Bourbaki

Corollary 2

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

Integration, chapter VII, § 1. Construction of a Haar measure

Every relatively left-invariant measure is relatively right-invariant.

For, with the notations of Cor. 1,

δ(s)(χ⋅μ)=(δ(s)χ)⋅(δ(s)μ)=(χ(s)χ)⋅(ΔG(s)μ)=(χΔG)(s)(χ⋅μ). \delta(s)(\chi \cdot \mu) = (\delta(s)\chi) \cdot (\delta(s)\mu) = (\chi(s)\chi) \cdot (\Delta_G(s)\mu) = (\chi \Delta_G)(s)(\chi \cdot \mu).

On account of Cor. 2, we shall henceforth speak of relatively invariant measures on G, without further specification. The relatively invariant measures admit as special cases the left Haar measures and the right Haar measures. Given a relatively invariant measure ν\nu on G, it is convenient to distinguish between its left multiplier χ\chi and its right multiplier χ′\chi', defined by γ(s)ν=χ(s)−1ν\gamma(s)\nu = \chi(s)^{-1}\nu, δ(s)ν=χ′(s)ν\delta(s)\nu = \chi'(s)\nu. By (34), these multipliers satisfy the relation

χ′=χΔG. \chi' = \chi \Delta_G.

Still denoting by μ\mu a left Haar measure, we have

ν′=(χ⋅μ)′=χ˙⋅μ˙=(χ−1ΔG−1)⋅μ, \nu' = (\chi \cdot \mu)' = \dot{\chi} \cdot \dot{\mu} = (\chi^{-1} \Delta_G^{-1}) \cdot \mu,

thus ν′\nu' is relatively invariant with left multiplier χ−1ΔG−1\chi^{-1} \Delta_G^{-1} and right multiplier χ−1\chi^{-1}.

The concepts of negligible, locally negligible, measurable and locally integrable function are the same for every relatively invariant measure.

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

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Cites

Transcribed from the printed volume. Not translated.