Ken’ichi Ohshika is a professor of mathematics at Gakushuin University, and a professor emeritus at Osaka University. He receivedhis PhD from the University of Tokyo in 1989, and worked at Tokyo Institute of Technology, University of Tokyo, and Osaka University before joining Gakushuin University. His interests includes Kleinian groups, hyperbolic geometry, Teichmüller theory, and 3-manifolds. He has held visiting positions at Institut des Hautes Études Scientifiques (1990-1991, 1999-2000), University of Warwick (1988-89, 1993, 2007), Université de Montréal (1995), Hausdorff Research Institute of Mathematics (2010), Université de Strasbourg (2014, 2019) and the Korea Institute of Advanced Science (2009-2015). He is a recipient of the MSJ geometry prize (2012), and the author of more than 60 papers.Athanase Papadopoulos (born 1957) is Directeur de Recherche at the French Centre National de la Recherche Scientifique. His mainfields of interest are geometry and topology, the history and philosophy of mathematics, and mathematics and music. He has held visiting positions at the Institute for Advanced Study, Princeton (1984–85 and 1993–94), USC (1998–1999), CUNY (Ada Peluso Professor, 2014), Brown University (Distinguished visiting professor, 2017), Tsinghua University, Beijing (2018), Lamé Chair of the State University of Saint Petersburg (2019), and has had several month visits to the Max-Plank Institute for mathematics (Bonn), the Erwin Schrödinger Institute (Vienna), the Graduate Center of CUNY (New York), the Tata Institute (Bombay), Galatasaray University (Istanbul), the University of Florence (Italy), Fudan University (Shanghai), Gakushuin University (Tokyo) and Presidency University (Calcutta). He is the author of more than 200 published articles and 35 monographs and edited books.