Notations

Let $S$ be a shape, and $x_0$ be one of its points. We note $\Xi$ a cartesian equation of $S$ and $(U, \xi)$ a parameterization of $S$ in the neighborhood of $x_0$. Thus we note $(u_0, v_0)$ the element of $U$ which image through $\xi$ is $x_0$: $x_0 = \xi(u_0, v_0)$.

Let $C$ be a curve of cartesian equation $\Gamma$, contained in $S$ going though $x_0$, and $(T, \gamma)$ a parameterization of $C$ such that $x_0 = \gamma(t_0)$.