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Let $G$ be an abelian topological group and let $\hat{G}$ be its completion, i.e. the group containing the equivalence classes of all Cauchy sequences of $G$. What exactly is the topology of $\hat{G}$?

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For each neighborhood $N$ of zero in $G$, define a neighborhood $\hat{N}$ in $\hat{G}$ consisting of those equivalence classes for which all sequences in the class are eventually in $N$. This is a base (of neighborhoods of zero) for the new topology. –  GEdgar Sep 8 '12 at 16:06
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Another remark. Unless $G$ is metrizable, you cannot expect the "completion" by sequences to be complete in the sense of uniform space. So normally we would do completion by nets or by filters or similar. –  GEdgar Sep 8 '12 at 16:07
    
@GEdgar: How do we know that this $\hat{N}$ will be nonempty? –  Manos Sep 8 '12 at 16:11
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It contains many constant sequences. For general $G$ it could happen that every cauchy sequence is eventually constant, so that the sequential completion is nothing new. –  GEdgar Sep 8 '12 at 16:12
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@MSina: Do we really need a [completeness] tag? –  Asaf Karagila Mar 3 '13 at 18:16
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up vote 3 down vote accepted

formerly a remark

For each neighborhood $N$ of zero in G, define a neighborhood $\hat{N}$ in $\hat{G}$ consisting of those equivalence classes for which all sequences in the class are eventually in $N$. This is a base (of neighborhoods of zero) for the new topology.

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