Nathalie Sonnac is an economist. A senior lecturer at the Institut français de presse de l'université of Paris II University, she is a researcher at the CARISM and an associate researcher at the CREST/LEI. Jean Gabszewicz, an economist, is a lecturer at the Catholic University of Louvain. An internationally recognized specialist in mathematic economy, imperfect concurrence and industrial Economy, he is a member of several editorial comities of premier scientific journals in the USA and in Europe.