Solution

ID: past-exam-of-the-options-i-course-of-the-university-of-oxford/2012/f/f1/a/solution

The function is continuously differentiable when all its first partial derivatives exist on and are continuous there. Equivalently, has a Fréchet derivative at every point and the map is continuous. For a scalar-valued function,
On the open domain, continuous first partial derivatives ensure that this linear expression is the total derivative.

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