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Hilbert Spaces - University of Washington
sites.math.washington.eduHilbert Spaces 87 If y∈ M, then kx−yk2 = kPx−yk2 +kQxk2, which is clearly minimized by taking y= Px. If y∈ M⊥, then kx−yk2 = kPxk2+kQx−yk2, which is clearly minimized by taking y= Qx. Corollary. If Mis a closed subspace of a Hilbert space X, then (M⊥)⊥ = M. In general, for any A⊂ X, (A⊥)⊥ = span{A}, which is the smallest closed subspace of Xcontaining A,