\begin{align}
    \delta_{\theta}\left(\mathbf{r}\right) = \sum^{N_{\text{dof}}^{\text{lofi}}}_{i=1}\delta^{\text{\tiny NN}}_{i,t_k} \,\phi_i\left(\mathbf{r}\right),\quad \forall k=1,...,N^{\text{lofi}}_T.
\end{align}
