% (c) P.B.L. Meijer 1996. Latex source for formulas (2.2), (2.3) and (2.16):

\begin{equation}\label{ndv}
\tau_{2}(\sigma_{1,ik}\,,\,\sigma_{2,ik}) \;\;\frac {{\rm d^2} y_{ik}} {{\rm d} t^2}
\;+\; \tau_{1}(\sigma_{1,ik}\,,\,\sigma_{2,ik}) \;\;\frac {{\rm d} y_{ik}} {{\rm d} t}
\;+\; y_{ik} \;=\; {\cal F} ( s_{ik}, \delta_{ik} ) 
\end{equation}

\begin{eqnarray}\label{sum}
s_{ik} & \stackrel {\triangle}{=} & {\mbox{\boldmath $w$}}_{ik} \cdot {\mbox{\boldmath $y$}}_{k-1} - \theta_{ik} 
\;+\; {\mbox{\boldmath $v$}}_{ik} \cdot \frac {\textstyle{\rm d} {\mbox{\boldmath $y$}}_{k-1}} {\textstyle{\rm d} t} 
\nonumber \\ & = & \sum_{j=1}^{N_{k-1}} w_{ijk} \; y_{j,k-1} \;-\; \theta_{ik} 
\;+\; \sum_{j=1}^{N_{k-1}} v_{ijk} \; \frac {\textstyle{\rm d} y_{j,k-1} } {\textstyle{\rm d} t} 
\end{eqnarray}                                                      

\begin{eqnarray}\label{dev2}
{\cal F}_{2} ( s_{ik}, \delta_{ik} ) & \stackrel {\triangle}{=} &
\frac{\textstyle 1}{\textstyle \delta_{ik}^2} \;\ln 
\frac{ \textstyle \cosh \frac{\textstyle \delta_{ik}^2 (s_{ik}+1)}{\textstyle 2} }
     { \textstyle \cosh \frac{\textstyle \delta_{ik}^2 (s_{ik}-1)}{\textstyle 2} }
\end{eqnarray}