arXiv subjects

Sandro Bettin

Publications and source records attributed to Sandro Bettin.

At least 19 recordsLinked to original sources

arXiv2025

A lower bound for the number of Egyptian fractions

An Egyptian fraction is a sum of the form $1/n_1 + \cdots + 1/n_r$ where $n_1, \dots, n_k$ are distinct positive integers. We prove explicit lower bounds for the cardinality of the set $E_N$ of rational numbers that can be represented by Egyptian fractions with denominators not exceeding $N$. More precisely, we show that for every integer $k \geq 4$ such that $\ln_k N \geq 3/2$ it holds $$ \frac{\ln(|E_N|)}{\ln 2} \geq \Big(2 - \frac{3}{\ln_k N}\Big)\frac{N}{\ln N}\prod_{j=3}^{k} \ln_j N , $$ where $\ln_k$ denotes the $k$-th iterate of the natural logarithm. This improves on a previous result of Bleicher and Erdős who established a similar bound but under the more stringent condition $\ln_k N\geq k$ and with a leading constant of $1$. Furthermore, we provide some methods to compute the exact values of $|E_N|$ for large positive integers $N$, and we give a table of $|E_N|$ for $N$ up to $154$.

math.NT↗

arXiv2025

On products of sets of natural density one

In a previous work, Bettin, Koukoulopoulos, and Sanna prove that if two sets of natural numbers $A$ and $B$ have natural density $1$, then their product set $A \cdot B := \{ab : a \in A, b \in B\}$ also has natural density $1$. They also provide an effective rate and pose the question of determining the optimal rate. We make progress on this question by constructing a set $A$ of density 1 such that $A\cdot A$ has a ''large'' complement.

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arXiv2022

On quantum modular forms of non-zero weights

We study functions $f$ on $\mathbb Q$ which statisfy a ``quantum modularity'' relation of the shape $$ f(x+1)=f(x), \qquad f(x) - |x|^{-k} f(-1/x) = h(x) $$ where $h:\mathbb R_{\neq 0} \to \mathbb C$ is a function satisfying various regularity conditions. We study the case $\Re(k)\neq 0$. We prove the existence of a limiting function $f^*$ which extends continuously $f$ to $\mathbb R$ in some sense. This means in particular that in the $\Re(k)\neq0$ case the quantum modular form itself has to have at least a certain level of regularity. We deduce that the values $\{f(a/q), 1\leq a<q, (a, q)=1\}$, appropriately normalized, tend to equidistribute along the graph of $f^*$, and we prove that under natural hypotheses the limiting measure is diffuse. We apply these results to obtain limiting distributions of values and continuity results for several arithmetic functions known to satisfy the above quantum modularity: higher weight modular symbols associated to holomorphic cusp forms; Eichler integral associated to Maass forms; a function of Kontsevich and Zagier related to the Dedekind $η$-function; and generalized cotangent sums.

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arXiv2022

A weighted one-level density of the non-trivial zeros of the Riemann zeta-function

We compute the one-level density of the non-trivial zeros of the Riemann zeta-function weighted by $|ζ(\frac12+it)|^{2k}$ for $k=1$ and, for test functions with Fourier support in $(-\frac12,\frac12)$, for $k=2$. As a consequence, for $k=1,2$, we deduce under the Riemann hypothesis that $T(\log T)^{1-k^2+o(1)}$ non-trivial zeros of $ζ$, of imaginary parts up to $T$, are such that $ζ$ attains a value of size $(\log T)^{k+o(1)}$ at a point which is within $O(1/\log T)$ from the zero.

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arXiv2022

On the typical rank of elliptic curves over ${\mathbb Q}(T)$

We give upper bounds for the number of rational elliptic surfaces in some families having positive rank, obtaining in particular that these form a subset of density zero. This confirms Cowan's conjecture (arXiv:2009.08622v2) in the case $m,n\leq2$.

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arXiv2022

Limit laws for rational continued fractions and value distribution of quantum modular forms

We study the limiting distributions of Birkhoff sums of a large class of cost functions (observables) evaluated along orbits, under the Gauss map, of rational numbers in $(0,1]$ ordered by denominators. We show convergence to a stable law in a general setting, by proving an estimate with power-saving error term for the associated characteristic function. This extends results of Baladi and Vallée on Gaussian behaviour for costs of moderate growth. We apply our result to obtain the limiting distribution of values of several key examples of quantum modular forms. We show that central values of the Esterman function ($L$ function of the divisor function twisted by an additive character) tend to have a Gaussian distribution, with a large variance. We give a dynamical, "trace formula free" proof that central modular symbols associated with a holomorphic cusp form for $SL(2,{\bf Z})$ have a Gaussian distribution. We also recover a result of Vardi on the convergence of Dedekind sums to a Cauchy law, using dynamical methods.

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arXiv2021

Ranks of elliptic curves over $\mathbb{Q}(T)$ of small degree in $T$

We study elliptic surfaces over $\mathbb{Q}(T)$ with coefficients of a Weierstrass model being polynomials in $\mathbb{Q}[T]$ with degree at most 2. We derive an explicit expression for their rank over $\mathbb{Q}(T)$ depending on the factorization and other simple properties of certain polynomials. Finally, we give sharp estimates for the ranks of the considered families and we present several applications, among which there are lists of rational points, generic families with maximal rank and generalizations of former results.

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arXiv2020

Effective estimation of some oscillatory integrals related to infinitely divisible distributions

We present a practical framework to prove, in a simple way, two-terms asymptotic expansions for Fourier integrals $$ {\mathcal I}(t) = \int_{\mathbb R}({\rm e}^{itϕ(x)}-1) {\rm d} μ(x) $$ where $μ$ is a probability measure on $\mathbb{R}$ and $ϕ$ is measurable. This applies to many basic cases, in link with Levy's continuity theorem. We present applications to limit laws related to rational continued fractions coefficients.

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arXiv2020

A note on the natural density of product sets

Given two sets of natural numbers $\mathcal{A}$ and $\mathcal{B}$ of natural density $1$ we prove that their product set $\mathcal{A}\cdot \mathcal{B}:=\{ab:a\in\mathcal{A},\,b\in\mathcal{B}\}$ also has natural density $1$. On the other hand, for any $\varepsilon>0$, we show there are sets $\mathcal{A}$ of density $>1-\varepsilon$ for which the product set $\mathcal{A}\cdot\mathcal{A}$ has density $<\varepsilon$. This answers two questions of Hegyvári, Hennecart and Pach.

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arXiv2020

Averages of long Dirichlet polynomials

We consider the asymptotic behavior of the mean square of truncations of the Dirichlet series of $ζ(s)^k$. We discuss the connections of this problem with that of the variance of the divisor function in short intervals and in arithmetic progressions, reviewing the recent results on this topic. Finally, we show how these results can all be proved assuming a suitable version of the moments conjecture.

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arXiv2020

Modularity and value distribution of quantum invariants of hyperbolic knots

We obtain an exact modularity relation for the $q$-Pochhammer symbol. Using this formula, we show that Zagier's modularity conjecture for a knot $K$ essentially reduces to the arithmeticity conjecture for $K$. In particular, we show that Zagier's conjecture holds for hyperbolic knots $K\neq 7_2$ with at most seven crossings. For $K=4_1$, we also prove a complementary reciprocity formula which allows us to prove a law of large numbers for the values of the colored Jones polynomials at roots of unity. We conjecture a similar formula holds for all knots and we show that this is the case if one assumes a suitable version of Zagier's conjecture.

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arXiv2020

Two arithmetic applications of perturbations of composition operators

We estimate the spectral radius of perturbations of a particular family of composition operators, in a setting where the usual choices of norms do not account for the typical size of the perturbation. We apply this to estimate the growth rate of large moments of a Thue-Morse generating function and of the Stern sequence. This answers in particular a question of Mauduit, Montgomery and Rivat (2018).

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arXiv2020

Greedy approximations by signed harmonic sums and the Thue--Morse sequence

Given a real number $τ$, we study the approximation of $τ$ by signed harmonic sums $σ_N(τ) := \sum_{n \leq N}{s_n(τ)}/n$, where the sequence of signs $(s_N(τ))_{N \in\mathbb{N}}$ is defined "greedily" by setting $s_{N+1}(τ) := +1$ if $σ_N(τ) \leq τ$, and $s_{N+1}(τ) := -1$ otherwise. Precisely, we compute the limit points and the decay rate of the sequence $(σ_N(τ)-τ)_{N \in \mathbb{N}}$. Moreover, we give an accurate description of the behavior of the sequence of signs $(s_N(τ))_{N\in\mathbb{N}}$, highlighting a surprising connection with the Thue--Morse sequence.

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arXiv2020

Partial sums of the cotangent function

Nous prouvons l'existence de formules de réciprocité pour des sommes de la forme $\sum_{m=1}^{k-1} f(\frac{m}k) \cot(π\frac{mh}k)$, où $f$ est une fonction $C^1$ par morceaux, qui met en évidence un phénomène d'alternance qui n'apparaît pas dans le cas classique où $f(x) = x$. Nous déduisons des majorations de ces sommes en termes du développement en fraction continue de $h/k$. We prove the existence of reciprocity formulae for sums of the form $\sum_{m=1}^{k-1}f(\frac{m}{k})\cot(π\frac{m h}k)$ where $f$ is a piecewise $C^1$ function, featuring an alternating phenomenon not visible in the classical case where $f(x)=x$. We deduce bounds for these sums in terms of the continued fraction expansion of $h/k$.

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arXiv2019

Counting Egyptian fractions

For any integer $N \geq 1$, let $\mathfrak{E}_N$ be the set of all Egyptian fractions employing denominators less than or equal to $N$. We give upper and lower bounds for the cardinality of $\mathfrak{E}_N$, proving that $$ \frac{N}{\log N} \prod_{j = 3}^{k} \log_j N<\log(\#\mathfrak{E}_N) < 0.421\, N, $$ for any fixed integer $k\geq 3$ and every sufficiently large $N$, where $\log_j x$ denotes the $j$-th iterated logarithm of $x$.

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arXiv2019

Mixed moments of characteristic polynomials of random unitary matrices

Following the work of Conrey, Rubinstein and Snaith and Forrester and Witte we examine a mixed moment of the characteristic polynomial and its derivative for matrices from the unitary group U(N) (also known as the CUE) and relate the moment to the solution of a Painleve differential equation. We also calculate a simple form for the asymptotic behaviour of moments of logarithmic derivatives of these characteristic polynomials evaluated near the unit circle.

math-ph↗

arXiv2018

A note on the dimension of the largest simple Hecke submodule

For $k\ge 2$ even, let $d_{k,N}$ denote the dimension of the largest simple Hecke submodule of $S_{k}(Γ_0(N); \mathbb{Q})^\text{new}$. We show, using a simple analytic method, that $d_{k,N} \gg_k \log\log N / \log(2p)$ with $p$ the smallest prime co-prime to $N$. Previously, bounds of this quality were only known for $N$ in certain subsets of the primes. We also establish similar (and sometimes stronger) results concerning $S_{k}(Γ_0(N), χ)$, with $k \geq 2$ an integer and $χ$ an arbitrary nebentypus.

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arXiv2018

Small values of signed harmonic sums

For every $τ\in\mathbb{R}$ and every integer $N$, let $\mathfrak{m}_N(τ)$ be the minimum of the distance of $τ$ from the sums $\sum_{n=1}^N s_n/n$, where $s_1, \ldots, s_n \in \{-1, +1\}$. We prove that $\mathfrak{m}_N(τ) < \exp\!\big(-C(\log N)^2\big)$, for all sufficiently large positive integers $N$ (depending on $C$ and $τ$), where $C$ is any positive constant less than $1/\log 4$.

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