arXiv subjects

Niccolo Torri

Publications and source records attributed to Niccolo Torri.

11 recordsLinked to original sources

arXiv2025

Non-directed polymers in random environments with range penalties: the high dimensional case

We study a non-directed polymer model in random environments. The polymer is modeled by a simple symmetric random walk $S$ on $\mathbb{Z}^d$ with $d\geq2$, and the random environment is modeled by i.i.d. random variables whose tail probability decays polynomially. The interaction between the polymer and the random environment is captured by a Gibbs transform: at time $N$, the law of $S$ is tilted by the factor $\exp(\sum_{x\in\mathcal{R}_N}(\beta\omega_x-h))$, where $\mathcal{R}_N$ is the range of $S$ up to time $N$, $\beta\geq0$ is the inverse temperature, and $h\in\mathbb{R}$ is an external field. By appropriately tuning $\beta=\beta_N$ and $h=h_N$, we establish the phase diagram, analyze the fluctuations of $S$ under the Gibbs transform, and derive the scaling limits of the (logarithmic) partition function. This paper is a follow-up work of arxiv.org/abs/2101.05949. The main novelty and challenge arise from tuning the external field $h$, which brings in various range penalties, unlike in arxiv.org/abs/2101.05949, where $h$ is fixed and serves only as a centering term for the random environment.

math.PR↗

arXiv2023

Hydrodynamic limit of the Schelling model with spontaneous Glauber and Kawasaki dynamics

In the present article we consider the Schelling model, an agent-based model describing a segregation dynamics when we have a cohabitation of two social groups. As for several social models, the behaviour of the Schelling model was analyzed along several directions, notably by exploiting theoretical physics tools and computer simulations. This approach led to conjecture a phase diagram in which either different social groups were segregated in two large clusters or they were mixed. In this article, we describe and analyze a perturbation of the Schelling model as a particle systems model by adding a Glauber and Kawasaki dynamics to the original Schelling dynamics. As far as the authors know, this is the first rigorous mathematical analysis of the perturbed Schelling model. We prove the existence of an hydrodynamic limit described by a reaction-diffusion equation with a discontinuous non-linear reaction term. The existence and uniqueness of the solution is non trivial and the analysis of the limit PDE is interesting in its own. Based on our results, we conjecture, as in other variations of this model, the existence of a phase diagram in which we have a mixed, a segregated and a metastable segregation phase. We also describe how this phase transition can be viewed as a transition between a relevant and irrelevant disorder regime in the model.

math.PR↗

arXiv2022

Prudent walk in dimension six and higher

We study the high-dimensional uniform prudent self-avoiding walk, which assigns equal probability to all nearest-neighbor self-avoiding paths of a fixed length that respect the prudent condition, namely, the path cannot take any step in the direction of a previously visited site. We prove that the prudent self-avoiding walk converges to Brownian motion under diffusive scaling if the dimension is large enough. The same result is true for weakly prudent walk in dimension d>5. A challenging property of the high-dimensional prudent walk is the presence of an infinite-range self-avoidance constraint. Interestingly, as a consequence of such a strong self-avoidance constraint, the upper critical dimension of the prudent walk is five, and thus greater than for the classical self-avoiding walk.

math.PR↗

arXiv2022

Scaling limits of tree-valued branching random walks

We consider a branching random walk (BRW) taking its values in the $\mathtt{b}$-ary rooted tree $\mathbb W_{ \mathtt{b}}$ (i.e. the set of finite words written in the alphabet $\{ 1, \ldots, \mathtt{b} \}$, with $\mathtt{b}\! \geq \! 2$). The BRW is indexed by a critical Galton--Watson tree conditioned to have $n$ vertices; its offspring distribution is aperiodic and is in the domain of attraction of a $γ$-stable law, $γ\in (1, 2]$. The jumps of the BRW are those of a nearest-neighbour null-recurrent random walk on $\mathbb W_{ \mathtt{b}}$ (reflection at the root of $\mathbb W_{ \mathtt{b}}$ and otherwise: probability $1/2$ to move closer to the root of $\mathbb W_{ \mathtt{b}}$ and probability $1/(2\mathtt{b})$ to move away from it to one of the $\mathtt{b}$ sites above). We denote by $\mathcal R_{\mathtt{b}} (n)$ the range of the BRW in $\mathbb W_{ \mathtt{b}}$ which is the set of all sites in $\mathbb W_{\mathtt{b}}$ visited by the BRW. We first prove a law of large numbers for $\# \mathcal R_{\mathtt{b}} (n)$ and we also prove that if we equip $\mathcal R_{\mathtt{b}} (n)$ (which is a random subtree of $\mathbb W_{\mathtt{b}}$) with its graph-distance $d_{\mathtt{gr}}$, then there exists a scaling sequence $(a_n)_{n\in \mathbb N}$ satisfying $a_n \! \rightarrow \! \infty$ such that the metric space $(\mathcal R_{\mathtt{b}} (n), a_n^{-1}d_{\mathtt{gr}})$, equipped with its normalised empirical measure, converges to the reflected Brownian cactus with $γ$-stable branching mechanism: namely, a random compact real tree that is a variant of the Brownian cactus introduced by N. Curien, J-F. Le Gall and G. Miermont.

math.PR↗

arXiv2020

One-dimensional polymers in random environments: stretching vs. folding

In this article we study a \emph{non-directed polymer model} on $\mathbb Z$, that is a one-dimensional simple random walk placed in a random environment. More precisely, the law of the random walk is modified by the exponential of the sum of "rewards" (or penalities) $\beta \omega_x -h$ sitting on the range of the random walk, where $(\omega_x)_{x\in \mathbb Z}$ are i.i.d.\ random variables (the disorder), and where $\beta\geq 0$ (disorder strength) and $h\in \mathbb{R}$ (external field) are two parameters. When $\beta=0,h>0$, this corresponds to a random walk penalized by its range; when $\beta>0, h=0$, this corresponds to the "standard" polymer model in random environment, except that it is non-directed. In this work, we allow the parameters $\beta,h$ to vary according to the length of the random walk, and we study in detail the competition between the \emph{stretching effect} of the disorder, the \emph{folding effect} of the external field (if $h\ge 0$), and the \emph{entropy cost} of atypical trajectories. We prove a complete description of the (rich) phase diagram. For instance, in the case $\beta>0, h=0$ of the non-directed polymer, if $\omega_x$ ha a finite second moment, we find a transversal fluctuation exponent $\xi=2/3$, and we identify the limiting distribution of the rescaled log-partition function.

math.PR↗

arXiv2018

Directed polymers in heavy-tail random environment

We study the directed polymer model in dimension ${1+1}$ when the environment is heavy-tailed, with a decay exponent $α\in(0,2)$. We give all possible scaling limits of the model in the weak-coupling regime, i.e., when the inverse temperature temperature $β=β_n$ vanishes as the size of the system $n$ goes to infinity. When $α\in(1/2,2)$, we show that all possible transversal fluctuations $\sqrt{n} \leq h_n \leq n$ can be achieved by tuning properly $β_n$, allowing to interpolate between all super-diffusive scales. Moreover, we determine the scaling limit of the model, answering a conjecture by Dey and Zygouras [cf:DZ] - we actually identify five different regimes. On the other hand, when $α<1/2$, we show that there are only two regimes: the transversal fluctuations are either $\sqrt{n}$ or $n$. As a key ingredient, we use the Entropy-controlled Last Passage Percolation (E-LPP), introduced in a companion paper [cf:BT_ELPP].

math.PR↗

arXiv2018

Beyond Hammersley's Last-Passage Percolation: a discussion on possible local and global constraints

Hammersley's Last-Passage Percolation (LPP), also known as Ulam's problem, is a well-studied model that can be described as follows: consider $m$ points chosen uniformly and independently in $[0,1]^2$, then what is the maximal number $\mathcal{L}_m$ of points that can be collected by an up-right path? We introduce here a generalization of this standard LPP, in order to allow for more general constraints than the up-right condition (a $1$-Lipschitz condition after rotation by $45^{\circ}$). We focus more specifically on two cases: (i) when the constraint is a $γ$-Hölder (local) condition, we call it H-LPP; (ii) when the constraint is a path-entropy (global) condition, we call it E-LPP. These generalizations also allows us to deal with non-directed LPP. We develop motivations for directed and non-directed constrained LPP, and we give the correct order of $\mathcal{L}_m$ in a general manner.

math.PR↗

arXiv2018

Entropy-controlled Last-Passage Percolation

In the present article we consider a natural generalization of Hammersley's Last Passage Percolation (LPP) called Entropy-controlled Last Passage Percolation (E-LPP), where points can be collected by paths with a global (entropy) constraint which takes in account the whole structure of the path, instead of a local ($1$-Lipschitz) constraint as in Hammersley's LPP. The E-LPP turns out to be a key ingredient in the context of the directed polymer model when the environment is heavy-tailed, which we consider in the related paper [Berger and Torri, 2018]. We prove several estimates on the E-LPP in continuous and in discrete settings, which are of interest on their own. We give applications in the context of polymers in heavy-tail environment which are essentials tools in [Berger and Torri, 2018]: we show that the limiting variational problem conjectured by [Dey and Zygouras, 2016] (Conjecture 1.7) is finite, and we prove that the discrete variational problem converges to the continuous one, generalizing techniques used by [Auffinger-Louidor, 2011] and [Hambly and Martin, 2007].

math.PR↗

arXiv2016

Collapse transition of the interacting prudent walk

This article is dedicated to the study of the 2-dimensional interacting prudent self-avoiding walk (referred to by the acronym IPSAW) and in particular to its collapse transition. The interaction intensity is denoted by $β>0$ and the set of trajectories consists of those self-avoiding paths respecting the prudent condition, which means that they do not take a step towards a previously visited lattice site. The IPSAW interpolates between the interacting partially directed self-avoiding walk (IPDSAW) that was analyzed in details in, e.g., [Zwanzig and Lauritzen, 1968], [Brak et al., 1992], [Carmona, Nguyen and Pétrélis, 2013-2016], and the interacting self-avoiding walk (ISAW) for which the collapse transition was conjectured in [Saleur, 1986]. Three main theorems are proven. We show first that IPSAW undergoes a collapse transition at finite temperature and, up to our knowledge, there was so far no proof in the literature of the existence of a collapse transition for a non-directed model built with self-avoiding path. We also prove that the free energy of IPSAW is equal to that of a restricted version of IPSAW, i.e., the interacting two-sided prudent walk. Such free energy is computed by considering only those prudent path with a general north-east orientation. As a by-product of this result we obtain that the exponential growth rate of generic prudent paths equals that of two-sided prudent paths and this answers an open problem raised in e.g., [Bousquet-Mélou, 2010] or [Dethridge and Guttmann, 2008]. Finally we show that, for every $β>0$, the free energy of ISAW itself is always larger than $β$ and this rules out a possible self-touching saturation of ISAW in its conjectured collapsed phase.

math.PR↗

arXiv2015

Universality for the pinning model in the weak coupling regime

We consider disordered pinning models, when the return time distribution of the underlying renewal process has a polynomial tail with exponent $α\in (1/2,1)$. This corresponds to a regime where disorder is known to be relevant, i.e. to change the critical exponent of the localization transition and to induce a non-trivial shift of the critical point. We show that the free energy and critical curve have an explicit universal asymptotic behavior in the weak coupling regime, depending only on the tail of the return time distribution and not on finer details of the models. This is obtained comparing the partition functions with corresponding continuum quantities, through coarse-graining techniques.

math.PR↗

arXiv2014

Pinning Model with Heavy Tailed Disorder

We study the so-called pinning model, which describes the behavior of a Markov chain interacting with a distinguished state. The interaction depends on an external source of randomness, called disorder, which can attract or repel the Markov chain path. We focus on the case when the disorder is heavy-tailed, with infinite mean, while the return times of the Markov chain have a stretched-exponential distribution. We prove that the set of times at which the Markov chain visits the distinguished state, suitably rescaled, converges in distribution to a limit set, which depends only on the disorder and on the interplay of the parameters. We also show that there exists a random threshold below which the limit set is trivial. As a byproduct of our techniques, we improve and complete a result of Auffinger and Louidor on the directed polymer in a random environment with heavy tailed disorder.

math.PR↗