Affiliation: | Universit?t Osnabrück, Fachbereich Mathematik/Informatik, D-49069 Osnabrück, Germany (e-mail: peter@mathematik.uni-osnabrueck.de), DE University of Twente, Faculty of Mathematical Sciences, P.O. Box 217, 7500 AE Enschede, The Netherlands (e-mail: j.l.hurink@math.utwente.nl), NL Memorial University of Newfoundland, Faculty of Business Administration, St. John's, Newfoundland, Canada (e-mail: wkubiak@morgan.ucs.mun.ca), CA
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