Affiliation: | 1. Faculty of Mathematics and Computer Science, Ovidius University, Bd. Mamaia 124, 900527, Constan?a, Romania 2. Simion Stoilow Institute of Mathematics of the Romanian Academy, Research Group of the Project ID-PCE-2011-1023, P.O. Box 1-764, 014700, Bucharest, Romania 3. Fachbereich Mathematik, Universit?t Duisburg-Essen, Campus Essen, 45117, Essen, Germany 4. School of Mathematics, Institute for Research in Fundamental Sciences (IPM), P.O. Box 19395-5746, Tehran, Iran
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