## Appendix A  
The matrices \([c]\), \([c_{p}]\), and \([\Delta c]\) in the numerical examples are as follows:  
Two degrees-of-freedom system, case1:  
\[[c]=\left[ \begin{array}{cc}6.6&-0.6\\ -0.6&0.6\end{array} \right]\!,\ \left[c_{p}\right]=\left[ \begin{array}{cc}4.7395&-0.879 1\\ -0.8791&1.1581\end{array} \right]\!,\ [\Delta c]=\left[ \begin{array}{cc}1.8605&0.2791 \\ 0.2791&-0.5581\end{array} \right]\]  
Case2:  
\[[c]=\left[ \begin{array}{cc}7.58&-0.18\\ -0.18&0.18\end{array} \right]\!,\ \left[c_{p}\right]=\left[ \begin{array}{cc}4.4172&-0.6544 \\ -0.6544&1.1288\end{array} \right]\!,\ [\Delta c]=\left[ \begin{array}{cc}3.1628&0.4744 \\ 0.4744&-0.9488\end{array} \right]\]  
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