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arXiv 1011.4151math.PR
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关于Lévy过程的上确界律

On the law of the supremum of Lévy processes

Loïc Chaumont

摘要:

我们证明了Lévy过程$X$在确定时间$t$之前的总体上确界$\bar{X}_t=\sup_{s\le t}X_s$的规律等价于平均占用测度$μ_t(dx)=\int_0^t\p(X_s\in dx)\,ds$,只要0对于两个开半线$(-\infty,0)$和$(0,\infty)$都是正则的.在这种情况下,$\p(\bar{X}_t\in dx)$对某些(因此对所有)$t>0$是绝对连续的,当且仅当$X$的预解式测度是绝对连续的。我们还研究了半线$(-\infty,0)$或$(0,\infty)$之一的0不规则的情况。然后给出了$(\bar{X}_t,X_t)$,$(g_t,\bar{X}_t)$和$(g_t,\bar{X}_t,X_t)$的绝对连续性判据,其中$g_t$是上确界出现在$t$之前的时刻.这些结果的证明使用了联合律$\p(g_t\in ds,X_t\in dx,\bar{X}_t\in dy)$在上确界和下确界的反射过程的偏移测度的入口律方面的表达式。作为一种应用,这项法律在某些特定情况下(部分)明确规定。

英文摘要:

We show that the law of the overall supremum $\bar{X}_t=\sup_{s\le t}X_s$ of a Lévy process $X$ before the deterministic time $t$ is equivalent to the average occupation measure $μ_t(dx)=\int_0^t\p(X_s\in dx)\,ds$, whenever 0 is regular for both open halflines $(-\infty,0)$ and $(0,\infty)$. In this case, $\p(\bar{X}_t\in dx)$ is absolutely continuous for some (and hence for all) $t>0$, if and only if the resolvent measure of $X$ is absolutely continuous. We also study the cases where 0 is not regular for one of the halflines $(-\infty,0)$ or $(0,\infty)$. Then we give absolute continuity criterions for the laws of $(\bar{X}_t,X_t)$, $(g_t,\bar{X}_t)$ and $(g_t,\bar{X}_t,X_t)$, where $g_t$ is the time at which the supremum occurs before $t$. The proofs of these results use an expression of the joint law $\p(g_t\in ds,X_t\in dx,\bar{X}_t\in dy)$ in terms of the entrance law of the excursion measure of the reflected process at the supremum and that of the reflected process at the infimum. As an application, this law is made (partly) explicit in some particular instances.

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