Existence and Uniqueness of Sigma-Algebra Generated by Collection of Mappings
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Theorem
Let $I$ be an indexing set.
Let $\family {\struct {X_i, \Sigma_i} }_{i \mathop \in I}$ be a family of measurable spaces.
Let $X$ be a set.
Let $\family {f_i: X \to X_i}_{i \mathop \in I}$ be a family of mappings.
Then $\map \sigma {f_i: i \in I}$, the $\sigma$-algebra generated by $\family {f_i}_{i \mathop \in I}$, exists and is unique.
Proof
By Characterization of Sigma-Algebra Generated by Collection of Mappings:
- $\ds \map \sigma {f_i: i \in I} = \map \sigma {\bigcup_{i \mathop \in I} \map {f_i^{-1} } {\Sigma_i} }$
where the second is a $\sigma$-algebra generated by a collection of subsets.
The result follows from applying Existence and Uniqueness of Sigma-Algebra Generated by Collection of Subsets.
$\blacksquare$
Sources
- 2005: René L. Schilling: Measures, Integrals and Martingales ... (previous) ... (next): $7.5$