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Let $X$ be a continuous time stochastic process, and denote by $\mathcal F_t$ its natural filtration. We define $\mathcal F_z = \mathcal F_0$ for all $z \leq 0$.

$X$ is said to be strongly predictable if there exists some $r > 0$, and an $\mathcal F_{t-r}$ adapted process $Y$ such that

$$\lim_{T \to \infty} \mathbb E \left [\int_{T}^\infty |X_t - Y_t| \, dt \right ] = 0.$$

Question: Suppose $X$ is a strongly predictable martingale. Is it true that $X_t$ converges almost surely as $t \to \infty$?

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$\newcommand{\F}{\mathcal{F}}$The answer is yes.

Indeed, by time rescaling, without loss of generality (wlog) $r=1$.

Take any random variable (r.v.) $Z$ with $EZ=0$, and let $Z'$ be an independent copy of $Z$. Then, by Jensen's inequality, for any real $z$ we have $E|Z|\le E|Z-Z'|=E|(Z-z)-(Z'-z)|\le2E|Z-z|$, so that \begin{equation*} E|Z|\le2E|Z-z|. \tag{1} \end{equation*}

Letting $E_t:=E(\cdot|\F_t)$ and using (1) with $Z=X_t-X_{t-1}$ and real $t\ge1$, we get \begin{equation*} \begin{aligned} E_{t-1}|X_t-X_{t-1}|&\le2E_{t-1}|(X_t-X_{t-1})-(Y_t-X_{t-1})| \\ &=2E_{t-1}|X_t-Y_t|, \end{aligned} \end{equation*} since $Y_t-X_{t-1}$ is $\F_{t-1}$-measurable. So, $E|X_t-X_{t-1}|\le2|X_t-Y_t|$. So, for any real $u\ge0$ and any natural $T$, \begin{equation*} S_u:=\sum_{n=T}^\infty E|X_{n+u}-X_{n-1+u}|\le2\sum_{n=T}^\infty E|X_{n+u}-Y_{n+u}| \end{equation*} and hence \begin{equation*} \begin{aligned} \int_0^1 du\, S_u &\le2\sum_{n=T}^\infty \int_0^1 du\, E|X_{n+u}-Y_{n+u}| \\ & =2\int_T^\infty dt\, E|X_t-Y_t|<\infty \end{aligned} \end{equation*} if $T$ is large enough, by your displayed condition. So, $S_u<\infty$ for some $u\in[0,1]$. By time shift, wlog $u=1$. So, \begin{equation*} \sum_{n=T}^\infty E|X_{n+1}-X_n|=S_1<\infty. \tag{2} \end{equation*} So, $(X_n)$ converges in $L^1$ and hence $(E|X_n|)$ is bounded. So, by Doob's martingale convergence theorem, \begin{equation*} X_n\to Y \tag{3} \end{equation*} as $n\to\infty$ almost surely (a.s.) for some (real-valued) r.v. $Y$.

Next, by Doob's martingale inequality, \begin{equation*} P(\max_{t\in[n,n+1]}|X_t-X_n|>h)\le\frac{E|X_{n+1}-X_n|}h \end{equation*} for any real $h>0$. So, by the Borel–Cantelli lemma and (2), for each real $h>0$ a.s. there will be only finitely many natural $n$ such that $\max_{t\in[n,n+1]}|X_t-X_n|>h$. That is, $\max_{t\in[n,n+1]}|X_t-X_n|\to0$ a.s. as $n\to\infty$.

Thus, by (3), $X_t\to Y$ a.s. as $t\to\infty$, as claimed.

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  • $\begingroup$ Thank you for your answer! The use of Jensen’s inequality was very nice. $\endgroup$
    – Nate River
    Commented Jan 16, 2022 at 15:33

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