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Suppose $𝑏:\mathbb R^d \to \mathbb R^d, \sigma:\mathbb R^d \to \mathbb R^{d\times d}$ are measurable functions and satisfy \begin{equation*} 2\langle π‘₯βˆ’π‘¦,𝑏(π‘₯)βˆ’π‘(𝑦)\rangle +\|\sigma(π‘₯)βˆ’\sigma(𝑦)\|^2 \le 𝐾|π‘₯βˆ’π‘¦| \rho(|π‘₯βˆ’π‘¦|),\ \ \ \ {(1)}\end{equation*} for all $π‘₯,𝑦 \in \mathbb R^d$, where $\rho:[0, \infty)β†’[0,\infty)$ is some appropriate function. In addition, assume that the SDE $$𝑑𝑋(𝑑)=𝑏(𝑋(𝑑))𝑑𝑑+ \sigma(𝑋(𝑑))π‘‘π‘Š(𝑑)$$ is well-posed in the strong sense.

We now consider the coupled process $$\begin{aligned} & 𝑑𝑋(𝑑)=𝑏(𝑋(𝑑))𝑑𝑑+\sigma(𝑋(𝑑))π‘‘π‘Š(𝑑), \\ & π‘‘π‘Œ(𝑑)=𝑏(π‘Œ(𝑑))𝑑𝑑+\sigma(π‘Œ(𝑑))π‘‘π‘Š(𝑑),\end{aligned}$$ where $π‘Š$ is a $𝑑$-dimensional standard Brownian motion. Is it true that for all $t\geq 0$ the following estimate is true? \begin{equation*} 𝑑|𝑋(𝑑)βˆ’π‘Œ(𝑑)|≀\rho(|𝑋(𝑑)βˆ’π‘Œ(𝑑)|)𝑑𝑑+\left\langle \frac{𝑋(𝑑)βˆ’π‘Œ(𝑑)}{|𝑋(𝑑)βˆ’π‘Œ(𝑑)|},(\sigma(𝑋(𝑑))βˆ’\sigma(π‘Œ(𝑑)))π‘‘π‘Š(𝑑)\right\rangle.\ {(2)}\end{equation*}

Using Ito's formula, we have $$ 𝑑|𝑋(𝑑)βˆ’π‘Œ(𝑑)|^2=(2\langle 𝑋(𝑑)βˆ’π‘Œ(𝑑),𝑏(𝑋(𝑑))βˆ’π‘(π‘Œ(𝑑))\rangle + \|\sigma(𝑋(𝑑))βˆ’\sigma(π‘Œ(𝑑))\|^2)𝑑𝑑+2\langle 𝑋(𝑑)βˆ’π‘Œ(𝑑),(\sigma(𝑋(𝑑))βˆ’\sigma(π‘Œ(𝑑)))π‘‘π‘Š(𝑑)\rangle.$$ Using (1), and applying Ito's formula to the process $|𝑋(𝑑)βˆ’π‘Œ(𝑑)|^2$ using the function $𝑓(π‘Ÿ)=π‘Ÿ^{1/2}, π‘Ÿ\ge 0,$ we can formally derive (2). But there is a problem, as $𝑓$ is not differentiable at 0. But I don't know how to rigorously prove or disprove (2).

Thanks a lot for your help. (I posted this question in Stack Exchange, but no answer yet).

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  • $\begingroup$ Does it help to mollify, with a sequence of smooth functions $F_n(x,y) \to |x-y|$ in some appropriate fashion? Apply Ito's formula for each $F_n$ and try to pass to the limit. You'll probably have to bound the quadratic variation of $\left|\nabla F_n(X_t,Y_t) - \frac{X_t - Y_t}{|X_t - Y_t|} \right|$ which seems like it should be possible if $F_n$ is chosen well. $\endgroup$ Commented Jan 16 at 1:43
  • $\begingroup$ Thanks a lot. I think I can use the classical Yamada & Watanabe technique (e.g., section 5.2 of Karatzas and Shreve) to derive (2). But is it possible to prove it using Ito-Tanaka formula? $\endgroup$
    – epsilon
    Commented Jan 16 at 5:20

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