K. Laursen, and A. Salter. DRUID Working Papers, 03. DRUID, Copenhagen Business School, Department of Industrial Economics and Strategy/Aalborg University, Department of Business Studies, (2003)
W. Langdon. SEN-R9907. Centrum voor Wiskunde en Informatica, CWI, P.O. Box 94079, Kruislaan 413, NL-1090 GB
Amsterdam, The Netherlands, (11 April 1999)