Published online by Cambridge University Press: 05 June 2012
This chapter builds directly on the last in developing a duality for the adjustment cost model of the firm. In particular, the current value form of the H-J-B equation given in Theorem 19.3 will be exploited to develop a method to derive the duality properties of the adjustment cost model of the firm. Moreover, we will establish envelope results that will allow the explicit construction of the feedback or closed-loop forms of the investment demand, variable input demand, and output supply functions, given a functional form for the current value optimal value function with known properties. The importance of such a development is monumental in dynamic economic theory for the reasons well summarized by Epstein (1981, page 82):
In static models, duality is a convenience. Demand functions cannot generally be determined explicitly from the technology but they are defined implicitly by first order conditions which can serve as the basis for estimation, though perhaps requiring complicated simultaneous equations techniques. Explicit solutions for calculus of variations problems are even rarer and the implicit representation of solutions generally involves a second order nonlinear differential equation (system) and non-trivial boundary conditions. The differential equation system can serve as the basis for estimation only if the generally unrealistic assumption is made that the firm does not revise its plans for several periods and continues along the same optimal path. Thus duality is indispensable for empirical work based on functional forms that are too complicated to be derived directly from the technology as explicit solutions of a problem of intertemporal optimization.
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