Institution: | 1. Università degli Studi di Perugia, Dipartimento di Matematica e Informatica, Via Vanvitelli 1, 06123 Perugia, Italy;2. Institute of Mathematics “Simion Stoilow” of the Romanian Academy, 014700 Bucharest, Romania;3. Department of Mathematics, University of Craiova, Street A.I. Cuza No. 13, 200585 Craiova, Romania |