[Math] Equal in distribution but unequal almost everywhere

measure-theoryprobability distributionsprobability theory

If this question has been asked, I apologize but I could not find it.

I was wondering if it was possible construct $X$, $Y$ two iid rv's such that they equal in distribution, i.e. $P_X(B) = P(X^{-1}(B)) = P(Y^{-1}(B)) = P_Y(B), B \in Borel(R)$, but differ every a.s., so that $X(\omega) \neq Y(\omega), \forall \omega \in \Omega$, except on a set of measure zero. I realize that likely one has to use the Axiom of Choice, but I am unsure about how to proceed.

I see here " If X and Y are equal almost surely, then they have the same distribution, but the reverse direction is not correct " that equality in distribution does not mean that $P(X = Y) = 1$ even if $X,Y$ are iid rv's.

Any advice/links would be greatly appreciated!

Best Answer

If rv $U$ is uniformly distributed on $(-1,1)$ then so is $-U$.

They have the same distribution but $U=-U\iff U=0$ and $P(U=0)=0$.

If you want this under the extra condition that the random variables are independent, then let $(X,Y)$ be uniformly distributed on $(0,1)^2$. Then $X$ and $Y$ are independent and both uniformly distributed on $(0,1)$ (so they are iid). However: $P(X=Y)=0$.

Related Question