Institution: | 1. Department of Pure Mathematics, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK;2. School of Mathematical Sciences, University of Nottingham, University Park, Nottingham, NG7 2RD, UK;3. Department of Physics and Astronomy, University of Turku, FI-20014 Turku, Finland;4. Department of Mathematics and Statistics, University of Turku, FI-20014 Turku, Finland |