# Closed Linear Subspaces Closed under Intersection

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## Theorem

Let $V$ be a topological vector space.

Let $\family {M_i}_{i \mathop \in I}$ be an $I$-indexed family of closed linear subspaces of $V$.

Then $M := \ds \bigcap_{i \mathop \in I} M_i$ is also a closed linear subspace of $V$.

## Proof

By Set of Linear Subspaces is Closed under Intersection, $M$ is a linear subspace of $V$.

By Topology Defined by Closed Sets, the intersection of closed sets is again closed.

As the $M_i$ are all closed, it follows that $M$ is closed.

Hence $M$ is a closed linear subspace of $V$.

$\blacksquare$