The cost of capital, corporation finance and the theory of investment: a refinement

The proof of Proposition I in the work of Modigliani and Miller (MM) (1958 Modigliani, F. and Miller, M. H. 1958. The cost of capital, corporation finance, and the theory of investment. American Economic Review, 48: 261–97. [Web of Science ®] , [Google Scholar]) is based on the mechanism of arbitrage. Two cases are considered: first, the case where the value of the levered firm is larger than that of the unlevered one; second, the case where the value of the levered firm is smaller than that of the unlevered one. The first case involves the investor engaging in personal borrowing. This article shows that the amount borrowed is greater than the amount envisaged by MM, and that the proof of Proposition I is slightly altered.