body: x 2 = ∑ i = 3 n ( ∂ u ∂ x i d x i ∧ d x 1 ∧ d x 2 ) {\displaystyle {\begin{aligned}d\sigma &=du\wedge dx^{1}\wedge dx^{2}\\&=\left(\sum _{i=1}^{n}{\frac {\partial u}{\partial x^{i}}}\,dx^{i}\right)\we