1. Fakultät für Mathematik, Universität Bielefeld, Postfach 100131, 33501 Bielefeld, Germany
2. Department of Mathematical and Statistical Sciences, 632 CAB, University of Alberta, Edmonton, AB, T6G 2G1, Canada
3. Department of Mathematical Sciences, MacEwan University, 10700 – 104 Avenue, Edmonton, AB, T5J 4S2, Canada
4. Department of Mathematics, Trent University, Peterborough, Ontario, Canada
5. Institute of Mathematics “Simon Stoilow”, Bucharest, Romania