\begin{equation}\label{FRF_xi3} \displaystyle |\widetilde{H}|=\frac{1/k}{\sqrt{\displaystyle\left(1-\left(\frac{\omega}{\omega_{0}}\right)^{2}\right)^{2}+4\xi^{2}\left(\frac{\omega}{\omega_{0}}\right)^{2}}} \end{equation}