Hello,
given a system of curvilinear coordinates x_i=x_i(u_1,\ldots,u_n); u_i=u_i(x_1,\ldots,x_n) and considering the position vector \mathbf{r}=x_1\mathbf{e}_1+\ldots+x_n\mathbf{e}_n there is the well-known identity that defines the reciprocal frame:
\frac{\partial \mathbf{r}}{\partial u_i...