Suppose that f is such that for any closed, oriented surface S, S f n d S = 0. (See
Chapter 7, Problem 26(choose chapter or problem)
Suppose that f is such that for any closed, oriented surface S, S f n d S = 0. (See Exercise 25 for the definition of the normal derivative f/n.) Show that then 2 f x 2 + 2 f y2 + 2 f z2 = 0 (i.e., that f is harmonic).
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