1. Department of Mathematics, Volga State University of Water Transport 1 , 5A, Nesterov str., Nizhny Novgorod 603950, Russia
2. Department of Control Theory, Lobachevsky State University of Nizhny Novgorod 2 , 23, Gagarin Ave., 603950 Nizhny Novgorod, Russia
3. Department of Mathematics and Statistics, Georgia State University 3 , P.O. Box 4110, Atlanta, Georgia 30302-410, USA