First Author:   Walter Trockel Institute of Mathematical Economics (IMW) IMWT@wiwi.uni-bielefeld.de   Institute of Mathematical Economics (IMW), Bielefeld University, P.O.Box 100131 Bielefeld, 33501, Germany   +49 521 106 6909(Phone) +49 521 106 6909(Fax) http://www.wiwi.uni-bielefeld.de/~imw/Members/WTrockel/