egin{equation}
ddot{y}_i=frac{S}{
hoDelta z^2}
left[(y_{i 1} y_{i-1}-2y_i) frac{A_i-A_{i-1}}{A_i} imes(y_{i 1} y_{i-1} Phi_{i-1})
(Phi_i-Phi_{i-1})
ight]
end{equation}
0
egin{equation}
ddot{y}_i=frac{S}{
hoDelta z^2}
left[(y_{i 1} y_{i-1}-2y_i) frac{A_i-A_{i-1}}{A_i} imes(y_{i 1} y_{i-1} Phi_{i-1})
(Phi_i-Phi_{i-1})
ight]
end{equation}