Valérie Berthé is a CNRS Research Director at LIAFA (Laboratoire d'Informatique Algorithmique: Fondements et Applications) at the Université Paris Diderot, Paris 7. She is Deputy Director of the Fondation Sciences Mathématiques de Paris and has authored 70 journal or conference papers. Her main research interests are tilings, numeration systems, substitutive dynamicals systems and symbolic dynamics. Michel Rigo is a full professor in the Department of Mathematics at the Université de Liège, Belgium, and head of the research group in discrete mathematics. He has authored more than 50 journal or conference papers and two books. His main research interests are formal language theory, numeration systems and combinatorics on words.