Affiliation: | 1.Faculty of Mathematics and Computer Science,University of Bucharest,Bucharest,Romania;2.Mathematics Department,Politehnica University of Bucharest,Bucharest,Romania;3.Bucharest,Romania;4.Department of Mathematics and Computer Science,Petroleum-Gas University of Ploie?ti,Ploie?ti,Romania;5.Research Center in Geometry, Topology and Algebra,University of Bucharest,Bucharest,Romania |