Sigma-Algebra Closed under Symmetric Difference
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Theorem
Let $\struct {X, \Sigma}$ be a measurable space.
Let $A, B \in \Sigma$.
Then the symmetric difference $A \Delta B$ is contained in $\Sigma$.
Proof
From Sigma-Algebra Closed under Set Difference, we have:
- $A \setminus B \in \Sigma$
and:
- $B \setminus A \in \Sigma$
Since $\sigma$-algebras are closed under countable union, we have:
- $\paren {A \setminus B} \cup \paren {B \setminus A} \in \Sigma$
From the definition of symmetric difference, we have:
- $A \Delta B = \paren {A \setminus B} \cup \paren {B \setminus A}$
so:
- $A \Delta B \in \Sigma$
$\blacksquare$