Complex dynamics on a Monopoly Market with Discrete Choices and Network Externality Denis Phan 1 and Jean-Pierre Nadal 2 1 ENST de Bretagne and ICI Universit´ e de Bretagne Occidentale (denis.phan@enst-bretagne.fr, http://www-eco.enst-bretagne.fr/∼phan) 2 Laboratoire de Physique Statistique, Ecole Normale Sup´ erieure, 24 rue Lhomond, 75231 Paris cedex 05, France (nadal@lps.ens.fr, http://www.lps.ens.fr/∼nadal) Abstract In this paper, we explore the effects of the introduction of localised externalities through interaction structures upon local and global properties of the simplest mar- ket model: the discrete choice model with a single homogeneous product and a single seller (monopoly). Following Kirman, the resulting market is viewed as a complex interactive system. We use an ACE (Agent based Computational Economics) ap- proach to investigate the market mechanisms and underline in what way the knowl- edge of generic properties of complex adaptive system dynamics can enhance our perception of the market mechanism in the numerous cases where individual deci- sions are inter-related. R´ esum´ e Dans ce papier, nous explorons les effets de l’introduction d’externalit´ es transmises par des structures d’interactions sur les propri´ et´ es locales et globales du plus simple mod` ele de march´ e : choix discret, produit homog` ene, monopole. A la suite de Kirman, le march´ e ainsi d´ ecrit peut ˆ etre consid´ er´ e comme un syst` eme com- plexe interactif. Nous utilisons une approche multi-agents (ACE) pour explorer les m´ ecanismes de march´ e et monter de quelle mani` ere les propri´ et´ es g´ en´ eriques des syst` emes complexes adaptatifs peuvent am´ eliorer notre perception des ph´ enom` enes de march´ es lorsque les choix des agents sont interd´ ependants. 10e Rencontre Internationale ACSEG (Approches Connexionnistes en Sciences Economiques et de Gestion) Universit´ e de Nantes, 20 et 21 novembre 2003 1 Introduction In this paper, we explore the effects of the introduction of interaction structures (struc- tured externalities) upon local and global properties of the simplest market model: the discrete choice model (Anderson et al. [1]) with a single homogeneous product and a single seller (the monopoly case). Following Kirman [12, 13, 14] such a market is viewed as a complex interactive system with a communication network. First part of the present study relies on numerical simulations, making use of ’Moduleco’, a multi-agent plate- form [25, 17, 18]. 1