Conditional Expectation of Measurable Random Variable
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Theorem
Let $\struct {\Omega, \Sigma, \Pr}$ be a probability space.
Let $\GG \subseteq \Sigma$ be a sub-$\sigma$-algebra.
Let $X$ be a integrable random variable that is $\GG$-measurable.
Let $\expect {X \mid \GG}$ be a version of the conditional expectation of $X$ given $\GG$.
Then:
- $\expect {X \mid \GG} = X$ almost everywhere.
Proof
We show that $X$ is a version of $\expect {X \mid \GG}$.
Then since conditional expectation is essentially unique by Existence and Essential Uniqueness of Conditional Expectation Conditioned on Sigma-Algebra, we will obtain:
- $\expect {X \mid \GG} = X$ almost everywhere.
Note that $X$ is integrable and $\GG$-measurable by hypothesis, with:
- $\ds \int_A X \rd \Pr = \int_A X \rd \Pr$
So $X$ satisfies the conditions of conditional expectation, and so:
- $\expect {X \mid \GG} = X$ almost everywhere.
$\blacksquare$
Sources
- 1991: David Williams: Probability with Martingales ... (previous) ... (next): $9.7$: Properties of conditional expectation: a list