Robust output regulation of singular nonlinear systems via a nonlinear internal model

The robust output regulation problem for singular nonlinear systems has been studied recently under a restrictive assumption that the solution of the regulator equations is polynomial. This assumption essentially limits the nonlinear systems to contain only polynomial nonlinearities. In this note, we will further show that the polynomial assumption can be replaced by a much milder condition. The new condition applies to a larger class of nonpolynomial nonlinear systems, thus significantly improving the existing result.