Let $G$ be the usual: a topological Abelian group with a topology induced by a countable neighbourhood basis $G_n$ of zero such that $G = G_1 \supset G_2 \supset \dots$. Let $\widehat{G}$ denote the completion of $G$.
In a proof I am using $ \widehat{G/G_n} \cong G/G_n$
(Atiyah-MacDonald, page 105, Corollary 10.4.)
Can you tell me if the following way is the correct way of thinking about it:
We know that $\widehat{G} \cong \varprojlim_n G/G_n$
so that $\widehat{G}$ are all sequences $\vec{g}$ in $G$ with $g_n$ in $G_n \subset G_{n-1} \subset \dots \subset G_1 = G$. Then $\widehat{G/G_n}$ are all sequences in $G/G_n$ with $g_n$ in $G_n/G_n = \{0\}$. Hence $\widehat{G/G_n}$ looks like all sequences that are zero after $n$. But this is just what sequences in $G/G_n$ look like.
I guess I should somehow say "in the inverse limit of $G/G_n$".