Dr Marc Barthelemy is a former student of the École Normale Superieure of Paris and graduated at the University of Paris with a thesis in theoretical physics. His research focuses on complex systems with an emphasis on cities and networks. MB is research director at the Institute of Theoretical Physics (CEA) in Saclay and a member of the Center of Social Analysis and Mathematics (CAMS) at the Ecole des Hautes Etudes en Sciences Sociales (EHESS). Dr Vincent Verbavatz is a former student of the École polytechnique and graduated at the University of Paris-Saclay with a thesis in statistical physics. His research focuses the modelling of cities, notably of the modelling of urban growth. VV is an associate researcher at Institut des systèmes complexes de Paris Île-de-France.