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Regulator equations arise in studying the output regulation problem for nonlinear systems. The solvability of the regulator equations is the necessary condition for that of the output regulation problem. It has been shown under various assumptions that the solvability of the regulator equations can be reduced to that of a center manifold equation defined by the zero dynamics of a composite system consisting of the plant and exosystem. In this paper we further show that, for a quite general class of nonlinear systems, the solvability of the regulator equations can be reduced to that of a center manifold equation if the relative degree of the composite system at the origin exists.