Bourbaki

Proposition 13

Tag 0BLJ, permanent. Cite it as [Bourbaki, Tag 0BLJ].

General Topology, chapter III, § 4. Groups operating properly on a topological space; compact ness in topological groups and spaces with operators

Let G be a locally compact group and let H be a closed subgroup of G. Then the homogeneous space G/HG/H is locally compact and paracompact.

Since G/HG/H is Hausdorff (§ 2, no. 5, Proposition 13) it is locally compact, by Proposition 9 of no. 5 applied to H operating on G on the right. Thus it remains to show that G/HG/H is paracompact. Let V be a symmetric compact neighbourhood of e in G, and let G0=V∞G_0 = V^\infty be the subgroup of G generated by V. G0G_0 is open (§ 2, no. 1, Corollary to Proposition 4) and operates continuously on G/HG/H (§ 2, no. 5, Proposition 12). If we can show that each of the orbits G0.z (z∈G/H)G_0.z \ (z \in G/H) is an open subset of G/HG/H and a countable union of compact sets, then it will follow that G/HG/H is the topological sum of the distinct orbits G0.zG_0.z and is therefore paracompact (Chapter I, , no. 10, Theorem 5). That G0.zG_0 . z is open in G/HG/H follows from the facts that G0G_0 is open in GG and the equivalence relation x−1y∈Hx^{-1} y \in H is open (§2\S 2, no. 4, Lemma 2). On the other hand, G0zG_0 z is the union of the sets Vn.zV^n . z (n≥1n \geq 1), and since VnV^n is compact in GG and G/HG/H is Hausdorff, Vn.zV^n . z is compact. This completes the proof

Bourbaki puts the proof directly after the statement, so what is above is the statement and its proof together.

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