Nonlinear output tracking control problem for a class of non-triangular underactuated systems is discussed by
backstepping method. For the non-triangular underactuated systems, if errors in the backstepping design procedure are
defined the same as errors of lower triangular systems, not all the error dynamics are able to be stabilized. Therefore, the
concept of contact function is introduced. Then errors in the backstepping design procedure are constructed differently from
errors of lower-triangular systems. State feedback tracking control controller are designed to guarantee that system errors are
globally asymptotically stable. Simulation results show the effectiveness of the proposed design method.