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The robust servomechanism problem (alternatively, output regulation problem) of the class of nonlinear systems in lower triangular form has been extensively studied in recent years. The semi-global solution was first given by either state feedback or output feedback. The global solution by the state feedback was given very recently. However, the global solution by the output feedback has long been an open problem. In this paper, we present a set of solvability conditions of the global robust servomechanism problem for this class of nonlinear systems by the output feedback control.