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ARGENTINA
San Luis, University of Sant Luis, Conicet, Dept. of \tab Mathematics, prof. E. Marchi, 1987.
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AUSTRIA
Laxenburg, International Institute for Applied Systems Analysis, prof. Young, 1981.
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CANADA
Montreal, Concordia University, Dept. of quantitative Methods, prof. G. Pederzoli, 1983.
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CHINA
Dalian, University of Dalian, Institut of Light and Technology, prof. Zhang Sheng-Kai, 1991.
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COLOMBIA
Bogota, University de los Andes, Dept. de Matematica, prof. L.J. Ferro, 2008, 2009.
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GERMANIA
Munich, University of Munich, prof. Manfred Holler.
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LATVIA
Riga, University of Latvia, Dept. of Economics, prof. T. Muravskaya, 1993.
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FRANCE
Marseille, Universit\'e9s de la M\'e9diterran\'e9e, EHESS, CNRS, prof. V. Merlin, 2011.
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ITALY
Universities of Alessandria, Brescia, Catania, Foggia, Genova ,
Macerata, Messina, Milano (Bicocca, Bocconi, Cattolica, Politecnico),
Modena, Napoli (Federico II), Pisa (Scuola Normale), Roma (LUISS),
Trieste, Udine.
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NETHERLANDS
Maastricht, Dept. of Economics and Business Administration, University of Maastricht, prof. Hans Peters, 1998.
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POLAND
Krakow, AGH University of Science and Management, Faculty of Management, prof. Z Lucki, 2004.
Warsaw, Systems Research Institute of the Polish Academy of Sciences, prof. J. Holubiec, 1989, 1992, 2011.
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SPAIN
Manresa, Universitat Polit\'e9cnica de Catalunya, Dept. de Matematica Aplicada, prof. J. Freixas, 2000.
Terrassa, Universitat Polit\'e9cnica de Catalunya, Dept. de Matematica Aplicada, prof. F. Carreras, 2000.
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U.S.A.
Monterey, CA, Naval Postgraduate School, Dept. of Math., prof. Guillermo Owen, 2007.
Newark, NJ, University of Newark, Dept. of Mathematics, prof. L. Papayanopoulos, 1995.
Philadelphia, PA, University of Pennsylvania, Dept. of Mathematics, prof. R.E. Kilhlstrom, 1980.
Princeton, NJ, Princeton University, Dept. of Mathematics, prof. H.W. Kuhn, 1980.