We appreciate S. M. Shahruz for his interest in our paper and for the useful comments on the numerical method for solving the transformed system  
Y¨t+eCtΛ2C2eCtYt=0,
 
Y0=X0,Y˙0=CX0+X˙0
(1)
As pointed out by Shahruz, the numerical method proposed in our paper is an approximate approach. The approximation comes from the assumption that the time-varying matrix eCtΛ2C2eCt is time-invariant within short time duration jh<t<j+1hj=0,1,2,,N where h is the time step, i.e.,  
eCt[Λ2C2]eCtejhC[Λ...
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