1. Department MathematikLehrstuhl für Angewandte Mathematik 2Cauerstr. 11 91058 Erlangen Germany
2. Saint‐Petersburg State UniversityUniversitetskaya nab. 7–9 St. Petersburg 199034 Russia
3. Institute for Problems in Mechanical Engineering of R.A.S.V.O., Bolshoj pr., 61 St. Petersburg 199178 Russia
4. Department of Mathematics and StatisticsUniversity of HelsinkiP.O. Box 68 00014 Helsinki Finland