\det{\left(\nabla{F}^{\tau}\nabla{F}\right)}=\sum\limits_{\left[I\right]}{\left(\det{\nabla{F}_I\right)^2} = 1 + \sum\limits_{i=1}^{n-1}{\left(\frac{\partial{f}}{\partial{x_i}}\right)^2}=1 + \left|\nabla{f}\right|^2