1. Mathematical Biology Unit, Okinawa Institute of Science and Technology, Okinawa Industrial Technology Center Annex, 12-2 Suzaki, Uruma, Okinawa 904-2234, Japan
2. Department of Mathematics, Tokai University, Hiratsuka City, Kanagawa Pref., 259-1292, Japan
3. Institut de mathématiques de Bourgogne (UMR CNRS 5584), 9, avenue Savary, F-21078 Dijon, France
4. ENSEEIHT-IRIT (UMR CNRS 5505), 2, rue Camichel, F-31071 Toulouse, France