In this study it is shown that the nontrivial hyperbolic fixed point of a nonlinear dynamical system, which is formulated by means of the adaptive expectations,udcorresponds to the unstable equilibrium of Harrod. We prove that this nonlinear dynamical (in the sense of Harrod) model is structurally stable under suitable economicudconditions. In the case of structural stability, small changes of the functions (C1-perturbations of the vector field) describing the expected and the true timeudvariation of the capital coefficients do not influence the qualitative properties of the endogenous variables, that is, although the trajectories may slightly change,udtheir structure is the same as that of the unperturbed one, and therefore these models are suitable for long-time predictions. In this situation the critique of Lucasudor Engel is not valid. There is no topological conjugacy between the perturbed and unperturbed models; the change of the growth rate between two levels may require different times for the perturbed and unperturbed models.
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