1. Université Sorbonne Paris Nord, Laboratoire Analyse, Géométrie & Applications, UMR 7539 Institut Galilée, 99 avenue J.B. Clément, 93430 Villetaneuse, France
2. Institute for Mathematical Sciences, Renmin University of China, 100872 Beijing, P.R. China
3. Department of Mathematics and Statistics, Concordia University, 1455 De Maisonneuve Blvd. W., Montreal, Canada