interactions include migration flows, commercial trade, information flows, knowledge exchange etc.

Three principles of interaction families are distinguished, each corresponding to a family of functions :

  • A principle of proximity : it is the most classical one linked to central functions . It concerns activities whose production is intended to the regional market, i.e. neighboring towns and cities (commercial, services and some manufacturing industries). The basic principle is the same whatever the spatio-temporal context, but the associated ranges will of course vary according to this context. There is competition between cities occurs when their zones of influence are overlapping.

  • A territorial principle : it concerns typically administrative activities which take place in the frame of political boundaries. It could concern the specific functions of the capital in the framework of the national territory or that of a regional capital. The administrative services which are produced will supply the demand of all the cities and towns of the corresponding region or country.

  • A network principle : it is associated to very specialized activities activities with large range and whose development depends on the relative position of the city in a system of relationships rather than on spatial proximities.