Characteristic Function of Intersection/Variant 2
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Theorem
Let $A, B \subseteq S$.
Let $\chi_{A \mathop \cap B}$ be the characteristic function of their intersection $A \cap B$.
Then:
- $\chi_{A \mathop \cap B} = \min \set {\chi_A, \chi_B}$
Proof
By Characteristic Function Determined by 1-Fiber, it suffices to show that:
- $\min \set {\map {\chi_A} s, \map {\chi_B} s} = 1 \iff s \in A \cap B$
By definition of characteristic function, we have:
- $\min \set {\map {\chi_A} s, \map {\chi_B} s} = 1$
- $\map {\chi_A} s = \map {\chi_B} s = 1$
because $\chi_A, \chi_B$ take values in $\set {0, 1}$.
By definition of $\chi_A$ and $\chi_B$, this is equivalent to the statement that both $s \in A$ and $s \in B$.
That is:
- $s \in A \cap B$
by definition of set intersection.
$\blacksquare$
Sources
- 2005: René L. Schilling: Measures, Integrals and Martingales ... (previous) ... (next): $\S 2$: Problem $5 \ \text{(vi)}$