Affiliation: | aMaastricht University, Quantitative Economics, P.O. Box 616, 6200 MD Maastricht, The Netherlands;bIBM T.J. Watson Research Center, P.O. Box 218, Yorktown Heigths, NY 10598, USA;cUniversity of Twente, Applied Mathematics, P.O. Box 217, 7500 AE Enschede, The Netherlands |