Affiliation: | 1. Department of Mathematics, Swansea University, Singleton Park, Swansea SA2 8PP, UK;2. Instytut Matematyczny, Polska Akademia Nauk, ul. ?niadeckich 8, Warszawa, 00-956, Poland;3. Department of Theoretical Physics and Informatics, University of ?ód?, Pomorska 149/153, 90-236 ?ód?, Poland |