Affiliation: | aDepartment of Mathematics, University of Duisburg-Essen, Forsthausweg 2, D-47057 Duisburg, Germany;bFaculty of Mathematics and Natural Sciences, University of Wuppertal, Gaußstraße 20, D-42119 Wuppertal, Germany;cDepartment of Mathematics, Technical University, Str. C. Daicoviciu, 15, RO-400020 Cluj-Napoca, Romania |