Proposition: Given any nonempty closed convex set C and an arbitrary vector x in R^J, there is a unique member Pcx of C closest, in the sense of the two-norm, to x. The vector Pcx is called the orthogonal (or metric) projection of x onto C and the operator Pc the orthogonal projection onto C.
Question: Given a point s in a convex set C, where are the points x for which s = Pcx?
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