Bourbaki

Exercise 14, § 5

Integration, IV, p. 129

Let (An)(A_n) be a sequence of subsets of XX such that, for every index nn, there exists a measurable set Bn⊃AnB_n \supset A_n, the BnB_n being pairwise disjoint. Show that

μ∗(⋃nAn)=∑nμ∗(An)andμ∗(⋃nAn)=∑nμ∗(An). \mu^*\left( \bigcup_n A_n \right) = \sum_n \mu^*(A_n) \quad \text{and} \quad \mu_*\left( \bigcup_n A_n \right) = \sum_n \mu_*(A_n).
Solution

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Transcribed from the printed volume. Not translated.