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Suppose $X \subset L^1_{([0,1])}$ is the subspace consisting of all square-integrable functions. I have to show that $X$ is not a closed subset of $L^1_{([0,1])}$. How do I go about doing this? What exactly do I need to prove?

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    I adjusted the title to be less confusing.2012-04-25

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There are closed subspaces of $L^1[0,1]$ consisting of square-integrable functions (e.g. every finite-dimensional subspace is closed). However, the closed graph theorem shows that on every $L^1$-closed subspace of $L^2$ the two norms $\|\cdot\|_1$ and $\|\cdot\|_2$ are eqivalent. This may help in concrete situations.