Affiliation: | 1.Department of Mathematics and Informatics, University of Oradea, University Street 1, 410087, Oradea, Romania ;2.Department of Mathematics, Wilfrid Laurier University, 75 University Ave W, Waterloo, Ontario, N2L 3C5, Canada ;3.Departamento de Matemática, Instituto Superior Técnico, Av. Rovisco Pais 1049-001, Lisboa, Portugal ; |