Let be a symplectic manifold. It said to be exact if for some one-form on .

(1) If is exact, then there is a canonical isomorphism between the v.f. and 1-forms. In particular, there exists a v.f. such that . Then we have , and , and .

(2) Suppose there exists a vector field on such that its Lie-derivative (notice the difference with ). Then Cartan’s formula says that , where . So is exact, and .

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