1. Department of Applied Mathematics and Computer Science; Ghent University; Krijgslaan 281 S9; B-9000 Gent; Belgium
2. Department of Mathematics; Ghent University; Krijgslaan 281 S22; B-9000 Gent; Belgium
3. Dipartimento di Matematica e Informatica; Universitá degli Studi della Basilicata; Viale dell'Ateneo Lucano; 10, I-85100 Potenza; Italy