Which gives X =X⊥⊥ X = X ⊥⊥, as required. (←) (←) If X =X⊥⊥ X = X ⊥⊥ then X X is a closed linear subspace of a Hilbert space. You could try the proof of the converse for yourself. Hint: use a sequence in X X together with the definition of the orthogonal complement, show that the limit point is in X X or X⊥⊥ X ⊥⊥. More @Wikipedia
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