1. Institute of Mathematics, University of Bialystok ul. Ciolkowskiego 1M, 15-245 Bialystok, Poland
2. Department of Mathematics and Computer Science, The University of Southern Denmark, Campusvej 55, DK-5230 Odense M, Denmark
3. Department of Mathematics, University of Oslo, P.O. Box 1053 Blindern, 0316 Oslo, Norway