[1] Fernuniv, Fac Math & Comp Sci, IZ, Univ Str 1, D-58084 Hagen, Germany
[2] Ryerson Univ, Dept Math, Toronto, ON, Canada
[3] Univ Claude Bernard Lyon 1, Univ Lyon, CNRS, UMR 5205,LIRIS, Lyon, France
[4] Dalhousie Univ, Dept Math & Stats, Halifax, NS, Canada