“…In the case of resonance, i.e., ω is a rational number, the above result cannot be applied directly. For this case, there also are many results, see [2], [3], [7], [8], [22], [23], [24] and the references therein. For example, Mansilla [7] obtained some sufficient conditions for stability and instability of the trivial solution by using Moser's twist theorem and Liapunov theorem, respectively.…”