1. Department of Mathematics, National Kaohsiung Normal University, Kaohsiung 82444, Taiwan
2. Dipartimento di Matematica, Universitá della Calabria, Arcavacata di Rende, 87036 Cosenza, Italy
3. Department of Mathematics, Babeş-Bolyai University Cluj-Napoca, Kogălniceanu Street, No. 1, 400084 Cluj-Napoca, Romania
4. Academy of Romanian Scientists, Splaiul Independenţei Street No. 54, 050094 Bucharest, Romania