Solution

ID: past-exam-of-the-mathematics-course-of-the-year-1-of-the-natural-sciences-course-of-the-university-of-cambridge/2007/paper-2/section-b/5/a/solution

If with continuous second partial derivatives of , then and . The symmetry of second derivatives gives the necessary condition for an exact differential form:
This condition is also locally sufficient on a small rectangle for continuously differentiable coefficients; global sufficiency additionally needs suitable topology of the domain.

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