Bourbaki

Corollary 3

Tag 096D, permanent. Cite it as [Bourbaki, Tag 096D].

Integration, V, p. 76, § 7. Integration with respect to an induced measure

Let θ\theta be a complex measure on T; then

∣θ∣X=∣θX∣. |\theta|_X = |\theta_X|.

Set ∣θ∣=μ|\theta| = \mu and apply Cor. 2 on taking gg to be a complex function of absolute value 1 such that θ=g⋅μ\theta = g \cdot \mu (§ 5, No. 5, Cor. 3 of Th. 2); then θX=gX⋅μX\theta_X = g_X \cdot \mu_X; but gXg_X is a function of absolute value 1, and the formula (6) follows from Prop. 2 of § 5, No. 2.

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.