Affiliation: | 1. Department of Mathematics and Computer Science, University of Oradea, 410087, Oradea, Romania 2. Department of Mathematics, University of Duisburg-Essen, Forsthausweg 2, D-47048, Duisburg, Germany 3. Department of Mathematics, University of Bochum, Universit?tsstr. 150, D-44780, Bochum, Germany 4. Department of Mathematics, Transilvania University, Str. Eroilor 29, RO-500 036, Bra?ov, Romania 5. Department of Mathematics, Lucian Blaga University of Sibiu, RO-550012, Sibiu, Romania
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