1. Université de Lyon, Université Lyon 1, CNRS, UMR 5208 Institut Camille Jordan, Batiment du Doyen Jean Braconnier, 43, blvd du 11 Novembre 1918, F-69200 Villeurbanne Cedex, France. E-mail: bourgeat@mcs.univ-lyon1.fr
2. Department of Mathematics, University of Zagreb, Bijenička 30, 10000 Zagreb, Croatia. E-mail: emarusic@math.hr
3. Narvik University College, HiN, Postbox 385, Lodve Langes Gate 2, Narvik N-8505, Norway and P.N. Lebedev Physical Institute RAS, Leninski Prospect 53, Moscow 117 333, Russia. E-mail: andrey@sci.lebedev.ru