1. School of Mathematics and Statistics; University of St Andrews; North Haugh, St Andrews Fife KY16 9SS United Kingdom
2. Center for Research and Development in Mathematics and Applications; Department of Mathematics; University of Aveiro; 3810-193 Aveiro Portugal
3. Département de Mathématique; Université Libre de Bruxelles; C.P. 216 - Algèbre et Combinatoire Boulevard du Triomphe 1050 Brussels Belgium
4. Department of Applied Mathematics; Wentworth Institute of Technology; Boston MA 02115 USA