X is an Inner product space and U is a subset(space) of X. U^\perp is the space of all vertices orthogonal to all the elements of U. Give an example of a space U^\perp\perp is not a subset of U. Can it be from l^2?
If U is a subspace, then U^{\perp\perp} is its closure (by the Hahn-Banach theorem), so you may take any subspace that is not closed as an example. For instance, if X=\ell^2, then you may take U to be the space of all finitely supported sequences.
If U is a subspace, then U^{\perp\perp} is its closure (by the Hahn-Banach theorem), so you may take any subspace that is not closed as an example. For instance, if X=\ell^2, then you may take U to be the space of all finitely supported sequences.