I am working on the control of underactuated mechanical systems (UMSs) using backstepping method. UMSs are classified into triangular normal forms (strict-feedback and strict-feedforward) and nontriangular form (byrnes-isidori normal form). In particular, a strict-feedback system permits applying backstepping method. How about nontriangular form? Is it possible to apply backstepping to this class? Can anyone help me?