Affiliation: | 1. Mathematics Section, College of Science and Technology, Hongik University, Sejong, 339-701, Republic of Korea 2. Department of Mathematics, Technical University of Cluj-Napoca, 28 Memorandumului, 400114, Cluj-Napoca, Romania 3. Department of Mathematics, ETH-Zürich, Ramistrasse 101, 8092, Zurich, Switzerland
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