\begin{equation*} 
\widehat{\widetilde  C}_{12,b}(t)=\sum_{|k|<L_{c}}\left(1-\frac{|k|}{L_{c}}\right) c_{k}\left(\widetilde 
 C^{(ij)}_{n, N, M}\right) e^{\mathrm{i} k \frac{2\pi}{T}t},
\end{equation*} 
