Condition for Independence from Product of Expectations/Corollary
Corollary to Condition for Independence from Product of Expectations
Let $\struct {\Omega, \Sigma, \Pr}$ be a probability space.
Let $X$ and $Y$ be independent discrete random variables on $\struct {\Omega, \Sigma, \Pr}$.
Then:
- $\expect {X Y} = \expect X \expect Y$
assuming the latter expectations exist.
General Result
Let $X_1, X_2, \ldots, X_n$ be independent discrete random variables.
Then:
- $\ds \expect {\prod_{k \mathop = 1}^n {X_k} } = \prod_{k \mathop = 1}^n \expect {X_k}$
assuming the latter expectations exist.
Proof
From Condition for Independence from Product of Expectations, setting both $g$ and $h$ to the identity functions:
- $\forall x \in \R: \map g x = x$
- $\forall y \in \R: \map h y = y$
It follows directly that if $X$ and $Y$ are independent, then:
- $\expect {X Y} = \expect X \expect Y$
assuming the latter expectations exist.
$\blacksquare$
Note on Converse
Note that the converse of the corollary does not necessarily hold.
Let $X$ and $Y$ be discrete random variables on $\struct {\Omega, \Sigma, \Pr}$ such that:
- $\expect {X Y} = \expect X \expect Y$
Then it is not necessarily the case that $X$ and $Y$ are independent.
Sources
- 1986: Geoffrey Grimmett and Dominic Welsh: Probability: An Introduction ... (previous) ... (next): $\S 3.3$: Independence of discrete random variables: Theorem $3 \text{C}$