Searcharxiv⌕ Search

arXiv subjects

Uri Shapira

Publications and source records attributed to Uri Shapira.

At least 19 recordsLinked to original sources
arXiv2025

Geometric and arithmetic aspects of approximation vectors

Let $θ\in\mathbb{R}^d$. We associate three objects to each approximation $(p,q)\in \mathbb{Z}^d\times \mathbb{N}$ of $θ$: the projection of the lattice $\mathbb{Z}^{d+1}$ to the hyperplane of the first $d$ coordinates along the approximating vector $(p,q)$; the displacement vector $(p - qθ)$; and the residue classes of the components of the $(d + 1)$-tuple $(p, q)$ modulo all primes. All of these have been studied in connection with Diophantine approximation problems. We consider the asymptotic distribution of all of these quantities, properly rescaled, as $(p, q)$ ranges over the best approximants and $ε$-approximants of $θ$, and describe limiting measures on the relevant spaces, which hold for Lebesgue a.e. $θ$. We also consider a similar problem for vectors $θ$ whose components, together with 1, span a totally real number field of degree $d+1$. Our technique involve recasting the problem as an equidistribution problem for a cross-section of a one-parameter flow on an adelic space, which is a fibration over the space of $(d + 1)$-dimensional lattices. Our results generalize results of many previous authors, to higher dimensions and to joint equidistribution.

math.NT↗
arXiv2025

Escape of mass of the Thue-Morse sequence

Every Laurent series in $\mathbb{F}_q\left(\left(t^{-1}\right)\right)$ has a continued fraction expansion whose partial quotients are polynomials. De Mathan and Teulié proved that the degrees of the partial quotients of the left-shifts of every quadratic Laurent series are unbounded. Shapira and Paulin and Kemarsky improved this by showing that certain sequences of probability measures on the space of lattices in the plane $\mathbb{F}_q\left(\left(t^{-1}\right)\right)^2$ exhibit positive escape of mass and conjectured that this escape is full -- that is, that these probability measures converge to zero. We disprove this conjecture by analysing in detail the case of the Laurent series over $\mathbb{F}_2$ whose sequence of coefficients is the Thue-Morse sequence. The proof relies on the discovery of explicit symmetries in its number wall.

math.NT↗
arXiv2024

On the rate of convergence of continued fraction statistics of random rationals

We show that the statistics of the continued fraction expansion of a randomly chosen rational in the unit interval, with a fixed large denominator $q$, approaches the Gauss-Kuzmin statistics with polynomial rate in $q$. This improves on previous results giving the convergence without rate. As an application of this effective rate of convergence, we show that the statistics of a randomly chosen rational in the unit interval, with a fixed large denominator $q$ and prime numerator, also approaches the Gauss-Kuzmin statistics. Our results are obtained as applications of improved non-escape of mass and equidistribution statements for the geodesic flow on the space $SL_2(\mathbb{R})/SL_2(\mathbb{Z})$.

math.DS↗
arXiv2024

Badly approximable grids and k-divergent lattices

For an m by n real matrix A, we investigate the set of badly approximable targets for A as a subset of the m-torus. It is well known that this set is large in the sense that it is dense and has full Hausdorff dimension. We investigate the relationship between its measure and Diophantine properties of A. On the one hand, we give the first examples of a non-singular matrix A such that the set of badly approximable targets has full measure with respect to some non-trivial algebraic measure on the torus. For this, we use transference theorems due to Jarnik and Khintchine, and the parametric geometry of numbers in the sense of Roy. On the other hand, we give a novel Diophantine condition on A that slightly strengthens non-singularity, and show that under the assumption that A satisfies this condition, the set of badly approximable targets is a null-set with respect to any non-trivial algebraic measure on the torus. For this we use naive homogeneous dynamics, harmonic analysis, and a novel concept we refer to as mixing convergence of measures.

math.NT↗
arXiv2023

K-divergent lattices

We introduce a novel concept in topological dynamics, referred to as $k$-divergence, which extends the notion of divergent orbits. Motivated by questions in the theory of inhomogeneous Diophantine approximations, we investigate this notion in the dynamical system given by a certain flow on the space of unimodular lattices in $\mathbb{R}^d$. Our main result is the existence of $k$-divergent lattices for any $k\geq 0$. In fact, we utilize the emerging theory of parametric geometry of numbers and calculate the Hausdorff dimension of the set of $k$-divergent lattices.

math.DS↗
arXiv2022

Translates of S-arithmetic orbits and applications

We prove that certain sequences of periodic orbits of the diagonal group in the space of lattices equidistribute. As an application we obtain new information regarding the sequence of best approximations to certain vectors with algebraic coordinates. In order to prove these results we generalize the seminal work of Eskin Mozes and Shah about the equidistribution of translates of periodic measures from the real case to the S-arithmetic case.

math.DS↗
arXiv2021

Translates of rational points along expanding closed horocycles on the modular surface

We study the limiting distribution of the rational points under a horizontal translation along a sequence of expanding closed horocycles on the modular surface. Using spectral methods we confirm equidistribution of these sample points for any translate when the sequence of horocycles expands within a certain polynomial range. We show that the equidistribution fails for generic translates and a slightly faster expanding rate. We also prove both equidistribution and non-equidistribution results by obtaining explicit limiting measures while allowing the sequence of horocycles to expand arbitrarily fast. Similar results are also obtained for translates of primitive rational points.

math.DS↗
arXiv2021

Linnik's problem in fiber bundles over quadratic homogeneous varieties

We compute the statistics of $SL_{d}(\mathbb{Z})$ matrices lying on level sets of an integral polynomial defined on $SL_{d}(\mathbb{R})$, a result that is a variant of the well known theorem proved by Linnik about the equidistribution of radially projected integral vectors from a large sphere into the unit sphere. Using the above result we generalize the work of Aka, Einsiedler and Shapira in various directions. For example, we compute the joint distribution of the residue classes modulo $q$ and the properly normalized orthogonal lattices of primitive integral vectors lying on the level set $-(x_{1}^{2}+x_{2}^{2}+x_{3}^{2})+x_{4}^{2}=N$ as $N\to\infty$, where the normalized orthogonal lattices sit in a submanifold of the moduli space of rank-$3$ discrete subgroups of $\mathbb{R}^{4}$.

math.NT↗
arXiv2019

Limiting Distributions of Translates of Divergent Diagonal Orbits

We define a natural topology on the collection of (equivalence classes up to scaling of) locally finite measures on a homogeneous space and prove that in this topology, pushforwards of certain infinite volume orbits equidistribute in the ambient space. As an application of our results we prove an asymptotic formula for the number of integral points in a ball on some varieties as the radius goes to infinity.

math.DS↗
arXiv2019

Dynamics on the space of 2-lattices in 3-space

We study the dynamics of $SL_3(\mathbb{R})$ and its subgroups on the homogeneous space $X$ consisting of homothety classes of rank-2 discrete subgroups of $\mathbb{R}^3$. We focus on the case where the acting group is Zariski dense in either $SL_3(\mathbb{R})$ or $SO(2,1)(\mathbb{R})$. Using techniques of Benoist and Quint we prove that for a compactly supported probability measure $μ$ on $SL_3(\mathbb{R})$ whose support generates a group which is Zariski dense in $SL_3(\mathbb{R})$, there exists a unique $μ$-stationary probability measure on $X$. When the Zariski closure is $SO(2,1)(\mathbb{R})$ we establish a certain dichotomy regarding stationary measures and discover a surprising phenomenon: The Poisson boundary can be embedded in $X$. The embedding is of algebraic nature and raises many natural open problems. Furthermore, motivating applications to questions in the geometry of numbers are discussed.

math.DS↗
arXiv2018

On continued fraction expansions of quadratic irrationals in positive characteristic

Let $P$ be a prime polynomial in the variable $Y$ over a finite field and let $f$ be a quadratic irrational in the field of formal Laurant series in the variable $Y^{-1}$. We study the asymptotic properties of the degrees of the coefficients of the continued fraction expansion of quadratic irrationals such as $P^nf$ and prove results that are in sharp contrast to the analogue situation in zero characteristic.

math.DS↗
arXiv2017

Equidistribution of divergent orbits of the diagonal group in the space of lattices

We consider divergent orbits of the group of diagonal matrices in the space of lattices in Euclidean space. We define two natural numerical invariants of such orbits: The discriminant - an integer - and the type - an integer vector. We then study the question of the limit distributional behaviour of these orbits as the discriminant goes to infinity. Using entropy methods we prove that for divergent orbits of a specific type, virtually any sequence of orbits equidistribute as the discriminant goes to infinity. Using measure rigidity for higher rank diagonal actions we complement this result and show that in dimension 3 or higher only very few of these divergent orbits can spend all of their life-span in a given compact set before they diverge.

math.DS↗
arXiv2017

Equidistribution of divergent orbits and continued fraction expansion of rationals

We establish an equidistribution result for push-forwards of certain locally finite algebraic measures in the adelic extension of the space of lattices in the plane. As an application of our analysis we obtain new results regarding the asymptotic normality of the continued fraction expansions of most rationals with a high denominator as well as an estimate on the length of their continued fraction expansions. By similar methods we also establish a complementary result to Zaremba's conjecture. Namely, we show that given a bound M, for any large q, the number of rationals $p/q\in [0,1]$ for which the coefficients of the continued fraction expansion of p/q are bounded by M is $o(q^{1-ε})$ for some $ε>0$ which depends on M.

math.DS↗
arXiv2017

Dimension bound for badly approximable grids

We show that for almost any vector $v$ in $\mathbb{R}^n$, for any $ε>0$ there exists $δ>0$ such that the dimension of the set of vectors $w$ satisfying $\liminf_{k\to\infty} k^{1/n} \ge ε$ (where $<\cdot>$ denotes the distance from the nearest integer), is bounded above by $n-δ$. This result is obtained as a corollary of a discussion in homogeneous dynamics and the main tool in the proof is a relative version of the principle of uniqueness of measures with maximal entropy.

math.DS↗
arXiv2016

Stable lattices and the diagonal group

Inspired by work of McMullen, we show that any orbit of the diagonal group in the space of lattices accumulates on the set of stable lattices. As consequences, we settle a conjecture of Ramharter concerning the asymptotic behaviour of the Mordell constant, and reduce Minkowski's conjecture on products of linear forms to a geometric question, yielding two new proofs of the conjecture in dimensions up to 7.

math.DS↗
arXiv2016

Shapes of unit lattices and escape of mass

We study the collection of points on the modular surface obtained from the logarithm embeddings of the groups of units in totally real cubic number fields. We conjecture that this set is dense and show that its closure contains countably many explicit curves and give a strategy to show that it has non-empty interior. The results are obtained by constructing explicit families of orders (generalizing the so called simplest cubic fields) and calculating their groups of units. We also address the question of escape of mass for the compact orbits of the diagonal group associated to these orders.

math.NT↗