Steinberg symbol

In algebra, the Hausdorff completion $\hat{G}$ of a group G with filtration $G_n$ is the inverse limit $\underleftarrow{\lim }G/G_n$ of the discrete group $G/G_n$. A basic example is a profinite completion. The image of the canonical map $G\to \hat{G}$ is a Hausdorff topological group and its kernel is the intersection of all $G_n$: i.e., the closure of the identity element. The canonical homomorphism $\mathrm{gr}(G)\to \mathrm{gr}(\hat{G})$ is an isomorphism.

The concept is named after Felix Hausdorff.

References

• Nicolas Bourbaki, Commutative algebra

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