Let $X_n \leq Y_n$ and both converge in distribution $X_n, Y_n \overset{d}{\longrightarrow}F$. Does $|Y_n – X_n| \overset{p}{\longrightarrow} 0\,$

probabilityprobability theoryrandom variables

Let $(X_n)$ and $(Y_n)$ be sequences of random variables such that $X_n \leq Y_n$ for all $n \in \mathbb N$. Let $F$ be an arbitrary distribution function.

Suppose both sequences converge in distribution to $F$, i.e.
$$
P(X_n \leq c) \underset{n \to \infty}{\longrightarrow} F(c) \quad \text{and} \quad P(Y_n \leq c) \underset{n \to \infty}{\longrightarrow} F(c)
$$

for all continuity points $c$ of $F$.

Does it then hold that $|Y_n – X_n| \overset{p}{\longrightarrow} 0\,$?

Thoughts

I'm aware that the weak convergence of random sequences to the same distribution doesn't generally imply this convergence in probability. (Just set $X_n := X$ and $Y_n := Y$ for i.i.d. $X$ and $Y$ with a non-degenerate distribution.) But I'm curious whether it does under the inequality assumption $X_n \leq Y_n$.

This seems intuitive, and I'm having trouble thinking up a counterexample. On the other hand, I haven't been able to prove the statement. So maybe it's time to add yet another counterexample to my collection 😉

Best Answer

Fix $\delta>0$. Choose $K>0$ such that $P(|Y_n|\le K)\ge 1 -\delta$ and $P(|X_n|\le K)\ge 1 -\delta$ for all $n$. (We can do this, because both sequences are tight.) Let $g(x)=(x\wedge K)\vee(-K)$. Then \begin{align} P(|Y_n - X_n| \ge \varepsilon) &\le P(|Y_n - g(Y_n)| \ge \varepsilon/3) + P(|g(Y_n) - g(X_n)| \ge \varepsilon/3) + P(|g(X_n) - X_n| \ge \varepsilon/3)\\ &\le 2\delta + P(|g(Y_n) - g(X_n)| \ge \varepsilon/3). \end{align} But $g$ is nondecreasing, so $g(Y_n)\ge g(X_n)$, and $g$ is bounded and continuous, so we have $$ E|g(Y_n) - g(X_n)| = E[g(Y_n) - g(X_n)] = E[g(Y_n)] - E[g(X_n)] \to 0. $$ Hence, $g(Y_n)-g(X_n)\to 0$ in $L^1$, and therefore also in probability. Letting $n\to\infty$ in the first inequality gives $$ \limsup_{n\to\infty} P(|Y_n - X_n| \ge \varepsilon) \le 2\delta. $$ Letting $\delta\to0$ shows that $|Y_n-X_n|\to0$ in probability.