1. Faculty of Engineering and Natural Sciences; Sabanci University; Orhanli-Tuzla 34956 Istanbul Turkey
2. Dipartimento di Matematica e Informatica; University of Perugia; Via Vanvitelli 1 - 06123 Perugia Italy
3. Dipartimento di Matematica; University of Trento; Via Sommarive 14 - 38123 Povo (TN) Italy