1. Institute of Mechanical Engineering Problems, Russian Academy of Sciences, S. Petersburg, Russia. E-mail: srgnazarov@yahoo.co.uk
2. Institut Elie Cartan, Laboratoire de Mathématiques, Université Henri Poincaré Nancy 1, B.P. 239, 54506 Vandoeuvre lés Nancy Cedex, France. E-mail: Jan.Sokolowski@iecn.u-nancy.fr, URL: http://www.iecn.u-nancy.fr/~sokolows/
3. Universität Kassel, Heinrich Plett Str. 40, D-34132 Kassel, Germany. E-mail: specovi@mathematik.uni-kassel.de, URL: http://www.mathematik.uni-kassel.de/~specovi/