# Proof with probability inequalities and infinite sequences

I am reading Berkes et al. (2003) about the GARCH model. Could someone help me figure out the proof of one lemma in the paper?

$\mathbf{Lemma.}$ If $\{\xi_k, 0\leq\xi_k<\infty\}$ is a sequence of identically distributed random variables satisfying $$\mathrm{E}\log^{+}|\xi_0|<\infty, \tag{1}$$ then $\sum_{k=0}^{\infty}z^{k}\xi_k$ converges with probability one for any $|z|<1$. Note, $\log ^{+} x=\log x$ if $x>1$, and $0$ otherwise.

$\mathbf{Proof.}$ By the Borel-Cantelli lemma it is enough to prove that, for any $\zeta>1$, $$\sum_{k=1}^{\infty}P\{|\xi_{k}|>\zeta^{k}\}<\infty. \tag{2}$$

The distribution of $\xi_k$ does not depend on $k$, so \begin{align} \sum_{k=1}^{\infty} P\{|\xi_{k}|>\zeta^{k}\} &=P\{\log^{+}|\xi_k|>k\log\zeta\}\nonumber \\ &=\sum_{k=1}^{\infty} P\{\log^{+}|\xi_0|>k\log\zeta\} \tag{3} \\[10pt] &\leq\mathrm{E} \log^{+} |\xi_0|/\log \zeta, \tag{4} \end{align} and thus $(1)$ implies $(2)$.

$\Box$

$\mathbf{Question.}$ It seems to me that $(3)$ does not imply $(4)$. It is natural to apply the Markov inequality to $(3)$ and we have $\sum_{k=0}^{\infty} P\{\log^{+}|\xi_0|>k\log\zeta\}\leq \sum_{k=1}^{\infty}\frac{\mathrm{E}\log^{+}|\xi_0|}{k\log\zeta}$. Since the harmonic sequence, $\sum_{k=1}^{\infty}\frac{1}{k}$, does not converge, we cannot get $(4)$ by using the Markov inequality. Did I miss something here?

References:

• You should add a reference to the paper. – Juho Kokkala Aug 5 '16 at 3:53
• I think I figured it out by using $\mathrm{E}X=\int_{0}^{\infty} P(X>s)ds$ for any nonnegative random variable $X$. – JRBNB Aug 5 '16 at 15:23
• Could you edit the title to highlight the actual problem? I don't know the terminology of these inequalities, but probably you do, so you could include a relevant name. Having GARCH in the title is not useful, IMHO. Also, the GARCH tag seems irrelevant as the inequality is probably not intrinsically specific to GARCH models, is it? – Richard Hardy Aug 5 '16 at 17:00
• My original title is 'Is this proof wrong?' And the only tag I used is 'probability'. Some people help me make modifications. If you like you also can do it. – JRBNB Aug 5 '16 at 17:33

I think I asked a dumb question. The proof is right. The missing step in the proof is as follows.

\begin{align} &~~~~\sum_{k=1}^{\infty} P\{\frac{\log^{+}|\xi_0|}{\log\zeta}>k\}\\ &\leq \sum_{k=1}^{\infty} \int_{k-1}^k P\{\frac{\log^{+}|\xi_0|}{\log\zeta}>s \}ds\\ &=\mathrm{E} \log^{+} |\xi_0|/\log \zeta, \end{align}

Your argument does not invalidate their argument. Basically, they say $A<B$, where $B<\infty$, while you are saying, $A<B\times C$, where $C=\infty$. Both arguments are compatible and your argument does not invalidate theirs.

• @JRBNB I don't know whether they used it or not. In fact, you should provide a link to the paper (specially because you are copy-pasting bits of it). My point is about the material you just posted. Your inequallity corresponds to $A< B\times C$, where $A < B$ is their inequality. – Kevin Levrone Aug 4 '16 at 23:31
• @JRBNB I have to go now. I suggest you post your questions when you have more time to interact and show more interest in your own questions. Good luck with your reading. – Kevin Levrone Aug 4 '16 at 23:37