arXiv subjects

Alexandre Legrand

Publications and source records attributed to Alexandre Legrand.

9 recordsLinked to original sources

arXiv2026

The localized phase of pinning models with correlated Gaussian disorder

We demonstrate that the results established for the localized regime of the pinning model with independent disorder -- notably the $\mathcal{C}^\infty$ regularity of the free energy, the scaling of the largest excursion between pinned sites, and the Central Limit Theorems for both the contact number and its mean -- can be generalized to translation-ergodic environments under the hypothesis that disorder is Gaussian. Our results are obtained assuming only summability of the charges' covariances. The two key ingredients for our proofs are the Birkhoff-sum approach introduced in Giacomin, Zamparo (2024) for independent disorder, but uniquely suited to handle correlated environments, and decorrelation tools such as the general and powerful Nelson's Gaussian hypercontractivity. We also rely on the Bernstein blocking method to prove a Central Limit Theorem under dependence. Additionally, we develop further techniques specifically tailored to the one-dimensional structure of the pinning model.

math.PR↗

arXiv2025

A random polymer approach to the weak disorder phase of the vertex reinforced jump process

In this paper, we study the transient phase of the Vertex Reinforced Jump Process (VRJP) in dimension $d\geq 3$. In Sabot, Zeng (2019), the authors introduce a positive martingale and show that the VRJP is recurrent if and only if that martingale converges to $0$. On $\mathbb{Z}^d$, $d\ge 3$, with constant conductances $W$, it can be shown that there is a critical value $0 W_c(\mathbb{Z}^d)$. On the other hand, the VRJP martingale can be interpreted as the partition function of a non-directed polymer with a very specific $1$-dependent random potential. In this paper, we focus on the question of the $L^p$ integrability of the VRJP martingale, which is related to the (diffusive) behavior of the VRJP. First, taking inspiration from the work of Junk (2022) for directed polymers in $\mathbb{Z}^{1+d}$, we prove that on the half-space $\mathbb{H}_d$ of $\mathbb{Z}^d$, for all $W>W_c(\mathbb{H}_d)$ there is some $\delta>0$ such that the VRJP martingale is in $L^{1+\delta}$. Second, we prove that, in dimension $d\geq 4$, the VRJP martingale is in $L^{p}$ for all $p>1$ above the ``slab critical point'' $W_c^{\mathrm{slab}} (\mathbb{Z}^d) = \lim_{m\to\infty} W_c(\mathbb{Z}^{d-1} \times \{-m,\ldots,m\})$. We also propose some related conjectures.

math.PR↗

arXiv2024

Some FKG inequalities for stochastic processes

This paper is interested in proving correlation inequalities of the FKG-type for various stochastic processes in continuous time. The pivotal tool which yields these correlation inequalities is an approximation with (possibly conditioned) Markov chains and random walks. In particular, we prove FKG inequalities for L\'evy processes, Bessel processes and several conditioned Brownian processes. As a side result, we also provide a necessary and sufficient condition for a random walk distribution in $\bbZ$ to satisfy the well-known ``FKG lattice condition''.

math.PR↗

arXiv2024

Recurrence and transience of the critical random walk snake in random conductances

In this paper we study the recurrence and transience of the $\mathbb{Z}^d$-valued branching random walk in random environment indexed by a critical Bienaym\'e-Galton-Watson tree, conditioned to survive. The environment is made either of random conductances or of random traps on each vertex. We show that when the offspring distribution is non degenerate with a finite third moment and the environment satisfies some suitable technical assumptions, then the process is recurrent up to dimension four, and transient otherwise. The proof is based on a truncated second moment method, which only requires to have good estimates on the quenched Green's function.

math.PR↗

arXiv2024

Time-inhomogeneous N-particle Branching Brownian Motion and the continuous random energy model

The $N$-particle branching Brownian motion ($N$-BBM) is a branching Markov process which describes the evolution of a population of particles undergoing reproduction and selection. It has attracted a lot of interest due to its relations to the study of front propagation phenomena on the one hand, and to (hierarchical) physical $p$-spin models on the other hand, among which the continuous random energy model (CREM). This paper investigates the asymptotic displacement of the $N$-BBM in a time-inhomogeneous setting, and when the time horizon $T$ and the number of particles $N$ jointly tend to infinity. We estimate the maximal displacement of the process up to the second order, and show that the latter undergoes a transition at the scale $\log N\approx T^{1/3}$. In particular when $\log N\ll T^{1/3}$ we recover the Brunet-Derrida behavior which was proven in a time-homogeneous setting and for $T\to+\infty$ then $N\to+\infty$. Furthermore, our results can also be interpreted from the perspective of algorithmic optimisation on some spin glass models, since the time-inhomogeneous $N$-BBM can be seen as the realization of an optimization procedure called beam search on the aforementioned CREM. The CREM has been proven by L. Addario-Berry and the second author to undergo an algorithm hardness threshold phenomenon, and the results of the present paper describe precisely the efficiency of the beam search algorithm around that threshold.

math.PR↗

arXiv2022

Scaling limit of the disordered generalized Poland--Scheraga model for DNA denaturation

The Poland--Scheraga model, introduced in the 1970's, is a reference model to describe the denaturation transition of DNA. More recently, it has been generalized in order to allow for asymmetry in the strands lengths and in the formation of loops: the mathematical representation is based on a bivariate renewal process, that describes the pairs of bases that bond together. In this paper, we consider a disordered version of the model, in which the two strands interact via a potential $\beta V(\hat\omega_i,\bar\omega_j)+h$ when the $i$-th monomer of the first strand and the $j$-th monomer of the second strand meet. Here, $h\in\mathbb R$ is a homogeneous pinning parameter, $(\hat\omega_i)_{i\geq 1}$ and $(\bar\omega_j)_{j\geq 1}$ are two sequences of i.i.d.~random variables attached to each DNA strand, $V(\cdot,\cdot)$ is an interaction function and $\beta>0$ is the disorder intensity. Our main result finds some condition on the underlying bivariate renewal so that, if one takes $\beta,h\downarrow0$ at some appropriate (explicit) rate as the length of the strands go to infinity, the partition function of the model admits a non-trivial, i.e. disordered, scaling limit. This is known as an \textit{intermediate disorder} regime and is linked to the question of disorder relevance for the denaturation transition. Interestingly and surprisingly, the rate at which one has to take $\beta\downarrow0$ depends on the interaction function $V(\cdot,\cdot)$ and on the distribution of $(\hat\omega_i)_{i\geq 1}$, $(\bar\omega_j)_{j\geq 1}$. On the other hand, the intermediate disorder limit of the partition function, when it exists, is universal: it is expressed as a chaos expansion of iterated integrals against a Gaussian process~$\mathcal{M}$, which arises as the scaling limit of the field $(e^{\beta V(\hat\omega_i,\bar\omega_j)})_{i,j\geq 0}$ and exhibits strong correlations on lines and columns.

math.PR↗

arXiv2021

A sharp asymptotics of the partition function for the collapsed interacting partially directed self-avoiding walk

In the present paper, we investigate the collapsed phase of the interacting partially-directed self-avoiding walk (IPDSAW) that was introduced in Zwanzig and Lauritzen (1968). We provide sharp asymptotics of the partition function inside the collapsed phase, proving rigorously a conjecture formulated in Guttmann (2015) and Owczarek et al. (1993). As a by-product of our result, we obtain that, inside the collapsed phase, a typical IPDSAW trajectory is made of a unique macroscopic bead, consisting of a concatenation of long vertical stretches of alternating signs, outside which only finitely many monomers are lying.

math.PR↗

arXiv2020

Surface transition in the collapsed phase of a self-interacting walk adsorbed along a hard wall

The present paper is dedicated to the 2-dimensional Interacting Partially Directed Self Avoiding Walk constrained to remain in the upper-half plan and interacting with the horizontal axis. The model has been introduced in \cite{F90} to investigate the behavior of a homopolymer dipped in a poor solvent and adsorbed along a horizontal hard wall. It is known to undergo a collapse transition between an extended phase, inside which typical configurations of the polymer have a large horizontal extension (comparable to their total size), and a collapsed phase inside which the polymer looks like a globule. In the present paper, we establish rigorously that inside the collapsed phase, a surface transition occurs between an adsorbed-collapsed regime where the bottommost layer of the globule is pinned at the hard wall, and a desorbed-collapsed regime where the globule wanders away from the wall. To prove the existence of this surface transition and exhibit its associated critical curve, we display some sharp asymptotics of the partition function for a slightly simplified version of the model.

math.PR↗

arXiv2019

Influence of disorder on DNA denaturation: the disordered generalized Poland-Scheraga model

The Poland-Scheraga model is a celebrated model for the denaturation transition of DNA, which has been widely used in the bio-physical literature to study, and investigated by mathematicians. In the original model, only opposite bases of the two strands can be paired together, but a generalized version of this model has recently been introduced, and allows for mismatches in the pairing of the two strands, and for different strand lengths. This generalized Poland-Scheraga (gPS) model has only been studied recently in the case of homogeneous interactions, then with disordered interactions perturbed by an i.i.d. field. The present paper considers a disordered version of the gPS model which is more appropriate to depict the inhomogeneous composition of the two strands (in particular interactions are perturbed in a strongly dependent manner): we study the question of the influence of disorder on the denaturation transition, and our main results provide criteria for disorder (ir)-relevance, both in terms of critical points and of order of the phase transition. Surprisingly, we find that criteria for disorder relevance depend on the law of the disorder field. We discuss this with regards to Harris' prediction for disordered systems.

math.PR↗