Aurelian Cernea
Faculty of Mathematics and Computer Science,
University of Bucharest,
Academiei 14, 010014 Bucharest, Romania
Academy of Romanian Scientists,11 Splaiul Independentei 54,
050094 Bucharest, Romania
Vasile Staicu http://sweet.ua.pt/vasile Department of Mathematics
University of Aveiro
3810-193 Aveiro
Portugal Portugal