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Having moved in the z -direction from z$_{0}$ to z$_{1}$ we now compute the point y$_{2}$ so that ( y$_{2}$, z$_{1}$ ) stays on the isocurve with R$_{p,q}$ ( n, n/ 2 , 1) = 1. A small change in z requires only a small change in y to preserve this property. 

With w$_{0}$ = W ( z$_{0}$ e z$_{0}$/ $^{2}$/ 2 ) we have the series expansion ( z$_{0}$ + x ) exp (( z$_{0}$ + x ) / 2) x 

giving an approximation for small x . We want to set 

and thus we have 

<img src='content_image/367610.jpg'>

<img src='content_image/367612.jpg'>

<img src='content_image/367611.jpg'>

