1. Mathematics and Mechanics Faculty; St.Petersburg State University; Petershoff, Universtetskii pr., 28, 198504 Saint Petersburg Russia
2. Institute of Mechanical Engineering Problems; Academy of Science St. Petersburg; V.O., Bolshoi pr., 61, 199178 Saint Petersburg Russia
3. Physical Faculty; St. Petersburg State University; Ul'yanovskaya., 3, Petrodvorets, 198504 Saint Petersburg Russia
4. Department of Mathematics and Statistics; University of Helsinki; P.O. Box 68, FI-00014 Helsinki Finland