Bourbaki

Proposition 19

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

Integration, chapter VIII, § 4. Convolution of measures and functions

— *Let GG be a locally compact group, β\beta a relatively invariant positive measure ≠0\neq 0 on GG, B\mathscr{B} a base for the filter of neighborhoods of ee in GG, consisting of compact neighborhoods. For every V∈BV \in \mathscr{B}, let fVf_V be a continuous function ≥0\geq 0 on GG, with support contained in VV, such that ∫fVdβ=1\int f_V d\beta = 1. If μ\mu is a measure on GG then, in M(G)\mathscr{M}(G) equipped with the topology of compact convergence in H(G)\mathscr{H}(G),

μ=lim⁡V(μ∗fV)⋅β=lim⁡V(fV∗μ)⋅β, \mu = \lim_V (\mu * f_V) \cdot \beta = \lim_V (f_V * \mu) \cdot \beta,

the limit being taken with respect to the section filter of B\mathscr{B}.

For the topology of compact convergence in C(G)\mathscr{C}(G), fV⋅βf_V \cdot \beta tends to εe\varepsilon_e with respect to the section filter of B\mathscr{B} (§ 2, No. 7, Cor. 1 of Lemma 4). Therefore μ=lim⁡Vμ∗(fV⋅β)=lim⁡V(fV⋅β)∗μ\mu = \lim_V \mu * (f_V \cdot \beta) = \lim_V (f_V \cdot \beta) * \mu in M(G)\mathscr{M}(G) equipped with the topology of compact convergence in H(G)\mathscr{H}(G) (§ 3, No. 3, Cor. of Prop. 12).

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

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