1. Department of Applied Research, Institute of Mathematics of NAS of Ukraine, 3 Tereshchenkivs'ka Street, Kyiv-4 01601, Ukraine
2. Centre de Recherches Mathématiques, Université de Montréal, C.P.6128-Centre Ville, Montréal, QC, Canada H3C 3J7
3. Institute of Mathematics, University of Bialystok, Akademicka 2, 15-267 Bialystok, Poland