“…However, for special classes of nonlinear systems such as bilinear and quadraticbilinear systems, one can indeed use well-established system theoretical methods; see, e.g., [3], [5], [6]. A large class of nonlinear systems, such as those with smooth nonlinearities, e.g., exponential, trigonometric, can indeed be represented as quadratic-bilinear systems by introducing new variables (in a lifting map) [22], [14], [4], [33], [31]. Converting a nonlinear model to its equivalent quadratic form provides an opportunity to perform input-independent model reduction techniques where a system-theoretic norm is defined [5], [6].…”