# A closed subspace of a Banach space is a Banach space

How to prove that a closed subspace of a Banach space is a Banach space?

A subspace is closed if it contains all of its limit points. But in the proof of the above question how can use this idea to get a Cauchy sequence and show that it is convergent in the subspace?

• You don't need to get a Cauchy sequence. You just assume a sequence is Cauchy and prove it converges and the limit point is in the subspace. – Hui Yu Nov 19 '12 at 16:15

Let $C$ be a closed subspace of a Banach space $B$, let $x_n$ be a Cauchy sequence in $C$ (with respect to the norm on $B$). Then the limit $x$ of $x_n$ exists in $B$ since $B$ is complete. Also, if it is not already in $C$, it must be a limit point of $C$. But $C$ is closed and hence contains all its limit points. Hence $C$ is complete, too.