252 research outputs found
The spatial Λ -coalescent
No full textThis paper extends the notion of the Λ-coalescent of Pitman (1999) to the spatial setting. The partition elements of the spatial Λ-coalescent migrate in a (finite) geographical space and may only coalesce if located at the same site of the space. We characterize the Λ-coalescents that come down from infinity, in an analogous way to Schweinsberg (2000). Surprisingly, all spatial coalescents that come down from infinity, also come down from infinity in a uniform way. This enables us to study space-time asymptotics of spatial Λ-coalescents on large tori in d≥3 dimensions. Some of our results generalize and strengthen the corresponding results in Greven et al. (2005) concerning the spatial Kingman coalescent
Eternal multiplicative coalescent is encoded by its L\'evy-type processes
No full text32 pages, 4 figures, this is the revision of v1, as promised in the abstract of the (interim) version v2The multiplicative coalescent is a Markov process taking values in ordered . It is a mean-field process in which any pair of blocks coalesces at rate proportional to the product of their masses. In Aldous and Limic (1998) each extreme eternal version of the multiplicative coalescent was described in three different ways. One of these specifications matches the (marginal) law of to that of the ordered excursion lengths above past minima of , where is a certain L\'evy-type process which (modulo shift and scaling) has infinitesimal drift at time . Using a modification of the breadth-first-walk construction from Aldous (1997) and Aldous and Limic (1998), and some new insight from the thesis by Uribe (2007), this work settles an open problem (3) from Aldous (1997), in the more general context of Aldous and Limic (1998). Informally speaking, is entirely encoded by , and contrary to Aldous' original intuition, the evolution of time for does correspond to the linear increase in the constant part of the drift of . In the "standard multiplicative coalescent" context of Aldous (1997), this result was first announced by Armend\'ariz in 2001, and obtained in a recent preprint by Broutin and Marckert, who simultaneously account for the process of excess edge counts (or marks). The novel argument presented here is based on a sequence of relatively elementary observations. Some of its components (for example, the new dynamic random graph construction via "simultaneous" breadth-first walks) are of independent interest, and may be useful for obtaining more sophisticated asymptotic results on near critical random graphs and related processes
On moments of multiplicative coalescents
No full textKonarovskyi V, Limic V. On moments of multiplicative coalescents. ANNALES DE L INSTITUT HENRI POINCARE-PROBABILITES ET STATISTIQUES. 2024;60(3):2025-2045.We prove existence of all moments of the multiplicative coalescent at all times. We obtain as byproducts a number of related results which could be of general interest. In particular, we show the finiteness of the second moment of the l 2 norm for any extremal eternal version of multiplicative coalescent. Our techniques are in part inspired by percolation, and in part are based on tools from stochastic analysis, notably the semi-martingale and the excursion theory
Stochastic block model in a new critical regime and the interacting multiplicative coalescent
No full textKonarovskyi V, Limic V. Stochastic block model in a new critical regime and the interacting multiplicative coalescent. Electronic Journal of Probability. 2021;26: 30.This work exhibits a novel phase transition for the classical stochastic block model (SBM). In addition we study the SBM in the corresponding near-critical regime, and find the scaling limit for the component sizes. The two-parameter stochastic process arising in the scaling limit, an analogue of the standard Aldous' multiplicative coalescent, is interesting in its own right. We name it the (standard) Interacting Multiplicative Coalescent. To the best of our knowledge, this object has not yet appeared in the literature
A dynamical approach to spanning and surplus edges of random graphs
No full text28 pages, 9 figures, improved and upgraded version of arXiv:1703.02574Consider a finite inhomogeneous random graph running in continuous time, where each vertex has a mass, and the edge that links any pair of vertices appears with a rate equal to the product of their masses. The simultaneous breadth-first-walk introduced by Limic (2019) is extended in order to account for the surplus edge data in addition to the spanning edge data. Two different graph-based representations of the multiplicative coalescent, with different advantages and drawbacks, are discussed in detail. A canonical multi-graph from Bhamidi, Budhiraja and Wang (2014) naturally emerges. The presented framework will facilitate the understanding of scaling limits with surplus edges for near-critical random graphs in the domain of attraction of general (not necessarily standard) eternal augmented multiplicative coalescent
A novel approach to the giant component fluctuations
No full text20 pages, 3 figures, comments are welcome!We present a novel approach to study the evolution of the size (i.e. the number of vertices) of the giant component of a random graph process. It is based on the exploration algorithm called simultaneous breadth-first walk, introduced by Limic in 2019, that encodes the dynamic of the evolution of the sizes of the connected components of a large class of random graph processes. We limit our study to the variant of the Erdös-Rényi graph process with vertices where an edge connecting a pair of vertices appears at an exponential rate 1 waiting time, independently over pairs. We first use the properties of the simultaneous breadth-first walk to obtain an alternative and self-contained proof of the functional central limit theorem recently established by Enriquez, Faraud and Lemaire in the super-critical regime ( and c > 1). Next, to show the versatility of our approach, we prove a functional central limit theorem in the barely super-critical regime ( where t > 0 and is a sequence of positive reals that converges to 0 such that tends to )
Urnes interagissantes
No full textNous nous intéressons au comportement asymptotique de plusieurs urnes de type Polya fortement renforcées et interagissantes. Le principal de notre étude porte sur les renforcements exponentiels ou assimilés ainsi que sur les interactions temporelles, c'est-à-dire lors desquelles les urnes n'interagissent qu'à certains instants aléatoires. Dans ce cas, nous mettons en évidence une transition de phase selon la fréquence des interactions. Si celle ci est supérieure à 1/2, les urnes se fixent toutes sur la même couleur tandis que si elle est inférieure à 1/2, une couleur majoritaire se dégage mais certaines urnes peuvent continuer à tirer une autre couleur aux instants où il n'y a pas interaction. Lorsque le renforcement devient infini, nous pouvons calculer la loi du nombre d'urnes se comportant de cette dernière façon quand le nombre total d'urnes est égal à deux ou est un nombre impair.Quand l'interaction est totale, c'est-à-dire quand toutes les urnes interagissent à tout instant, nous montrons alors qu'un renforcement fort et croissant, mais plus nécessairement exponentiel, suffit à obtenir la fixation de toutes les urnes sur la même couleur.Pour finir, nous discutons brièvement du modèle d'interaction spatiale dans lequel les urnes sont situées sur les sommets d'un graphe et n'interagissent qu'avec leurs voisines. Nous dégageons alors quelques propriétés préliminaires concernant les sous-graphes susceptibles de se fixer sur une couleur avec une probabilité positive.We study the asymptotic behavior of several Polya-type strongly reinforced interacting urns. The main results deal with exponential or exponential-like reinforcements and temporal interactions, that is when the urns interact only at some random times. In that case, we show the existence of a transition of phase depending on the frequency of interactions. If this frequency is larger than 1/2, all the urns eventually fixate on the same color, while if it is smaller than 1/2, a majority color will be fixed after some finite random time but while some of the urns eventually draw only the majority color, there can be other urns that still draw other colors at times where there is no interactions. When the reinforcement becomes infinite, we can calculate the law of the number of urns of later type when the total number of urns is two or an odd integer greater than two.When the interaction is maximal, that is when all the urns interact at any time, we show that a strong and non-decreasing reinforcement, but not necessarily exponential, suffices to obtain the fixation of all the urns on the same color.At the end, we consider briefly the spatial interaction model in which the urns are located on the vertices of a graph and interact only with their neighbors. In that case, we discuss some properties of sub-graphs that can fixate on one color with positive probability
A playful note on spanning and surplus edges
No full text13 pages, 7 figuresConsider a (not necessarily near-critical) random graph running in continuous time. A recent breadth-first-walk construction is extended in order to account for the surplus edge data in addition to the spanning edge data. Two different graph representations of the multiplicative coalescent, with different advantages and drawbacks, are discussed in detail. A canonical multi-graph of Bhamidi, Budhiraja and Wang (2014) naturally emerges. The presented framework should facilitate understanding of scaling limits with surplus edges for near-critical random graphs in the domain of attraction of general (not necessarily standard) eternal multiplicative coalescent
A surprising Poisson process arising from a species competition model
No full textMotivated by the work of Tilman (Ecology 75 (1994) 2) and May and Nowak (J. Theoret. Biol. 170 (1994) 95) we consider a process in which points are inserted randomly into the unit interval and a new point kills each point to its left independently and with probability a. Intuitively this dynamic will create a negative dependence between the number of points in adjacent intervals. However, we show that the ensemble of points converges to a Poisson process with intensity 1/(a(1-x)), and the number of points at time t grows like (log t)/a.Poisson process Species competition Stationary measure
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