1. Department Mathematik Friedrich-Alexander-Universität Erlangen-Nürnberg; Cauerstr. 11 (03.322) 91058 Erlangen
2. Saint Petersburg State University; St. Petersburg State University; 7/9 Universitetskaya nab. St. Petersburg 199034 Russia
3. Institute of Problems of Mechanical Engineering V.O.; Bolshoj pr., 61 St. Petersburg 199178 Russia