Rule for Extracting Random Variable from Conditional Expectation of Product

Theorem
Let $\struct {\Omega, \Sigma, \Pr}$ be a probability space.

Let $\GG \subseteq \Sigma$ be a sub-$\sigma$-algebra.

Let $X$ and $Y$ be integrable random variables such that:


 * $X Y$ is integrable

and:


 * $Y$ is $\GG$-measurable.

Let $\expect {X \mid \GG}$ be a version of the conditional expectation of $X$ given $\GG$.

Let $\expect {X Y \mid \GG}$ be a version of the conditional expectation of $X Y$ given $\GG$.

Then:


 * $\expect {X Y \mid \GG} = Y \expect {X \mid \GG}$ almost everywhere.

Proof
Let $X$ and $Y$ be non-negative random variables.

We show first that the statement holds if $Y = 1_A$ for some $A \in \GG$.

We show that $Y \expect {X \mid \GG}$ is a version of the conditional expectation of $X Y$ given $\GG$.

We will then obtain the demand from Existence and Essential Uniqueness of Conditional Expectation Conditioned on Sigma-Algebra.

Then, for any $G \in \GG$ we have:

So we have:


 * $\expect {X Y \mid \GG} = Y \expect {X \mid \GG}$ almost everywhere

for all $Y$ of the form $1_A$ for $A \in \GG$.

Now let $Y$ be a general non-negative random variable.

Similarly, there exists an increasing sequence $\sequence {Y_n}_{n \mathop \in \N}$ of positive simple random variables such that:


 * $\ds \map Y \omega = \lim_{n \mathop \to \infty} \map {Y_n} \omega$

for $\omega \in \Omega$.

Then $\sequence {X Y_n}_{n \mathop \in \N}$ is an increasing sequence of random variables.

So, by the Conditional Monotone Convergence Theorem, we have:


 * $\ds \lim_{n \mathop \to \infty} \expect {X Y_n \mid \GG} = \expect {X Y \mid \GG}$ almost everywhere.

We also see that:


 * $\ds \lim_{n \mathop \to \infty} Y_n \expect {X \mid \GG} = Y \expect {X \mid \GG}$

Hence we have:


 * $\expect {X Y \mid \GG} = Y \expect {X \mid \GG}$ almost everywhere

when $X$ and $Y$ are non-negative.

Now, suppose that $X$ and $Y$ are general integrable random variables.

Then we have:

From Function Measurable iff Positive and Negative Parts Measurable, we have that:


 * $Y^+$ and $Y^-$ are $\GG$-measurable.

Then we have, since the theorem is known to hold for non-negative random variables:

Also known as
Applying this rule is often known as taking out what is known.