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Itai Benjamini

Publications and source records attributed to Itai Benjamini.

At least 19 recordsLinked to original sources
arXiv2026

The Uniform Gromov Hausdorff Gap Problem for Approximating Spheres by Finite Homogeneous Spaces

Let $S^n$ be the unit round sphere with its intrinsic angular metric, normalized so that $\operatorname{diam}S^n=\pi$. For finite homogeneous metric spaces $X$, put \[ \delta_n=\inf_X d_{GH}(X,S^n). \] The main open problem is whether $\inf_{n\ge2}\delta_n>0$. Gelander's theorem gives $\delta_n>0$ in each fixed dimension, but not uniformly. An abstract cross-polytope construction gives the universal upper bound $\delta_n\le\pi/4$. In the opposite direction, ChatGPT combines the passage from small Gromov--Hausdorff error to an approximate finite action on the sphere, logarithmic stability of approximate inner-product-preserving maps due to Cuesta, operator-norm stability of almost representations, and Green's width theorem for finite transitive sets. This gives the quantitative bound \[ \delta_n\ge \frac{c}{(1+\log(n+1))^2} \] for all sufficiently large $n$.

math.MG↗
arXiv2026

The Conclave Process

We introduce a stochastic model for the papal conclave in which $n$ cardinals vote repeatedly among themselves until one cardinal receives all the votes. In each round, the probability that a cardinal votes for a given candidate is proportional to the $\alpha$-th power of that candidate's vote count in the preceding round. For $\alpha=1$, the model reduces to the Wright-Fisher model and is dual to Kingman's n-coalescent. We reveal a sharp transition in the absorption time $\mathcal{T}$ at $\alpha=1$. It was known that when $\alpha=1$, $\mathcal{T}$ is typically of order $n$. We prove that for $\alpha>1$, it drops to order $\textit{loglog n.}$ In contrast, for $\alpha<1$, $\mathcal{T}$ is typically at least $\exp(\Omega(n))$. We also prove a sharp phase transition in the identity of the winner when $\alpha>1$. For every positive integer $k$, if $2^{1/k}<\alpha<2^{1/(k-1)}$ (where we write $2^{1/0} = +\infty$), with probability tending to 1 as $n\to\infty$, the eventual winner is the unique leader after round $k$. These results show that reinforced voting processes reach consensus remarkably quickly even for large electorates.

math.PR↗
arXiv2026

Euclidean vs Graph Metric: The Fixed-Source Problem

We prove that two fixed sources in the Euclidean plane can be realized by a bounded-degree planar unit-edge graph on a 10-net, with graph distance from each source agreeing with Euclidean distance up to a universal additive constant. We ask whether the analogous statement holds for three non-collinear sources, and prove a logarithmic obstruction for large ordered source sets in the coordinate-planar setting.

math.MG↗
arXiv2026

The Pile Process on a Cycle

Consider a finite particle system in which, at each step, one particle from a vertex with a lower neighboring vertex moves to a neighboring vertex of minimum height. For the cycle C_n, started with n particles at one vertex and no particles elsewhere, we prove that the expected stabilization time is at most a constant multiple of n^3 The deterministic transportation lower bound is of order n^2. Simulations suggest that the true order should be near n^3. Chat GPT was used to reincarnate notes from 2007.

math.PR↗
arXiv2026

Non-colliding billiards in the plane

We present an open problem about non-colliding freely moving hard disks in the Euclidean plane, together with related positive and negative partial results. The open problem is stated in a non-degenerate form: velocities are required to be pairwise distinct and their speeds are required to be uniformly bounded away from infinity. The positive deterministic result gives a bounded, injective, non-colliding velocity assignment for the integer lattice; after a common velocity shift, the speeds are also bounded away from zero. The negative result shows that no bounded continuous vector field on the whole plane can serve as a universal assignment satisfying the same separation inequality for all pairs of points at distance greater than one. We also record a space-time interpretation of the problem, relate it to packings by nonparallel cylinders in three dimensions, and formulate a corresponding topological-dynamical question for cylinder packings.

math.DS↗
arXiv2026

How fast does the range of simple random walk grow?

Consider a discrete-time simple random walk $(X_t)_{t\ge 0}$ on an infinite, connected, locally finite simple graph $G$, and let \[ R_t := |\{X_0,\ldots,X_t\}| \] denote its range. The main result of this revised note is that positive vertex isoperimetry already forces linear expected range, with no bounded-degree assumption: if \[ \iota_V(G) := \inf_{0<|S|<\infty} \frac{|\partial_V S|}{|S|} >0, \] then $\E_x R_t \ge c(G)(t+1)$ for every starting vertex $x$ and every $t\ge 0$. The proof is direct: vertex expansion implies an unweighted Dirichlet inequality, which in turn gives a uniform positive escape probability from every vertex. We also record a finite counterpart: in an $n$-vertex finite vertex expander, the expected hitting time of an independent stationary random target is $\Theta(n)$, again with no restriction on degrees. We also record a chain of geometrically growing lollipops for which \[ \E_o R_t \asymp t^{1/3}, \] so the subdiffusive exponent $1/3$ need not be accompanied by superdiffusive oscillations. In particular, for this graph the lower and upper logarithmic exponents of $\E_oR_t$ are both equal to $1/3$. Finally, since Barnes and Feige proved the sharp universal estimate $\E T_n= O(n^3)$ for the $n$-th discovery time, we move our elementary proof of the weaker bound $\E T_n=O(n^3\log n)$ to a later section as a short self-contained argument with a logarithmic loss. We close with a related bounded-degree mixing statement: if the lazy walk has worst-case mixing time $m$, then at least $c\sqrt m$ starting vertices are still noticeably unmixed at time $\lfloor m/2\rfloor$. This final result uses the same commute-time/effective-resistance control of connected sets that appears throughout the paper.

math.PR↗
arXiv2025

Phase transition for recurrence of stationary random walks on lamplighter groups

We introduce and study a class of random walks on lamplighter groups $H\wr G$, where $H$ is a nontrivial finitely generated group and $G$ is an infinite finitely generated group, called \textbf{stationary random walks}. At each step, the walk switches the lamp at its current position, moves in the base group with a drift towards the identity, and switches the lamp again at the new position. We show that when $G$ is virtually-$\mathbb{Z}$ and $H$ is finite, these walks exhibit a phase transition between recurrence and transience, while when~$G$ is not virtually-$\mathbb{Z}$ or $H$ is infinite, they are always transient. In the case $G=\mathbb{Z}$, we determine the exact critical parameter and provide a quantitative description of this phase transition.

math.PR↗
arXiv2025

Sums along the edges of bounded degree graphs

Let $G$ be a graph on $n$ vertices and $(H,+)$ be an abelian group. What is the minimum size ${\sf S}_H(G)$ of the set of all sums $A(u)+A(v)$ over all injections $A:V(G)\to H$? In 2012, the first author, Angel, the second author, and Lubetzky proved that, for expander graphs and $H=\mathbb{Z}$, this minimum is at least $Ω(\log n)$, and this bound is tight -- there exists a regular expander $G$ with ${\sf S}_{\mathbb{Z}}(G)=O(\log n)$. We prove that, for every constant $d\geq 3$, the random $d$-regular graph $\mathcal{G}_{n,d}$ has significantly larger sum-sets: with high probability, for every abelian group $H$, ${\sf S}_H(\mathcal{G}_{n,d})=Ω(n^{1-2/d})$. In particular, this proves that, for every $\varepsilon>0$, there exists a regular graph with $O(n)$ edges and with sum-sets of size at least $n^{1-\varepsilon}$, for all abelian groups. The bound ${\sf S}_H(\mathcal{G}_{n,d})=Ω(n^{1-2/d})$ is tight up to a polylogarithmic factor: We show that, for every $3\leq d\leq \ln n/ \ln \ln n$, there exists an abelian group $H$ such that, for every graph $G$ on $n$ vertices with maximum degree at most $d$, ${\sf S}_H(G) \leq n^{1-2/d}(\log n)^{O(1)}$. We also prove that, for $d\gg\ln^2 n$, with high probability, for every abelian group $H$, ${\sf S}_H(\mathcal{G}_{n,d})=n(1-o(1))$ and determine the second-order term, up to a polylogarithmic factor.

math.CO↗
arXiv2025

A very sharp threshold for first order logic distinguishability of random graphs

In this paper we find an integer $h=h(n)$ such that the minimum number of variables of a first order sentence that distinguishes between two independent uniformly distributed random graphs of size $n$ with the asymptotically largest possible probability $\frac{1}{4}-o(1)$ belongs to $\{h,h+1,h+2,h+3\}$. We also prove that the minimum (random) $k$ such that two independent random graphs are distinguishable by a first order sentence with $k$ variables belongs to $\{h,h+1,h+2\}$ with probability $1-o(1)$.

math.CO↗
arXiv2025

Majority dynamics on finite trees

For an arbitrary finite tree $T$, we find the exact value of the wort-case stabilisation time of majority dynamics on $T$. We also prove that for a perfect rooted cubic tree $T$ with diameter $D$ and uniformly random initial opinions, the dynamics stabilises in time $τ\in(D/4,D/3)$ with high probability.

math.CO↗
arXiv2025

On an epidemic model on finite graphs

We study a system of random walks, known as the frog model, starting from a profile of independent Poisson($λ$) particles per site, with one additional active particle planted at some vertex $\mathbf{o}$ of a finite connected simple graph $G=(V,E)$. Initially, only the particles occupying $\mathbf{o}$ are active. Active particles perform $t \in \mathbb{N} \cup \{\infty \}$ steps of the walk they picked before vanishing and activate all inactive particles they hit. This system is often taken as a model for the spread of an epidemic over a population. Let $\mathcal{R}_t$ be the set of vertices which are visited by the process, when active particles vanish after $t$ steps. We study the susceptibility of the process on the underlying graph, defined as the random quantity $\mathcal{S}(G):=\inf \{t:\mathcal{R}_t=V \}$ (essentially, the shortest particles' lifetime required for the entire population to get infected). We consider the cases that the underlying graph is either a regular expander or a $d$-dimensional torus of side length $n$ (for all $d \ge 1$) $\mathbb{T}_d(n)$ and determine the asymptotic behavior of $\mathcal{S} $ up to a constant factor. In fact, throughout we allow the particle density $λ$ to depend on $n$ and for $d \ge 2$ we determine the asymptotic behavior of $\mathcal{S}(\mathbb{T}_d(n))$ up to smaller order terms for a wide range of $λ=λ_n$.

math.PR↗
arXiv2025

Hypercube minor-universality

A graph $G$ is $m$-minor-universal if every graph with at most $m$ edges (and no isolated vertices) is a minor of $G$. We prove that the $d$-dimensional hypercube, $Q_d$, is $Ω\left(\frac{2^d}{d}\right)$-minor-universal, and that there exists an absolute constant $C >0$ such that $Q_d$ is not $\frac{C2^d}{\sqrt{d}}$-minor-universal. Similar results are obtained in a more generalized setting, where we bound the size of minors in a product of finite connected graphs. A key component of our proof is the following claim regarding the decomposition of a permutation of a box into simpler, one-dimensional permutations: Let $n_1, \dots, n_d$ be positive integers, and define $X := [n_1] \times \dots \times [n_d]$. We prove that every permutation $σ: X \to X$ can be expressed as $σ= σ_1 \circ \dots \circ σ_{2d-1}$, where each $σ_i$ is a one-dimensional permutation, meaning it fixes all coordinates except possibly one. We discuss future directions and pose open problems.

math.CO↗
arXiv2024

Coupled but distant

We construct a coupling of two random walks in 4 dimensions so that their traces do not intersect with positive probability.

math.PR↗
arXiv2023

On the Number of Shortest Paths in Graphs

It is proved that the number of shortest paths between two vertices of distance $t$ in a graph with degrees bounded by $Δ$ is at most $2 \cdot (\fracΔ{2})^t$. This improves upon the naïve $Δ(Δ-1) ^{t-1}$ bound.

math.CO↗
arXiv2023

Randomly twisted hypercubes -- between structure and randomness

Twisted hypercubes are generalizations of the Boolean hypercube, obtained by iteratively connecting two instances of a graph by a uniformly random perfect matching. Dudek et al. showed that when the two instances are independent, these graphs have optimal diameter. We study twisted hypercubes in the setting where the instances can have general dependence, and also in the particular case where they are identical. We show that the resultant graph shares properties with random regular graphs, including small diameter, large vertex expansion, a semicircle law for its eigenvalues and no non-trivial automorphisms. However, in contrast to random regular graphs, twisted hypercubes allow for short routing schemes.

math.CO↗
arXiv2023

Global information from local observations of the noisy voter model on a graph

We observe the outcome of the discrete time noisy voter model at a single vertex of a graph. We show that certain pairs of graphs can be distinguished by the frequency of repetitions in the sequence of observations. We prove that this statistic is asymptotically normal and that it distinguishes between (asymptotically) almost all pairs of finite graphs. We conjecture that the noisy voter model distinguishes between any two graphs other than stars.

math.CO↗
arXiv2022

Determining a Points Configuration from a Subset of the Pairwise Distances

We study rigidity without assuming general position. Given $n$ distinct labelled points and a set $\mathcal{P}\subseteq \binom{[n]}{2}$ of revealed pairs, we ask when the corresponding distances determine the configuration up to isometry. On the line, we prove an extremal result: if $|\mathcal{P}|=\Omega(n^{3/2})$, then there is an induced globally rigid subgraph on $\Omega(|\mathcal{P}|/n)$ vertices. In other words, any dense enough graph will contain a subset of labels whose locations can be determined from their distances up to isometry. To prove this, we establish a graph-theoretic result, which may be of independent interest: a dense graph in which every non-edge has few common neighbours contains a clique of size $\Omega(|E|/n)$. We also study random revealed pairs. For every labelled configuration $V$ of distinct points in $\mathbb{R}$, if each pair is revealed independently with probability $p=C\ln n/n$, where $C>1$, then the revealed distances determine $V$ w.h.p. We prove a similar result for $d\ge1$ under the mild non-degeneracy assumption that every subcollection of more than $\tau n$ points of $V\subseteq\mathbb R^d$ affinely spans $\mathbb R^d$, for some fixed $0<\tau<1$. In this case, every $C>1/(1-\tau)$ suffices. The same ideas also settle the weak-threshold form of a conjecture of Gir\~ao et al. for a giant reconstructable component, and substantially improve in this direction the work of Barnes et al. establishing such a component for $p>n^{-2/(d+4)}$.

math.MG↗