arXiv subjects

Thomas Duquesne

Publications and source records attributed to Thomas Duquesne.

28 records · Page 2Linked to original sources

arXiv2008

The Packing Measure of the Range of Super-Brownian Motion

We prove that the total range of Super-Brownian motion with quadratic branching mechanism has an exact packing measure with respect to the gauge function $g(r)=r^4 (\log \log1/r)^{-3}$ in super-critical dimensions $d\geq 5$. More precisely, we prove that the total occupation measure of Super-Brownian motion is equal to the $g$-packing measure restricted to its range, up to a deterministic multiplicative constant that only depends on space dimension $d$.

math.PR↗

arXiv2008

Continuum random trees and branching processes with immigration

We study a genealogical model for continuous-state branching processes with immigration with a (sub)critical branching mechanism. This model allows the immigrants to be on the same line of descent. The corresponding family tree is an ordered rooted continuum random tree with a single infinite end defined thanks to two continuous processes denoted by $(\overleftarrow{H}_t ;t\geq 0)$ and $(\overrightarrow{H}_t ;t\geq 0)$ that code the parts at resp. the left and the right hand of the infinite line of descent of the tree. These processes are called the left and the right height processes. We define their local time processes via an approximation procedure and we prove that they enjoy a Ray-Knight property. We also discuss the important special case corresponding to the size-biased Galton-Watson tree in the continuous setting. In the last part of the paper we give a convergence result under general assumptions for rescaled discrete left and right contour processes of sequences of Galton-Watson trees with immigration. We also provide a strong invariance principle for a sequence of rescaled Galton-Watson processes with immigration that also holds in the supercritical case.

math.PR↗

arXiv2006

The coding of compact real trees by real valued functions

This paper is a detailled study of the coding of real trees by real valued functions that is motivated by probabilistic problems related to continuum random trees. Indeed it is known since the works of Aldous (1993) and Le Gall (1991) that a continuous non-negative function $h$ on $[0,1]$ such that $h(0)=0$ can be seen as the contour process of a compact real tree. This particular coding of a compact real tree provides additional structures, namely a root that is the vertex corresponding to $0\in [0,1]$, a linear order inherited from the usual order on $[0,1]$ and a measure induced by the Lebesgue measure on $[0,1]$; of course, the root, the linear order and the measure obtained by such a coding have to satisfy some compatibility conditions. In this paper, we prove that any compact real tree equipped with a root, a linear order and a measure that are compatible can be encoded by a non-negative function $h$ defined on a finite interval $[0, M]$, that is assumed to be left-continuous with right-limit, without positive jump and such that $h(0+)=h(0)=0$. Moreover, this function is unique if we assume that the exploration of the tree induced by such a coding backtracks as less as possible. We also prove that a measure-change on the tree corresponds to a re-parametrization of the coding function. In addition, we describe several path-properties of the coding function in terms of the metric properties of the real tree.

math.PR↗

arXiv2005

The Hausdorff measure of stable trees

We study fine properties of the so-called stable trees, which are the scaling limits of critical Galton-Watson trees conditioned to be large. In particular we derive the exact Hausdorff measure function for Aldous' continuum random tree and for its level sets. It follows that both the uniform measure on the tree and the local time measure on a level set coincide with certain Hausdorff measures. Slightly less precise results are obtained for the Hausdorff measure of general stable trees.

math.PR↗

arXiv2005

Random Trees, Levy Processes and Spatial Branching Processes

We investigate the genealogical structure of general critical or subcritical continuous-state branching processes. Analogously to the coding of a discrete tree by its contour function, this genealogical structure is coded by a real-valued stochastic process called the height process, which is itself constructed as a local time functional of a Levy process with no negative jumps. We present a detailed study of the height process and of an associated measure-valued process called the exploration process, which plays a key role in most applications. Under suitable assumptions, we prove that whenever a sequence of rescaled Galton-Watson processes converges in distribution, their genealogies also converge to the continuous branching structure coded by the appropriate height process. We apply this invariance principle to various asymptotics for Galton-Watson trees. We then use the duality properties of the exploration process to compute explicitly the distribution of the reduced tree associated with Poissonnian marks in the height process, and the finite-dimensional marginals of the so-called stable continuous tree. This last calculation generalizes to the stable case a result of Aldous for the Brownian continuum random tree. Finally, we combine the genealogical structure with an independent spatial motion to develop a new approach to superprocesses with a general branching mechanism. In this setting, we derive certain explicit distributions, such as the law of the spatial reduced tree in a domain, consisting of the collection of all historical paths that hit the boundary.

math.PR↗

arXiv2005

Growth of Levy trees

We construct random locally compact real trees called Levy trees that are the genealogical trees associated with continuous-state branching processes. More precisely, we define a growing family of discrete Galton-Watson trees with i.i.d. exponential branch lengths that is consistent under Bernoulli percolation on leaves; we define the Levy tree as the limit of this growing family with respect to the Gromov-Hausdorff topology on metric spaces. This elementary approach notably includes supercritical trees and does not make use of the height process introduced by Le Gall and Le Jan to code the genealogy of (sub)critical continuous-state branching processes. We construct the mass measure of Levy trees and we give a decomposition along the ancestral subtree of a Poisson sampling directed by the mass measure.

math.PR↗

arXiv2005

Path decompositions for real Levy processes

Let $X$ be a real Lévy process and let $\Xpos $ be the process conditioned to stay positive. We assume that $ 0 $ is regular for $(-\infty, 0)$ and $(0, +\infty) $ with respect to $X$. Using elementary excursion theory arguments, we provide a simple probabilistic description of the reversed paths of $X$ and $\Xpos $ at their first hitting time of $ (x, +\infty)$ and last passage time of $ (-\infty, x ] $, on a fixed time interval $[0, t]$, for a positive level $x$. From these reversion formulas, we derive an extension to general Lévy processes of Williams' decomposition theorems, Bismut's decomposition of the excursion above the infimum and also several relations involving the reversed excursion under the maximum.

math.PR↗

arXiv2005

A limit theorem for the contour process of conditioned Galton-Watson trees

In this work, we study asymptotics of the genealogy of Galton--Watson processes conditioned on the total progeny. We consider a fixed, aperiodic and critical offspring distribution such that the rescaled Galton--Watson processes converges to a continuous-state branching process (CSBP) with a stable branching mechanism of index $α\in (1, 2]$. We code the genealogy by two different processes: the contour process and the height process that Le Gall and Le Jan recently introduced \cite{LGLJ1, LGLJ1}. We show that the rescaled height process of the corresponding Galton--Watson family tree, with one ancestor and conditioned on the total progeny, converges in a functional sense, to a new process: the normalized excursion of the continuous height process associated with the $α$-stable CSBP. We deduce from this convergence an analogous limit theorem for the contour process. In the Brownian case $α=2$, the limiting process is the normalized Brownian excursion that codes the continuum random tree: the result is due to Aldous who used a different method.

math.PR↗

arXiv2005

Continuum tree limit for the range of random walks on regular trees

Let $b$ be an integer greater than 1 and let $W^{\ee}=(W^{\ee}_n; n\geq 0)$ be a random walk on the $b$-ary rooted tree $\U_b$, starting at the root, going up (resp. down) with probability $1/2+ε$ (resp. $1/2 -ε$), $ε\in (0, 1/2)$, and choosing direction $i\in \{1, ..., b\}$ when going up with probability $a_i$. Here $å=(a_1, ..., a_b)$ stands for some non-degenerated fixed set of weights. We consider the range $\{W^{\ee}_n ; n\geq 0 \}$ that is a subtree of $\U_b $. It corresponds to a unique random rooted ordered tree that we denote by $τ_ε$. We rescale the edges of $τ_ε$ by a factor $\ee $ and we let $\ee$ go to 0: we prove that correlations due to frequent backtracking of the random walk only give rise to a deterministic phenomenon taken into account by a positive factor $γ(å)$. More precisely, we prove that $τ_ε$ converges to a continuum random tree encoded by two independent Brownian motions with drift conditioned to stay positive and scaled in time by $γ(å)$. We actually state the result in the more general case of a random walk on a tree with an infinite number of branches at each node ($b=\infty$) and for a general set of weights $å=(a_n, n\geq 0)$.

math.PR↗

arXiv2005

Probabilistic and fractal aspects of Levy trees

We investigate the random continuous trees called Lévy trees, which are obtained as scaling limits of discrete Galton-Watson trees. We give a mathematically precise definition of these random trees as random variables taking values in the set of equivalence classes of compact rooted R-trees, which is equipped with the Gromov-Hausdorff distance. To construct Lévy trees, we make use of the coding by the height process which was studied in detail in previous work. We then investigate various probabilistic properties of Lévy trees. In particular we establish a branching property analogous to the well-known property for Galton-Watson trees: Conditionally given the tree below level a, the subtrees originating from that level are distributed as the atoms of a Poisson point measure whose intensity involves a local time measure supported on the vertices at distance a from the root. We study regularity properties of local times in the space variable, and prove that the support of local time is the full level set, except for certain exceptional values of a corresponding to local extinctions. We also compute several fractal dimensions of Lévy trees, including Hausdorff and packing dimensions, in terms of lower and upper indices for the branching mechanism function $ψ$ which characterizes the distribution of the tree. We finally discuss some applications to super-Brownian motion with a general branching mechanism.

math.PR↗