Thursday 22 October 2015

Proof verification : $X_n to X$ in distribution, $Y_n to 0$ in probability $implies$ $X_nY_n to 0$ in probability



I have to show : $X_n \to X$ in distribution, $Y_n \to 0$ in probability $\implies$ $X_nY_n \to 0$ in probability.




Let $\alpha>0, \epsilon>0$. Then $\exists \delta>0$ such that $-\epsilon/\delta$, $\epsilon/\delta$ are continuity points of distribution of $X$ and $$P(|X|>\epsilon/\delta) \leq \alpha$$



Since $X_n \to X$ in distribution, $P(X_n \leq x) \to P(X \leq x)$ for all continuity points $($and in particular $-\epsilon/\delta$ and $\epsilon/\delta)$. There exists $N_1, N_2 \in \mathbb{N}$ such that
$$n \geq N_1 \implies |P(X_n \leq -\epsilon/\delta) - P(X \leq -\epsilon/\delta)|<\alpha$$
$$n \geq N_2 \implies |P(X_n \leq \epsilon/\delta) - P(X \leq \epsilon/\delta)|<\alpha$$
Let $N=\max\{N_1,N_2\}$. Then for $n \geq N$,




$$P(|X_n|>\epsilon/\delta)=1-P(|X_n| \leq \epsilon/\delta)=1-P(-\epsilon/\delta \leq X_n \leq \epsilon/\delta) = 1-P(X_n \leq \epsilon/\delta)+P(X_n < -\epsilon/\delta) = 1-P(X_n \leq \epsilon/\delta)+P(X_n \leq -\epsilon/\delta
\,\,[\text{by continuity}] \leq 1-P(X \leq \epsilon/\delta)+P(X \leq -\epsilon/\delta)+2\alpha = P(|X|>\epsilon/\delta)+2\alpha \leq 3\alpha$$



Since $Y_n \to 0$ in probability, $\exists$ $N_3 \in \mathbb{N}$ such that
$$n \geq N_3 \implies P(|Y_n|>\delta) \leq \alpha$$



Choose $N^{*}=\max\{N,N_3\}$. Note that
$$|X_nY_n|>\epsilon \implies |X_n|>\epsilon/\delta \,\,\text{or}\,\, |Y_n|>\delta$$
Hence,
$$P(|X_nY_n|>\epsilon) \leq P(|X_n|>\epsilon/\delta \,\,\text{or}\,\, |Y_n|>\delta) \leq P(|X_n|>\epsilon/\delta)+P(|Y_n|>\delta)$$

Thus,
$$n \geq N^{*} \implies P(|X_nY_n|>\epsilon) \leq 4\alpha$$
Since, $\alpha>0$ is arbitrary, $X_nY_n \to 0$ in probability.




Is the proof okay? I have a feeling that I have sort of over-killed it. Is it possible to write a shorter proof of the result? Thank you.


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