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Let $B(t)$ be a Standard Brownian Motion.

We need to prove that $\underset{t \to \infty}\limsup \dfrac{B\left(t\right)}{\sqrt{t}\cdot\ln(t)}=0$

I was thinking if we could use the Blumenthal's $0-1$ law along the lines of the proof for $\underset{t \rightarrow \infty}\limsup \dfrac{B(t)}{\sqrt{t}}=\infty$. But I am not sure about how to argue in this case.

Math Attack
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    Use Law of Iterated Logarithm for Brownian Motion. https://math.stackexchange.com/questions/3867634/brownian-motion-law-of-iterated-logarithm – Kavi Rama Murthy Apr 23 '23 at 23:52
  • Hint: Checking the problem you already know, which part of the proof do you have to modify, are there already estimates/arguments you can use from your lecture? – a.s. graduate student Apr 24 '23 at 14:57
  • There exists motions B such that B(t)/sqrt T log t > k for any k, right? So why would lim sup of that object be equal to 0? The max value achieved is arbitrarily high if we can pick the motion. You're not taking an integral over all motions, then I could see how that would be zero. Can someone help me understand that point related to this question? – Snared Apr 26 '23 at 02:10

2 Answers2

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Here is perhaps an easier way to argue without the use of Law of Iterated Logarithm or Stochastic integrals.

Let $M_{t}=\sup_{[0,t]}B_{s}$ denote the running max of Brownian Motion and it is a simple consequence of Reflection Principle that $M_{t}\sim |B_{t}|$

The idea is to show that $\frac{M_{s_{k}}}{\sqrt{s_{k}\ln(s_{k})}}<\epsilon$ almost surely for all large $k$ for some increasing sequence $s_{k}$. Then we want to use a sandwich argument by controlling $\frac{B_{t_{k}}}{\sqrt{t_{k}\ln(t_{k})}}$ by means of $\frac{M_{s_{k}}}{\sqrt{s_{k}\ln(s_{k})}}$ by sandwiching properly. This idea can even be used to conclude $\lim\sup\frac{B_{t}}{\sqrt{t\ln(\ln(t))}}\leq 1$ which shows one side of the law of iterated logarithm .

Let us consider a fixed sequence $s_{k}=e^{k}$

Then $P(\frac{M_{e^{k+1}}}{\sqrt{e^{k}\ln(e^{k})}}\geq \epsilon) = P(\frac{M_{e^{k+1}}}{\sqrt{e^{k+1}}}>\epsilon \sqrt{\frac{\ln(e^{k})}{e}}) = P(|X|>\epsilon \sqrt{\frac{\ln(e^{k})}{e}}) $ where $X\sim N(0,1)$ .

and $P(|X|>\sqrt{\frac{\ln(e^{k})}{e}})=2P(X>\sqrt{\frac{\ln(e^{k})}{e}})\leq 2\exp(-\frac{k}{2e}) $ using the gross but useful Gaussian Tail Estimate .

And we also have that $\displaystyle\sum_{k}2\exp(-\frac{k}{2e})<\infty$

Thus by Borel-Cantelli Lemma , only finitely many events $\{\frac{M_{e^{k+1}}}{\sqrt{e^{k}\ln(e^{k})}}\geq \epsilon\}$ occur and hence almost surely $\frac{M_{e^{k+1}}}{\sqrt{e^{k}\ln(e^{k})}}\leq \epsilon$ for all large enough $k$ .

Now let $t_{k}$ be an arbitrary real sequence such that $t_{k}\to\infty$

Then we choose indices $k_{l}$ such that $t_{k}\leq e^{k_{l}+1}$ and $\sqrt{t_{k}\ln(t_{k})}\geq \sqrt{e^{k_{l}}\ln(e^{k_{l}})}$

Then $B_{t_{k}}\leq M_{e^{k_{l}}+1}$ and hence

$$\frac{B_{t_{k}}}{\sqrt{t_{k}\ln(t_{k})}}\leq \frac{M_{e^{k_{l}+1}}}{\sqrt{e^{k_{l}}\ln(e^{k_{l}})}}\leq \epsilon$$ for all large enough $l,k$ .

Thus $\displaystyle\limsup_{k\to\infty}\frac{B_{t_{k}}}{\sqrt{t_{k}\ln(t_{k})}}\leq \epsilon$ .

Now as $t_{k}$ was an arbitrary sequence, you have

$$\lim\sup_{t\to\infty}\frac{B_{t}}{\sqrt{t\ln(t)}}\leq \epsilon$$ and this holds for all $\epsilon$ . Using the fact that $-B_{t}$ is also a Brownian Motion , you also have that $\lim\inf \frac{B_{t}}{\sqrt{t\ln(t)}}\geq -\epsilon$ for all $\epsilon$ . Thus you have your desired conclusion and actually even more , namely , $\lim_{t\to \infty}\frac{B_{t}}{\sqrt{t\ln(t)}}=0$

If you are allowed to use LIL , then the proof is much shorter(actually just a one step proof) .

A point to note is that my choice of the sequence $e^k$ is nothing special and was just made such that the tail bound is nice. I can easily use any $a^k$ for $a>1$.

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Let \begin{gather*} A(t)=1+\sqrt{t}\cdot\ln(t\vee 1), \quad t\ge 0, \tag{1}\\ M(t)=\int_0^t\frac{\mathrm{d}B(s)}{A(s)}, \quad t\ge 0. \tag{2} \end{gather*} Then $A$ is positive, continuous and increasing on $\mathbb{R}_+$, $M$ is a continuous martingale on $\mathbb{R}_+$ with \begin{align*} \langle M \rangle_t &=\int_0^t\frac{\mathrm{d}s}{(1+\sqrt{s}\cdot\ln(s\vee 1))^2}\\ &\le \int_0^e\frac{\mathrm{d}s}{(1+\sqrt{s}\cdot\ln(s\vee 1))^2} + \int_e^t\frac{\mathrm{d}s}{s(\ln(s))^2} \\ &\le e + 1.\\ \lim_{t\to\infty} \langle M \rangle_t &< +\infty. \end{align*} Hence \begin{equation*} M(+\infty)=\lim_{t\to\infty}M(t) \quad\text{ exists and $M(+\infty)$ is finite a.s.} \tag{3} \end{equation*}

Furthermore, \begin{equation*} \lim_{t\to\infty}\frac{M(t)}{A(t)}=0. \qquad \text{a.s.} \end{equation*} Now, from (2) and integration by parts, \begin{align*} B(t)&=\int_{0}^{t}A(s)\,\mathrm{d}M(s) =A(t)M(t)-\int_{0}^{t}M(s)\,\mathrm{d}A(s),\\ \frac{B(t)}{A(t)}&=M(t)-\frac{1}{A(t)}\int_{0}^{t}M(s)\,\mathrm{d}A(s). \end{align*} Hence, using (3) get \begin{equation*} \lim_{t\to\infty}\Big[\frac{1}{A(t)}\int_{0}^{t}M(s)\,\mathrm{d}A(s)\Big] =\lim_{t\to\infty}M(t) \tag{4} \end{equation*} and \begin{equation*} \lim_{t\to\infty}\frac{B(t)}{A(t)}=0. \quad \text{a.s.} \end{equation*} Meanwhile, \begin{align*} \lim_{t\to\infty}\frac{\sqrt{t}\ln(t)}{A(t)}=1, \end{align*} Hence, \begin{equation*} \lim_{t\to\infty}\frac{B(t)}{\sqrt{t}\ln(t)}=0.\quad \text{a.s.} \end{equation*}

Add in proof( of (4)): Fixing \begin{align*} \omega_0 & \in \Omega_0 \\ &\stackrel{\triangle}{=}\{\omega: M(+\infty,\omega)=\lim_{t\to\infty}M(t,\omega) \text{ exists and $M(+\infty,\omega)$ is finite }\}, \end{align*} then for each $\varepsilon>0$, there exists $T$ such that \begin{equation*} |M(t,\omega_0)-M(+\infty,\omega_0)|<\varepsilon,\quad \text{as $t>T$}. \end{equation*} Hence \begin{align*} &\Big|\frac{1}{A(t)-A(0) } \int_{0}^{t}M(s,\omega_0)\,\mathrm{d}A(s)-M(+\infty,\omega_0) \Big|\\ &\quad \le \frac{1}{A(t)-A(0)}\int_{0}^{t}|M(s,\omega_0)-M(+\infty,\omega_0)|\,\mathrm{d}A(s)\\ &\quad \le \frac{1}{A(t)-A(0)}\int_{0}^{T}|M(s,\omega_0)-M(+\infty,\omega_0)|\,\mathrm{d}A(s)\\ &\qquad\quad +\frac{1}{A(t)-A(0)} \int_{T}^{t}|M(s,\omega_0)-M(+\infty,\omega_0)|\,\mathrm{d}A(s)\\ &\quad \le \frac{1}{A(t)-A(0)} \int_{0}^{T}|M(s,\omega_0)-M(+\infty,\omega_0)|\,\mathrm{d}A(s) + \varepsilon, \end{align*} and \begin{equation*} \varlimsup_{t\to\infty} \Big|\frac{1}{A(t)-A(0)} \int_{0}^{t}M(s,\omega_0)\, \mathrm{d}A(s)-M(+\infty,\omega_0) \Big|\le \varepsilon. \end{equation*} Now letting $\varepsilon\downarrow 0$ get \begin{gather*} \varlimsup_{t\to\infty} \Big|\frac{1}{A(t)-A(0)}\int_{0}^{t}M(s,\omega_0)\, \mathrm{d}A(s)-M(+\infty,\omega_0) \Big|=0. \end{gather*} This also means \begin{equation*} \lim_{t\to\infty} \frac{1}{A(t)}\int_{0}^{t}M(s,\omega_0)\,\mathrm{d}A(s) = M(+\infty,\omega_0), \end{equation*} and \begin{equation*} \Omega_0\subset \Big\{\omega:\lim_{t\to\infty} \frac{1}{A(t)}\int_{0}^{t}M(s,\omega)\,\mathrm{d}A(s) = M(+\infty,\omega) \Big\}. \end{equation*}

JGWang
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