1. Department of Mathematics, Chelyabinsk State University, Brat'ev Kashirinykh Street 129, Chelyabinsk 454001, Russia
2. Institute of Mathematics and Mechanics, Ural Branch of the Russian Academy of Sciences, S. Kovalevskaja Street 16, Ekaterinburg 620990, Russia
3. UFR Mathématiques, Université de Rennes 1, 35042 Rennes Cedex, France