Past exam of the mathematics course of the University of Cambridge / 2012 / iii / Paper 17 / 2 / 1

The space of smooth differential forms of degree $p$ isThus a form assigns smoothly to each point an alternating $p$-linear function on its tangent space. Here ${\Omega }^0(M)=C^{\infty }(M)$, and forms of degree greater than $\dim M$ vanish. The wedge product of differential forms gives the graded exterior algebra of forms.

The exterior derivative is an $\mathbb{R}$-linear map of degree one satisfying the following axioms: $df(X)=X(f)$ for functions; for a $p$-form $\alpha$,and $d^2=0$. The sign is the graded Leibniz rule.

To construct it, write in a coordinate chartThis formula is linear, agrees with differentiation on functions and obeys the graded Leibniz rule. Its square vanishes because symmetric second partial derivatives are contracted against antisymmetric wedge products.

The construction is independent of the chart. For coordinates $y^j=y^j(x)$, the chain rule gives $d{y}^j={\sum }_i({\partial }_i{y}^j)d{x}^i$, and the coordinate formula gives $d(d{y}^j)=0$ by equality of mixed partial derivatives. Consequently, expressing a form as ${\sum }_I{b}_{I}d{y}^I$ and applying the $x$-coordinate formula gives exactly ${\sum }_{I}d{b}_I\wedge d{y}^I$, the $y$-coordinate formula. The local constructions agree on overlaps and therefore define a global exterior derivative.

For uniqueness, first derive locality from the axioms. If $\alpha$ vanishes near $p$, choose a smooth bump function $\chi$ supported in that neighborhood and equal to one near $p$. Then $\chi \alpha =0$, andimplies $(d\alpha {)}_p=0$. Thus the value depends only on the local germ of $\alpha$. Local coordinate functions can be extended using bump functions, so the same axioms apply to their germs. Since $d(d{x}^i)=d^2{x}^i=0$, the graded Leibniz rule forces precisely the coordinate formula above. The exterior derivative exists and is unique, as expressed by uniqueness of the exterior derivative from its axioms.