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Sébastien Martineau

Publications and source records attributed to Sébastien Martineau.

13 recordsLinked to original sources
arXiv2026

Stochastic domination and lifts of random variables in percolation theory

Consider some matrix waiting for its coefficients to be written. For each column, sample independently a Bernoulli random variable of some parameter $p$. Seeing all this and possibly using extra randomness, Alice then chooses one spot in each column, in any way she wants. When the Bernoulli random variable of some column is equal to 1, the number 1 is written in the chosen spot. When the Bernoulli random variable of a column is 0, nothing is done on this column. We prove that, using extra randomness, it is possible for Bob to fill the empty entries with well chosen 0's and 1's so that the entries of the matrix are independent Bernoulli random variables of parameter $p$. We investigate various generalisations and variations of this problem, and use this result to revisit and generalise (nonstrict) monotonicity of the percolation threshold $p_c$ with respect to a form of graph-quotienting, namely fibrations. We also use this result to revisit the BK inequality. In a second part, which is independent of the first one, we revisit strict monotonicity of $p_c$ with respect to fibrations, a result that naturally requires more assumptions than its nonstrict counterpart. We reprove the bond-percolation case of the result of Martineau--Severo without resorting to essential enhancements, using couplings instead.

math.PR↗
arXiv2026

Percolation on graphs of polynomial growth is local: analyticity, supercritical sharpness, isoperimetry

We investigate locality of the supercritical regime for Bernoulli percolation on transitive graphs with polynomial growth, by which we mean the following. Take a transitive graph of polynomial growth $\mathscr{G}$ satisfying $p_c(\mathscr{G})<1$ and take $p>p_c(\mathscr{G})$. Let $\mathscr{H}$ be another such graph and assume that $\mathscr{G}$ and $\mathscr{H}$ have the same ball of radius $r$ for $r$ large. We prove that various quantities regarding percolation of parameter close to $p$ on $\mathscr{H}$ can be well understood from $(\mathscr{G},p)$ alone. This includes uniform versions of supercritical sharpness as well as the Kesten-Zhang bound on the probability of observing a large finite cluster: the constants involved can be chosen to depend only on $(\mathscr{G},p)$. We also prove that $θ_\mathscr{H}$ is an analytic function of $p$ in the whole supercritical regime and that, for a suitable $\varepsilon=\varepsilon(\mathscr{G},p)>0$, the analytic extension of $θ_\mathscr{H}$ to the $\varepsilon$-neighbourhood of $p$ in $\mathbb C$ is, uniformly, well approximated by the analytic extension of $θ_\mathscr{G}$. The proof relies on new results on the connectivity of minimal cutsets; in particular, we answer a question asked by Babson and Benjamini in 1999. We further discuss connections with the conjecture of non-percolation at criticality.

math.PR↗
arXiv2025

Percolative properties of the random coprime colouring

Given $u$ and $v$ in $\mathbb{Z}^d$, say that $u$ is visible from $v$ if the segment from $u$ to $v$ contains exactly two elements, which are $u$ and $v$. Take $X$ "uniformly at random in $\mathbb{Z}^d$" and colour each vertex $u$ of $\mathbb{Z}^d$ in white if $u$ is visible from $X$ and in black otherwise. Previous independent works of Pleasants-Huck and of the third author give a precise meaning to this definition. This paper is dedicated to the study of this random colouring from the point of view of percolation theory: given a reasonable graph structure on $\mathbb{Z}^d$, how many infinite black (resp. white) connected components are there?

math.PR↗
arXiv2023

Supercritical percolation on graphs of polynomial growth

We consider Bernoulli percolation on transitive graphs of polynomial growth. In the subcritical regime ($p p_c$) and prove the exponential decay of the truncated connection probabilities (probabilities that two points are connected by an open path, but not to infinity). This sharpness result was established by [CCN87] on $\mathbb Z^d$ and uses the difficult slab result of Grimmett and Marstrand. However, the techniques used there are very specific to the hypercubic lattices and do not extend to more general geometries. In this paper, we develop new robust techniques based on the recent progress in the theory of sharp thresholds and the sprinkling method of Benjamini and Tassion. On $\mathbb Z^d$, these methods can be used to produce a new proof of the slab result of Grimmett and Marstrand.

math.PR↗
arXiv2022

Locality of percolation for graphs with polynomial growth

Schramm's Locality Conjecture asserts that the value of the critical percolation parameter $p_c$ of a graph satisfying $p_c<1$ depends only on its local structure. In this note, we prove this conjecture in the particular case of transitive graphs with polynomial growth. Our proof relies on two recent works about such graphs, namely supercritical sharpness of percolation by the same authors and a finitary structure theorem by Tessera and Tointon.

math.PR↗
arXiv2021

Approximation on slabs and uniqueness for inhomogeneous percolation with a plane of defects

Let $ \mathbb{L}^{d} = ( \mathbb{Z}^{d},\mathbb{E}^{d} ) $ be the $ d $-dimensional hypercubic lattice. We consider a model of inhomogeneous Bernoulli percolation on $ \mathbb{L}^{d} $ in which every edge inside the $ s $-dimensional hyperplane $ \mathbb{Z}^{s} \times \{ 0 \}^{d-s} $, $ 2 \leq s < d $, is open with probability $ q $ and every other edge is open with probability $ p $. We prove the uniqueness of the infinite cluster in the supercritical regime whenever $ p \neq p_{c}(d) $, where $ p_{c}(d) $ denotes the threshold for homogeneous percolation, and that the critical point $ (p,q_{c}(p)) $ can be approximated on the phase space by the critical points of slabs, for any $ p < p_{c}(d) $.

math.PR↗
arXiv2019

On coprime percolation, the visibility graphon, and the local limit of the GCD profile

Colour an element of $\mathbb{Z}^d$ white if its coordinates are coprime and black otherwise. What does this colouring look like when seen from a "uniformly chosen" point of $\mathbb{Z}^d$? More generally, label every element of $\mathbb{Z}^d$ by its GCD: what do the labels look like around a "uniform" point of $\mathbb{Z}^d$? We answer these questions and generalisations of them, provide results of graphon convergence, as well as a "local/graphon" convergence. One can also investigate the percolative properties of the colouring under study.

math.PR↗
arXiv2018

Strict monotonicity of percolation thresholds under covering maps

We answer a question of Benjamini and Schramm by proving that under reasonable conditions, quotienting a graph strictly increases the value of its percolation critical parameter $p_c$. More precisely, let $\mathcal{G}=(V,E)$ be a quasi-transitive graph with $p_c(\mathcal{G})<1$, and let $G$ be a nontrivial group that acts freely on $V$ by graph automorphisms. Assume that $\mathcal{H}:=\mathcal{G}/G$ is quasi-transitive. Then one has $p_c(\mathcal{G})<p_c(\mathcal{H})$. We provide results beyond this setting: we treat the case of general covering maps and provide a similar result for the uniqueness parameter $p_u$, under an additional assumption of boundedness of the fibres. The proof makes use of a coupling built by lifting the exploration of the cluster, and an exploratory counterpart of Aizenman-Grimmett's essential enhancements.

math.PR↗
arXiv2017

Locally infinite graphs and symmetries

When one studies geometric properties of graphs, local finiteness is a common implicit assumption, and that of transitivity a frequent explicit one. By compactness arguments, local finiteness guarantees several regularity properties. It is generally easy to find counterexamples to such regularity results when the assumption of local finiteness is dropped. The present work focuses on the following problem: determining whether these regularity properties still hold when local finiteness is replaced by an assumption of transitivity. After recalling the locally finite situation, we show that there are Cayley graphs with infinite generating systems that have infinite diameter but do not contain any infinite geodesic ray. We also introduce a notion of generalised diameter. The generalised diameter of a graph is either an ordinal or "truly infinite" and captures the extension properties of geodesic paths. It is a finite ordinal if and only if the usual diameter is finite, and in that case the two notions agree. Besides, the generalised diameter is "truly infinite" if and only if the considered graph contains an infinite geodesic ray. We show that there exist Cayley graphs of abelian groups of arbitrary generalised diameter. Finally, we build Cayley graphs of abelian groups that have isomorphic balls of radius R for every R but are not globally isomorphic. This enables us to construct a non-transitive graph such that for every R and any vertices u and v, the ball of centre u and radius R is isomorphic to that of centre v and same radius.

math.CO↗
arXiv2016

On rotarily transitive graphs

From the point of view of discrete geometry, the class of locally finite transitive graphs is a wide and important one. The subclass of Cayley graphs is of particular interest, as testifies the development of geometric group theory. Recall that Cayley graphs can be defined as non-empty locally finite connected graphs endowed with a transitive group action such that any non-identity element acts without fixed point. We define a class of transitive graphs which are transitive in an "absolutely non-Cayley way": we consider graphs endowed with a transitive group action such that any element of the group acts with a fixed point. We call such graphs "rotarily transitive graphs", and we show that, even though there is no finite rotarily transitive graph with at least 2 vertices, there is an infinite locally finite connected rotarily transitive graph. The proof is based on groups built by Ivanov which are finitely generated, of finite exponent and have a small number of conjugacy classes. We also build infinite transitive graphs (which are not locally finite) any automorphism of which has a fixed point. This is done by considering "unit distance graphs" associated with the projective plane over suitable subfields of the real numbers.

math.CO↗
arXiv2015

Directed Diffusion-Limited Aggregation

In this paper, we define a directed version of the Diffusion-Limited-Aggregation model. We present several equivalent definitions in finite volume and a definition in infinite volume. We obtain bounds on the speed of propagation of information in infinite volume and explore the geometry of the infinite cluster. We also explain how these results fit in a strategy for proving a shape theorem for this model.

math.PR↗
arXiv2014

Ergodicity and indistinguishability in percolation theory

This paper explores the link between the ergodicity of the clus-ter equivalence relation restricted to its infinite locus and the indis-tinguishability of infinite clusters. It is an important element of the dictionary connecting orbit equivalence and percolation theory. This note starts with a short exposition of some standard material of these theories. Then, the classic correspondence between ergodicity and in-distinguishability is presented. Finally, we introduce a notion of strong indistinguishability that corresponds to strong ergodicity, and obtain that this strong indistinguishability holds in the Bernoulli case. We also define an invariant percolation that is not insertion-tolerant, sat-isfies the Indistinguishability Property and does not satisfy the Strong Indistinguishability Property.

math.PR↗