Affiliation: | 1. Department of Mathematics, Wroc?aw University of Environmental and Life Sciences, Grunwaldzka 53, 50-357, Wroc?aw, Poland 2. Institute of Mathematics and Computer Science, Wroc?aw University of Technology, Wybrze?e Wyspiańskiego 27, 50-370, Wroc?aw, Poland 3. Laboratoire de Mathématiques et Applications de Valenciennes, Université de Valenciennes et du Hainaut-Cambrésis, Le Mont Houy, Lamath-ISTV2, 59313, Valenciennes Cedex 9, France
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