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Shabnam Akhtari

Publications and source records attributed to Shabnam Akhtari.

At least 19 recordsLinked to original sources
arXiv2026

A Classification of Multiply Monogenic Quartic Orders

We study two-times monogenic quartic orders; i.e., those of the shape $\mathbb{Z}[\alpha] = \mathbb{Z}[\beta]$, with algebraic integers $\alpha$ and $\beta$ not $\mathbb{Z}$-equivalent. Two specific types, describing possible algebraic relation among monogenizers of two-times monogenic orders were defined by B\'erczes, Evetrse, Gy\H{o}ry, who proved under certain conditions on the Galois group of the normal closure of a given number field $K$, that there can be only finitely many two-times monogenic $\mathbb{Z}$-orders in the ring of integers $K$ which are not of these specific two types. In this article, we prove this fact for all quartic number fields.

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arXiv2025

Effective equidistribution of norm one elements in CM-fields

For a number field $K$ let $\mathcal{S}_K$ be the maximal subgroup of the multiplicative group $K^\times$ that embeds into the unit circle under each embedding of $K$ into the complex numbers. The group $\mathcal{S}_K$ can be seen as an archimedean counterpart to the group of units $\mathcal{O}_K^\times$ of the ring of integers $\mathcal{O}_K$. If $K=\mathbb{Q}(\mathcal{S}_K)$ is a CM-field then $\mathcal{S}_K/{\mathop{\rm Tor}\nolimits}(K^\times)$ is a free abelian group of infinite rank. If $K=\mathbb{Q}(\mathcal{S}_K)$ is not a CM-field then $\mathcal{S}_K=\{\pm 1\}$. In the former case $\mathcal{S}_K$ is the kernel of the relative norm map from $K^\times$ to the multiplicative subgroup $k^\times$ of the maximal totally real subfield $k$ of $K$.

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arXiv2024

On rational points on classifying stacks and Malle's conjecture

In this expository article, we compare Malle's conjecture on counting number fields of bounded discriminant with recent conjectures of Ellenberg--Satriano--Zureick-Brown and Darda--Yasuda on counting points of bounded height on classifying stacks. We illustrate the comparisons via the classifying stacks $B(\mathbb{Z}/n\mathbb{Z})$ and $B{μ_n}$.

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arXiv2023

A bound for the exterior product of $S$-units

We generalize an inequality for the determinant of a real matrix proved by A. Schinzel, to more general exterior products of vectors in Euclidean space. We apply this inequality to the logarithmic embedding of $S$-units contained in a number field $k$. This leads to a bound for the exterior product of $S$-units expressed as a product of heights. Using a volume formula of P. McMullen we show that our inequality is sharp up to a constant that depends only on the rank of the $S$-unit group but not on the field $k$. Our inequality is related to a conjecture of F. Rodriguez Villegas.

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arXiv2023

Small integral generators of totally complex number fields

Let $K$ be an algebraic number field and $H$ the absolute Weil height. Write $c_K$ for a certain positive constant that is an invariant of $K$. We consider the question: does $K$ contain an algebraic integer $\alpha$ such that both $K = \mathbb{Q}(\alpha)$ and $H(\alpha) \le c_K$? If $K$ has a real embedding then a positive answer was established in previous work. Here we obtain a positive answer if $\textrm{Tor}\bigl(K^{\times}\bigr) \not= \{\pm 1\}$, and so $K$ has only complex embeddings. We also show that if the answer is negative, then $K$ is totally complex, $\textrm{Tor}\bigl(K^{\times}\bigr) = \{\pm 1\}$, and $K$ is a Galois extension of its maximal totally real subfield. Further, we show that if $\mu \in O_K$ is not totally real, then there exists $\alpha$ in $O_K$ with $K = \mathbb{Q}(\alpha)$ and $H(\alpha) \le H(\mu)\thinspace c_K$.

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arXiv2022

Lower Bounds for Regulators of Number Fields in terms of their Discriminants

We prove inequalities that compare the regulator of a number field with its absolute discriminant. We refine some ideas in Silverman's work in 1984 where such general inequalities are first proven. In order to prove our main theorems, we combine these refinements with the authors' recent results on bounding the product of heights of relative units in a number field extension.

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arXiv2021

Independent relative units of low height

We prove inequalities that compare the relative regulator of an extension of number fields with a product of heights of multiplicatively independent relative units.

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arXiv2021

A positive proportion of locally soluble quartic Thue equations are globally insoluble

For any fixed nonzero integer $h$, we show that a positive proportion of integral binary quartic forms $F$ do locally everywhere represent $h$, but do not globally represent $h$. We order classes of integral binary quartic forms by the two generators of their ring of $\textrm{GL}_{2}(\mathbb{Z})$-invariants, classically denoted by $I$ and $J$.

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arXiv2021

Representation of integers by sparse binary forms

We will give new upper bounds for the number of solutions to the inequalities of the shape $|F(x , y)| \leq h$, where $F(x , y)$ is a sparse binary form, with integer coefficients, and $h$ is a sufficiently small integer in terms of the absolute value of the discriminant of the binary form $F$. Our bounds depend on the number of non-vanishing coefficients of $F(x , y)$. When $F$ is really sparse, we establish a sharp upper bound for the number of solutions that is linear in terms of the number of non-vanishing coefficients. This work will provide affirmative answers to a number of conjectures posed by Mueller and Schmidt in 1988, for special but important cases.

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arXiv2019

A positive proportion of Thue equations fail the integral Hasse principle

For any nonzero $h\in\mathbb{Z}$, we prove that a positive proportion of integral binary cubic forms $F$ do locally everywhere represent $h$ but do not globally represent $h$; that is, a positive proportion of cubic Thue equations $F(x,y)=h$ fail the integral Hasse principle. Here, we order all classes of such integral binary cubic forms $F$ by their absolute discriminants. We prove the same result for Thue equations $G(x,y)=h$ of any fixed degree $n \geq 3$, provided that these integral binary $n$-ic forms $G$ are ordered by the maximum of the absolute values of their coefficients.

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arXiv2018

Lower bounds for Mahler measure that depend on the number of monomials

We prove a new lower bound for the Mahler measure of a polynomial in one and in several variables that depends on the complex coefficients, and the number of monomials. In one variable our result generalizes a classical inequality of Mahler. In $M$ variables our result depends on $\mathbb{Z}^M$ as an ordered group, and in general our lower bound depends on the choice of ordering.

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arXiv2018

On the height of solutions to norm form equations

Let $k$ be a number field. We consider norm form equations associated to a full $O_k$-module contained in a finite extension field $l$. It is known that the set of solutions is naturally a union of disjoint equivalence classes of solutions. We prove that each nonempty equivalence class of solutions contains a representative with Weil height bounded by an expression that depends on parameters defining the norm form equation.

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arXiv2017

Lower Bounds for Heights in Relative Galois Extensions

The goal of this paper is to obtain lower bounds on the height of an algebraic number in a relative setting, extending previous work of Amoroso and Masser. Specifically, in our first theorem we obtain an effective bound for the height of an algebraic number $α$ when the base field $\mathbb{K}$ is a number field and $\mathbb{K}(α)/\mathbb{K}$ is Galois. Our second result establishes an explicit height bound for any non-zero element $α$ which is not a root of unity in a Galois extension $\mathbb{F}/\mathbb{K}$, depending on the degree of $\mathbb{K}/\mathbb{Q}$ and the number of conjugates of $α$ which are multiplicatively independent over $\mathbb{K}$. As a consequence, we obtain a height bound for such $α$ that is independent of the multiplicative independence condition.

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arXiv2017

Thue's inequalities and the hypergeometric method

Following a method originally due to Siegel, we establish upper bounds for the number of primitive integer solutions to inequalities of the shape $0<|F(x, y)| \leq h$, where $F(x , y) =(αx + βy)^r -(γx + δy)^r \in \mathbb{Z}[x ,y]$, $α$, $β$, $γ$ and $δ$ are algebraic constants with $αδ-βγ\neq 0$, and $r \geq 3$ and $h$ are integers. As an important application, we pay special attention to the binomial Thue's inequaities $|ax^r - by^r| \leq c$. The proofs are based on the hypergeometric method of Thue and Siegel and its refinement by Evertse.

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arXiv2017

Minkowski's theorem on independent conjugate units

We call a unit $β$ in a Galois extension $l/\mathbb{Q}$ a Minkowski unit if the subgroup generated by $β$ and its conjugates over $\mathbb{Q}$ has maximum rank in the unit group of $l$. Minkowski showed the existence of such units in every Galois extension. We will give a new proof to Minkowski's theorem and show that there exists a Minkowski unit $β\in l$ such that the Weil height of $β$ is comparable with the sum of the heights of a fundamental system of units of $l$. Our proof implies a bound on the index of the subgroup generated by the algebraic conjugates of $β$ in the unit group of $l$. If $k$ is an intermediate field such that \begin{equation*} \mathbb{Q} \subseteq k \subseteq l, \end{equation*} and $l/\mathbb{Q}$ and $k/\mathbb{Q}$ are Galois extensions, we prove an analogous bound for the subgroup of relative units.

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arXiv2016

Heights, Regulators and Schinzel's determinant inequality

We prove inequalities that compare the size of an S-regulator with a product of heights of multiplicatively independent S-units. Our upper bound for the S-regulator follows from a general upper bound for the determinant of a real matrix proved by Schinzel. The lower bound for the S-regulator follows from Minkowski's theorem on successive minima and a volume formula proved by Meyer and Pajor. We establish similar upper bounds for the relative regulator of an extension of number fields.

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arXiv2015

Representation of Small Integers by Binary Forms

We establish some upper bounds for the number of integer solutions to the Thue inequality $|F(x , y)| \leq m$, where $F$ is a binary form of degree $n \geq 3$ and with non-zero discriminant $D$, and $m$ is an integer. Our upper bounds are independent of $m$, when $m$ is smaller than $|D|^{\frac{1}{4(n-1)}}$. We also consider the Thue equation $|F(x , y)| = m$ and give some upper bounds for the number of its integral solutions. In the case of equation, our upper bounds will be independent of integer $m$, when $ m < |D|^{\frac{1}{2(n-1)}}$.

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