1. Department of Mathematics, University of Bielefeld, Germany; and Institute of Control Sciences, Russian Academy of Sciences, Moscow, Russia
2. Instituto de Matematicas, National Autonomous University of Mexico (UNAM), Unidad Oaxaca, Mexico
3. Faculty of Mathematics and Computer Science, University of Warmia and Mazury, Olsztyn, Poland
4. Department of Mathematics, Harvard University, Cambridge, Massachusetts, U.S.A.