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Published online by Cambridge University Press: 20 November 2018
In the theory of surface area one meets situations where a function z = f(x, y) which is defined and continuous on a closed rectangle E, is partially differentiable on E except on a subset of E of Lebesgue measure zero.
* Such functions are easy to construct. One takes a continuously partially differentiable function and merely "flattens " (i.e., replaces by a plane) a suitable sequence of subsets of S in a neighborhood of (P, f(P)).