Bourbaki

Proposition 22

Tag 08WG, permanent. Cite it as [Bourbaki, Tag 08WG].

Integration, chapter IV, § 5. Measurable functions and sets

— *Let F be a Banach space, X a locally compact space, E\mathscr{E} the set of bounded positive measures on X, μ\mu an element of E\mathscr{E}, and B\mathscr{B} a filter base on E\mathscr{E}. Assume that B\mathscr{B} converges vaguely to μ\mu and that ∥ν∥\| \nu \| tends to ∥μ∥\| \mu \| with respect to B\mathscr{B}. Let f be a mapping of X into F satisfying the following conditions:

(i) f is bounded, and is integrable for μ\mu and for every measure that belongs to some element of B\mathscr{B}; (ii) the set of points of discontinuity of f is μ\mu-negligible. Then ∫f dν\int f \, d\nu tends to ∫f dμ\int f \, d\mu with respect to B\mathscr{B}.

Let ε>0\varepsilon > 0. There exist elements a1,…,ana_1, \ldots, a_n in F, functions g1,…,gng_1, \ldots, g_n in K(X)\mathscr{K}(X), and a bounded continuous function h≥0h \geq 0 on X, such that

∣f−g1a1−⋯−gnan∣≤h≤2sup⁡∣f∣ |f - g_1 a_1 - \cdots - g_n a_n| \leq h \leq 2 \sup |f|

on X and ∫h dμ≤ε\int h \, d\mu \leq \varepsilon (Lemma 5). Let M=sup⁡∣f∣M = \sup |f|. There exist a compact subset K of X such that μ∗(X−K)≤ε\mu^*(X - K) \leq \varepsilon (§ 4, No. 7, Prop. 12 and No. 6, Th. 4), a compact neighborhood K′K' of K in X, and a continuous mapping h′h' of X into [0,2M][0, 2M] such that h′=hh' = h on K, h′=2Mh' = 2M on X−K′X - K'; replacing h′h' by sup⁡(h,h′)\sup(h, h'), we can suppose in addition that h′≥hh' \geq h. Then ∫(h′−h) dμ≤2Mμ∗(X−K)≤2Mε\int (h' - h) \, d\mu \leq 2M \mu^*(X - K) \leq 2M \varepsilon. On the other hand, h′=h1+2Mh' = h_1 + 2M, where h1∈K(X)h_1 \in \mathscr{K}(X). Taking into account § 4, No. 7, Prop. 12, the number

∫h′ dν=∫h1 dν+2M∥ν∥ tends to ∫h1 dμ+2M∥μ∥=∫h′ dμ with respect to B. There then exists an A∈B such that, for every ν∈A, \int h' \, d\nu = \int h_1 \, d\nu + 2M \|\nu\| \text{ tends to } \int h_1 \, d\mu + 2M \|\mu\| = \int h' \, d\mu \text{ with respect to } \mathscr{B}. \text{ There then exists an } A \in \mathscr{B} \text{ such that, for every } \nu \in A,
∣∫(g1a1+⋯+gnan) dν−∫(g1a1+⋯+gnan) dμ∣≤ε, \left| \int (g_1 a_1 + \cdots + g_n a_n) \, d\nu - \int (g_1 a_1 + \cdots + g_n a_n) \, d\mu \right| \leq \varepsilon,
∫h dν≤∫h′ dν≤∫h′ dμ+ε≤∫h dμ+2Mε+ε≤2(M+1)ε. \int h \, d\nu \leq \int h' \, d\nu \leq \int h' \, d\mu + \varepsilon \leq \int h \, d\mu + 2M \varepsilon + \varepsilon \leq 2(M + 1)\varepsilon.
These inequalities imply
∣∫f dν−∫f dμ∣≤ \left| \int f \, d\nu - \int f \, d\mu \right| \leq
∫h dν+∣∫(g1a1+⋯+gnan) dν−∫(g1a1+⋯+gnan) dμ∣+∫h dμ \int h \, d\nu + \left| \int (g_1 a_1 + \cdots + g_n a_n) \, d\nu - \int (g_1 a_1 + \cdots + g_n a_n) \, d\mu \right| + \int h \, d\mu
≤2(M+2)ε, \leq 2(M + 2)\varepsilon,
which proves the proposition.

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.