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Let $Y_1, Y_2, Y_3,\ldots,Y_n,\ldots$ be iid and bounded random variables with $E[Y_1]=0$. Define $X_n = Y_1+Y_2+ \cdots + Y_n$.

If $\Pr(Y_1 \neq 0) \gt 0$, then $ \limsup X_n = \infty$ with probability $1$.

The conclusion seems intuitive, but how would I approach it rigorously?

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  • $\begingroup$ Hint: If you assume $\limsup X_n$ is a finite number ... can you show this leads to a contradiction? $\endgroup$ – WetlabStudent May 18 '15 at 18:48
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Suppose that $ \limsup X_n = b$ for some $b \in \mathbb{R}$. We know that $|Y_i| \leq c$ for some $c \in \mathbb{R}^{+}$. So $X_n \leq nc$. Now for some large $N$, $n \geq N$ means that $X_n \leq b$. But we can choose a large enough $n$ (say $m$) such that $b < X_m \leq mc$.

Note: This is just my attempt. Please feel free to continue OP.

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    $\begingroup$ I'm not sure if this is meant as a hint or a proof. Hints are preferred for homework-style questions. That being said, your last sentence needs motivation: perhaps the OP can supply this. $\endgroup$ – ekvall May 19 '15 at 0:09

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