Affiliation: | 1. Faculty of Mathematics, “Al.I. Cuza” University of Ia?i, Ia?i, Romania;2. “Octav Mayer” Institute of Mathematics, Ia?i, Romania;3. University of Milan, Department of Mathematics, Milan, Italy;4. University of Guelph, Department of Mathematics and Statistics, Guelph, Ontario, Canada;5. University of Milan, Department of Economics, Management and Quantitative Methods, Milan, Italy |