1. School of Mathematics and Statistics, University College Dublin, Belfield, Dublin 4, Ireland
2. MACSI, Department of Mathematics and Statistics, University of Limerick, Limerick V94 T9PX, Ireland
3. Departament d’Enginyeria Informàtica i Matemàtiques, Universitat Rovira i Virgili, 43007 Tarragona, Catalonia, Spain