[4] Baumol, W.J. and Willig, R.D., 1981: Fixed Costs, Sunk Costs, Entry Barriers, and Sustainability of Monopoly, The Quarterly Journal of Economics, Vol.96, No.3.
[5] Baumol, W.J., Bailey, E.E., and Willig, R.D., 1977: Weak Invisible Hand Theorems on the Sustainability of Multiproduct Natural Monopoly, The American Economic Review, Vol.67, No.3.
[6] Stigler, G.J., 1951: The Division of Labor is Limited by the Extent of the Market, Journal of Political Economy, Vol.59, No.3.
[7] Williamson, O.E., 1975: Markets and Hierarchies, New York: Free Press.