Random variable greater than another random variable – measure theoretic argument

analysismeasure-theoryprobabilityproduct-measurerandom variables

Suppose $\xi,\eta$ are two independent, identical random variables that are non-degenerate. I want to show that $\mathbb{P}(\xi<\eta) = \mathbb{P}(\eta<\xi) > 0$ through a measure-theoretic argument. I think it's relatively easy to show for discrete or (absolutely) continuous random variables by considering sums or integrals, but I want to make an argument using measure theory so I can account for the case of mixed distributions.

To do so, I wanted to first understand the meaning of identical random variables. My interpretation is that $\xi,\eta$ are identical if $\mathbb{P}_\xi$ and $\mathbb{P}_\eta$, the pushforward measures on $\mathbb{R}$, are identical. But does that imply the mappings $\xi:\Omega\to\mathbb{R}$ and $\eta:\Omega\to\mathbb{R}$ are also identical?

Thus far, I've considered two approaches to this. One is to show that $\mathbb{P}_{\xi-\eta}(\mathbb{R}_+) = \mathbb{P}_{\xi-\eta}(\mathbb{R}_-) >0$, where $\mathbb{P}_{\xi-\eta}$ is the pushforward measure of the random variable $\xi-\eta$. Another way to approach this is to consider the product measure on $\mathbb{R}^2$ (since the random variables are continuous) and the pushforward measure it places on the region $\{(x,y)\in\mathbb{R}^2:x>y\}$. However, I'm not quite sure which approach is better / how I should further my analysis.

Best Answer

Suppose to the contrary that the statement does not hold. Then $$\Bbb{P}(\xi< \eta) = \Bbb{P}(\xi > \eta) = 0$$ which means that $\Bbb{P}(\xi = \eta) = 1$. Thus $\xi = \eta$ almost surely. But since $\xi$ and $\eta$ are independent, we have $$\Bbb{P}(\xi \in A, \eta\in B) = \Bbb{P}(\xi \in A) \Bbb{P}(\eta \in B) $$ for Borel sets $A,B$. Taking $A=B$, and using that $\xi =\eta$ almost surely we obtain $$\Bbb{P}(\xi \in A) = \Bbb{P}(\xi \in A)^2$$ such that $\Bbb{P}(\xi \in A)\in \{0,1\}$ for every Borel set $A$. Such a random variable must be constant almost surely, contradicting non-degeneracy.

Related Question