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arXiv 1106.2302math.PR
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Hurst指数小于1/2的分数阶布朗运动的广义二次协变差

The generalized quadratic covariation for fractional Brownian motion with Hurst index less than 1/2

Litan Yan, Chao Chen, Junfeng Liu

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22 pages
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摘要：

设$B^H$是具有Hurst指标$0<H<1/2$的分数布朗运动.本文研究了由$$ [f(B^H),B^H]^{(W)}_t=\lim_{ε\downarrow 0}\frac{2H}{ε^{2H}}\int_0^t\{f(B^{H}_{s+ε})-f(B^{H}_s)\}(B^{H}_{s+ε}- B^{H}_s)s^{2H-1}ds, $$定义的{ \it广义二次协变} $[f(B^H),B^H]^{(W)}$，其中极限是概率一致的，$x\mapsto f(x)$是确定性函数.本文构造了一个可测函数的Banach空间${\mathscr H}$，使得$L^2$中存在广义二次协变，并且在$f\in {\mathscr H}$的条件下，Bouleau-Yor恒等式为$$ [f(B^H),B^H]_t^{(W)}=-\int_{\mathbb {R}}f(x){\mathscr L}^{H}(dx,t) $$，其中${\mathscr L}^{H}(x,t)$是$B^H$的加权局部时.这使我们能够写出导数属于${\mathscr H}$的绝对连续函数的分数伊藤公式。这些也扩展到时间依赖的情况。

英文摘要：

Let $B^H$ be a fractional Brownian motion with Hurst index $0<H<1/2$. In this paper we study the {\it generalized quadratic covariation} $[f(B^H),B^H]^{(W)}$ defined by $$ [f(B^H),B^H]^{(W)}_t=\lim_{ε\downarrow 0}\frac{2H}{ε^{2H}}\int_0^t\{f(B^{H}_{s+ε})-f(B^{H}_s)\}(B^{H}_{s+ε}- B^{H}_s)s^{2H-1}ds, $$ where the limit is uniform in probability and $x\mapsto f(x)$ is a deterministic function. We construct a Banach space ${\mathscr H}$ of measurable functions such that the generalized quadratic covariation exists in $L^2$ and the Bouleau-Yor identity takes the form $$ [f(B^H),B^H]_t^{(W)}=-\int_{\mathbb {R}}f(x){\mathscr L}^{H}(dx,t) $$ provided $f\in {\mathscr H}$, where ${\mathscr L}^{H}(x,t)$ is the weighted local time of $B^H$. This allows us to write the fractional Itô formula for absolutely continuous functions with derivative belonging to ${\mathscr H}$. These are also extended to the time-dependent case.

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