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Abstract:
Theoretical Economics Paper
The Optimal Consumption Function in a Brownian Model of Accumulation Part A: The Consumption Function as Solution of a Boundary Value Problem Lucien Foldes March 1996 Paper No' TE/1996/297: Full Paper We consider a neoclassical model of optimal economic growth with c.r.r.a. utility in which the traditional deterministic trends representing population growth, technological progress, depreciation and impatience are replaced by Brownian motions with drift. When transformed to 'intensive' units, this is equivalent to a stochastic model of optimal saving with diminishing returns to capital. For the intensive model, we give sufficient conditions for optimality of a consumption plan (openloop control) comprising a finite welfare condition, a martingale condition for shadow prices and a transversality condition as t ? ?. We then replace these by conditions of optimality of a plan generated by a consumption function (closedloop control), i.e. a function H(z) expressing logconsumption as a timeinvariant, deterministic function of logcapital z. Making use of the exponential martingale formula we replace the martingale condition by a nonlinear, nonautonomous second order o.d.e. which an optimal consumption function must satisfy; this has the form H'(z) = F[H'(z),?(z),z], where ?(z) = exp{H(z)z}. Economic considerations suggest certain limiting values which H'(z) and ?(z) should satisfy as z ? ? ?, thus defining a twopoint boundary value problem (b.v.p.)  or rather, a family of problems, depending on the values of parameters. We prove two theorems showing that a consumption function which solves the appropriate b.v.p. generates an optimal plan. Proofs that a unique solution of each b.v.p. exists will be given in a separate paper (Part B). 
 
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