9Berg, S. V. and Tschirhart, J., 1988,Natural Monopoly Regutation, Cambridge University Press.
10Braeutigam, R. R., 1989, "Optimal Policies for Natural Monopoly" in R. Schmalensee and R. D. Willing(ed.), Handbook of Industrial Organization, Ⅱ , Elsevier Science Publishers B.V
