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> Is the topological group $(\mathbf{Q}\_p/\mathbf{Z}\_p)^{\oplus k}$, $k\ge 1$, a $\sigma$-compact topological group when endowed with its natural $p$-adic topology?
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More generally, I'm looking for a criterion for locally compact Hausdorff topological groups to not contain a nested exhaustive sequence of compact subgroups (i.e. locally compact Hausdorff topological groups $G$ with a countable family of nested compact subsets $K\_n$ such that $\bigcup\_{n\ge 0}K\_n = G$).


**Example** Real and complex Lie groups are $\sigma$-compact exactly when they are compact.


**Example** The same group $(\mathbf{Q}\_p/\mathbf{Z}\_p)^{\oplus k}$, endowed with the discrete topology. It is uncountable, so it's not $\sigma$-compact when endowed with the discrete topology, if I'm not missing anything.



$\sigma$-compactness of some locally compact Hausdorff topological groups
I agree with @MatthewDaws ... for example the real Lie group $(\mathbb R,+)$ with the usual topology is locally compact and sigma-compact but not compact.


Take any locally compact group $G$. There is a neighborhood $V$ of $e$ with compact closure. The subgroup $H$ generated by $V$ is an open, locally compact, sigma-compact subgroup of $G$. The coset space $G/H$ is discrete.




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The example. The group $\mathbb Q\_p$ of $p$-adic numbers is locally compact. The subgroup $\mathbb Z\_p$ of $p$-adic integers is an open, compact, subgroup. The quotient $\mathbb Q\_p/\mathbb Z\_p$ is countable and discrete.


Indeed,
$$
\mathbb Q\_p = \bigcup\_{n=0}^\infty p^n \mathbb Z\_p .
$$


For a fixed $k \in \mathbb N$, also $(\mathbf{Q}\_p/\mathbf{Z}\_p)^{\oplus k}$ is countable and discrete.


