Institution: | (1) School of Mathematics, La Trobe University, Bundoora, Victoria 3083, Australia and Department of Mathematics, M. Bel University, SK 974 01 Banská Bystrica, Slovakia, e-mail: haviar@bb.sanet.sk,, SK;(2) Mathematical Institute, University of Oxford, 24/29 St. Giles, Oxford OX1 3LB, e-mail: hap@maths.ox.ac.uk,, UK |