\begin{equation}
s_{ij}(t) = 
 \begin{cases} 
     \frac{A t}{\tau_f} + \overline{s_{ij}} - \frac{A}{2} & t\leq \tau_f \\
      \overline{s_{ij}} + \frac{A}{2} - \frac{A  (t-\tau_f)}{\tau_s} & \tau_f < t\leq \tau_s \\
   \end{cases}
\ ~,
\end{equation}
