Bourbaki

Remark 2

Tag 09FD, permanent. Cite it as [Bourbaki, Tag 09FD].

Integration, VII, p. 36, § 2. Quotient of a space by a group; homogeneous spaces

Suppose HH is compact and β\beta is normalized. The saturation of every compact subset of XX is compact. Therefore, if f∈K(X/H)f \in \mathscr{K}(X/H) then f∘π∈K(X)f \circ \pi \in \mathscr{K}(X); and, for every positive measure λ\lambda on X/HX/H, Prop. 5 c) gives

∫X(f∘π)(x) dλ#(x)=∫X/Hf(u) dλ(u). \int_X (f \circ \pi)(x)\, d\lambda^\#(x) = \int_{X/H} f(u)\, d\lambda(u).

In other words, λ\lambda is the image of λ#\lambda^\# under π\pi.

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

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Transcribed from the printed volume. Not translated.