Consider the inclusion $\phi: \mathbb{Z} \to \hat{\mathbb{Z}}$ where $\hat{\mathbb{Z}} : = \varprojlim_n\mathbb{Z}/n\mathbb{Z}$. Is the subspace topology on $\mathbb{Z}$ from this inclusion the discrete topology? Is this true for all $G \to \hat{G}$?
2026-03-26 04:50:27.1774500627
Is Z discrete in its profinite completion?
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No, the image of $Z$ is dense in its profinite completion which is a compact group. https://en.wikipedia.org/wiki/Profinite_group#Profinite_completion
You can't have an infinite discrete subset in a compact space.