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arXiv 0904.4175math.PR
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一个连续树值马尔可夫过程

A continuum-tree-valued Markov process

Romain Abraham, Jean-François Delmas

摘要:

我们提出了一个建设的Lévy连续随机树(CRT)与超临界连续状态分支过程使用所谓的探索过程和Girsanov定理。我们还扩展了修剪过程,这种超临界的情况下。设$ψ$为临界分支机制。我们设置了$ψ_θ(\cdot)=ψ(\cdot+θ)-ψ(θ)$。令$Θ=(θ_\infty,+\infty)$或$Θ=[θ_\infty,+\infty)$为$θ$的值集,其中$ψ_θ$为分支机制。剪枝过程允许构造一个递减的Lévy-CRT值马尔可夫过程$(\ct_θ,θ\inΘ)$,使得$\mathcal{T}_θ$具有分支机制$ψ_θ$ .如果是$θ>0$,则为亚临界;如果是$θ<0$,则为超临界。然后,我们考虑CRT的爆炸时间$A$:较小的(负)时间$θ$,其中$\mathcal{T}_θ$具有有限的质量。我们描述了$A$的规律以及在这个爆炸时刻之后CRT的分布。爆发后的CRT可以看作是一个条件不熄灭的CRT,它被修剪成与$A$相关的独立强度。我们还研究了CRT值过程在爆炸时间后的演化。这扩展了Aldous和Pitman关于Galton-Watson树的结果。对于特殊情况下的二次分支机制,我们表明,爆炸后的CRT的总质量表现得像一个稳定的从属指数为1/2的逆。这一结果与Aldous CRT碎片的标记碎片的大小有关。

英文摘要:

We present a construction of a Lévy continuum random tree (CRT) associated with a super-critical continuous state branching process using the so-called exploration process and a Girsanov's theorem. We also extend the pruning procedure to this super-critical case. Let $ψ$ be a critical branching mechanism. We set $ψ_θ(\cdot)=ψ(\cdot+θ)-ψ(θ)$. Let $Θ=(θ_\infty,+\infty)$ or $Θ=[θ_\infty,+\infty)$ be the set of values of $θ$ for which $ψ_θ$ is a branching mechanism. The pruning procedure allows to construct a decreasing Lévy-CRT-valued Markov process $(\ct_θ,θ\inΘ)$, such that $\mathcal{T}_θ$ has branching mechanism $ψ_θ$. It is sub-critical if $θ>0$ and super-critical if $θ<0$. We then consider the explosion time $A$ of the CRT: the smaller (negative) time $θ$ for which $\mathcal{T}_θ$ has finite mass. We describe the law of $A$ as well as the distribution of the CRT just after this explosion time. The CRT just after explosion can be seen as a CRT conditioned not to be extinct which is pruned with an independent intensity related to $A$. We also study the evolution of the CRT-valued process after the explosion time. This extends results from Aldous and Pitman on Galton-Watson trees. For the particular case of the quadratic branching mechanism, we show that after explosion the total mass of the CRT behaves like the inverse of a stable subordinator with index 1/2. This result is related to the size of the tagged fragment for the fragmentation of Aldous' CRT.

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