Lemma 6.1.11.label Let $X$ be a normed space and $Y$ a closed subspace. Then $X$ is complete if and only if $Y$ and the quotient $X/Y$ are complete
Lemma 6.1.11.label Let $X$ be a normed space and $Y$ a closed subspace. Then $X$ is complete if and only if $Y$ and the quotient $X/Y$ are complete