\begin{eqnarray*}
        \mathbf{z}_{t+1} & = & \overbrace{\mathbf{z}_{t}}^{=(\mathbf{z}_{t}-\mathbf{c}_{t})\Rfree}+\zeta_{t+1}  \\
        \mathbf{z}_{t+2} & = & \mathbf{z}_{t+1}+\zeta_{t+2}%+\Rfree \zeta_{t+1}
                      \\ & = & (\mathbf{z}_{t+1}-\mathbf{c}_{t+1})\Rfree + \zeta_{t+2} \\
                      \\ & = & (\mathbf{z}_{t}+\underbrace{\mathbf{z}_{t+1}-\mathbf{z}_{t}}_{\zeta_{t+1}}-\mathbf{c}_{t})\Rfree + \zeta_{t+2} \\
                      \\ & = & \mathbf{z}_{t}+\zeta_{t+1}\Rfree + \zeta_{t+2} \\
%        \mathbf{z}_{t+3} & = & \mathbf{z}_{t}+\Rfree^{2} \epsilon_{t+1} + \Rfree \epsilon_{t+2}
\\ \vdots       &   & \vdots \nonumber
\\  \mathbf{z}_{t+n} & = & \mathbf{z}_{t}+\underbrace{\sum_{s=1}^{n} \Rfree^{n-s}\zeta_{t+s}}_{\equiv \Delta^{n} \mathbf{z}_{t+n}}
\end{eqnarray*}
