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**closed form??**

let [tex] f:u \rightarrow R^n [/tex] be a differentiable function with a differentiable inverse [tex] f^{-1}: f(u) \rightarrow R^n [/tex]. if every closed form on u is exact, show that the same is true for f(u).

Hint: if dw=0 and [tex]f^{\star}w = d\eta, [/tex] consider [tex](f^{-1})^{\star}\eta. [/tex]

i don't know where to start with the problem. what is a closed form? what does it mean that "every closed form on u is exact"?