Affiliation: | 1. Department of Mathematical Sciences, Worcester Polytechnic Institute, 100 Institute Road, Worcester, MA?, 01609, USA 2. Department of Mathematics and Statistics, Loyola University Chicago, 1032 W. Sheridan Road, Chicago, IL?, 60660, USA 3. Department of Mathematics, University of Craiova, 200585?, Craiova, Romania 4. Research Group of the Project PN-II-ID-PCE-2011-3-0075, “Simion Stoilow” Institute of Mathematics of the Romanian Academy, P.O. Box 1-764, 014700?, Bucharest, Romania
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