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Florian Luca

Publications and source records attributed to Florian Luca.

At least 55 records · Page 3Linked to original sources
arXiv2023

Sequences of integers generated by two fixed primes

Let $p$ and $q$ be two distinct fixed prime numbers and $(n_i)_{i\geq 0}$ the sequence of consecutive integers of the form $p^a\cdot q^b$ with $a,b\ge 0$. Tijdeman gave a lower bound (1973) and an upper bound (1974) for the gap size $n_{i+1}-n_i$, with each bound containing an unspecified exponent and implicit constant. We will explicitly bound these four quantities. Earlier Langevin (1976) gave weaker estimates for (only) the exponents. Given a real number $α>1$, there exists a smallest number $m$ such that for every $n\ge m$, there exists an integer $n_i$ in $[n,nα)$. Our effective version of Tijdeman's result immediately implies an upper bound for $m$, which using the Koksma-Erdős-Turan inequality we will improve on. We present a fast algorithm to determine $m$ when $\max\{p,q\}$ is not too large and demonstrate it with numerical material. In an appendix we explain, given $n_i$, how to efficiently determine both $n_{i-1}$ and $n_{i+1}$, something closely related to work of Bérczes, Dujella and Hajdu.

math.NT↗
arXiv2023

Transcendence of Sturmian Numbers over an Algebraic Base

We consider numbers of the form $S_β(\boldsymbol{u}):=\sum_{n=0}^\infty \frac{u_n}{β^n}$ for $\boldsymbol{u}=\langle u_n \rangle_{n=0}^\infty$ a Sturmian sequence over a binary alphabet and $β$ an algebraic number with $|β|>1$. We show that every such number is transcendental. More generally, for a given base~$β$ and given irrational number~$θ$ we characterise the $\overline{\mathbb{Q}}$-linear independence of sets of the form $\left\{ 1, S_β(\boldsymbol{u}^{(1)}),\ldots,S_β(\boldsymbol{u}^{(k)}) \right\}$, where $\boldsymbol{u}^{(1)},\ldots,\boldsymbol{u}^{(k)}$ are Sturmian sequences having slope $θ$. We give an application of our main result to the theory of dynamical systems, showing that for a contracted rotation on the unit circle with algebraic slope, its limit set is either finite or consists exclusively of transcendental elements other than its endpoints $0$ and $1$. This confirms a conjecture of Bugeaud, Kim, Laurent, and Nogueira.

cs.FL↗
arXiv2023

Three essays on Machin's type formulas

We study three questions related to Machin's type formulas. The first one gives all two terms Machin formulas where both arctangent functions are evaluated $2$-integers, that is values of the form $b/2^a$ for some integers $a$ and~$b$. These formulas are computationally useful because multiplication or division by a power of two is a very fast operation for most computers. The second one presents a method for finding infinitely many formulas with $N$ terms. In the particular case $N=2$ the method is quite useful. It recovers most known formulas, gives some new ones, and allows to prove in an easy way that there are two terms Machin formulas with Lehmer measure as small as desired. Finally, we correct an oversight from previous result and give all Machin's type formulas with two terms involving arctangents of powers of the golden section.

math.NT↗
arXiv2023

Numbers of the form $k+f(k)$

For a function $f\colon \mathbb{N}\to\mathbb{N}$, let $$ N^+_f(x)=\{n\leq x: n=k+f(k) \mbox{ for some } k\}. $$ Let $τ(n)=\sum_{d|n}1$ be the divisor function, $ω(n)=\sum_{p|n}1$ be the prime divisor function, and $φ(n)=\#\{1\leq k\leq n: \gcd(k,n)=1 \}$ be Euler's totient function. We show that \begin{align*} &(1) \quad x \ll N^+_ω(x), \\ &(2) \quad x\ll N^+_τ(x) \leq 0.94x, \\ &(3) \quad x \ll N^+_φ(x) \leq 0.93x. \end{align*}

math.NT↗
arXiv2023

Bad witnesses for a composite number

We describe the average sizes of the set of bad witnesses for a pseudo-primality test which is the product of a multiple-rounds Miller-Rabin test by the Galois test.

math.NT↗
arXiv2023

On (almost) realizable subsequences of linearly recurrent sequences

In this note we show that if $(u_n)_{n\geqslant 1}$ is a simple linearly recurrent sequence of integers whose minimal recurrence of order $k$ involves only positive coefficients that has positive initial terms, then $(Mu_{n^s})_{n\geqslant 1}$ is the sequence of periodic point counts for some map for a suitable positive integer $M$ and $s$ any sufficiently large multiple of $k!$. This extends a result of Moss and Ward [The Fibonacci Quarterly 60 (2022), 40-47] who proved the result for the Fibonacci sequence.

math.NT↗
arXiv2023

On equal values of products and power sums of consecutive elements in an arithmetic progression

In this paper we study the Diophantine equation \begin{align*} b^k + \left(a+b\right)^k + &\left(2a+b\right)^k + \ldots + \left(a\left(x-1\right) + b\right)^k = \\ &y\left(y+c\right) \left(y+2c\right) \ldots \left(y+ \left(\ell-1\right)c\right), \end{align*} where $a,b,c,k,\ell$ are given integers under natural conditions. We prove some effective results for special values for $c,k$ and $\ell$ and obtain a general ineffective result based on Bilu-Tichy method.

math.NT↗
arXiv2023

On the iterates of shifted Euler's function

Let $φ$ be the Euler's function and fix an integer $k\ge 0$. We show that, for every initial value $x_1\ge 1$, the sequence of positive integers $(x_n)_{n\ge 1}$ defined by $x_{n+1}=φ(x_n)+k$ for all $n\ge 1$ is eventually periodic. Similarly, for every initial value $x_1,x_2\ge 1$, the sequence of positive integers $(x_n)_{n\ge 1}$ defined by $x_{n+2}=φ(x_{n+1})+φ(x_n)+k$ for all $n\ge 1$ is eventually periodic, provided that $k$ is even.

math.NT↗
arXiv2022

Numbers of the form $kf(k)$

For a function $f\colon \mathbb{N}\to\mathbb{N}$, define $N^{\times}_{f}(x)=\#\{n\leq x: n=kf(k) \mbox{ for some $k$} \}$. Let $τ(n)=\sum_{d|n}1$ be the divisor function, $ω(n)=\sum_{p|n}1$ be the prime divisor function, and $φ(n)=\#\{1\leq k\leq n: (k,n)=1 \}$ be Euler's totient function. We prove that \begin{gather*} \!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\! 1) \quad N^{\times}_τ(x) \asymp \frac{x}{(\log x)^{1/2}}; \\ 2) \quad N^{\times}_ω(x) = (1+o(1))\frac{x}{\log\log x}; \\ \!\!\!\!\!\!\!\!\! 3) \quad N^{\times}_φ(x) = (c_0+o(1))x^{1/2}, \end{gather*} where $c_0=1.365...$\,.

math.NT↗
arXiv2022

On the transcendence of a series related to Sturmian words

Let $b$ be an algebraic number with $|b|>1$ and $\mathcal{H}$ a finite set of algebraic numbers. We study the transcendence of numbers of the form $\sum_{n=0}^\infty \frac{a_n}{b^n}$ where $a_n \in \mathcal{H}$ for all $n\in\mathbb{N}$. We assume that the sequence $(a_n)_{n=0}^\infty$ is generated by coding the orbit of a point under an irrational rotation of the unit circle. In particular, this assumption holds whenever the sequence is Sturmian. Our main result shows that all numbers of the above form are transcendental. We moreover give sufficient conditions for a finite set of such numbers to be linearly independent over~$\bar{\mathbb{Q}}$.

math.NT↗
arXiv2022

Skolem Meets Schanuel

The celebrated Skolem-Mahler-Lech Theorem states that the set of zeros of a linear recurrence sequence is the union of a finite set and finitely many arithmetic progressions. The corresponding computational question, the Skolem Problem, asks to determine whether a given linear recurrence sequence has a zero term. Although the Skolem-Mahler-Lech Theorem is almost 90 years old, decidability of the Skolem Problem remains open. The main contribution of this paper is an algorithm to solve the Skolem Problem for simple linear recurrence sequences (those with simple characteristic roots). Whenever the algorithm terminates, it produces a stand-alone certificate that its output is correct -- a set of zeros together with a collection of witnesses that no further zeros exist. We give a proof that the algorithm always terminates assuming two classical number-theoretic conjectures: the Skolem Conjecture (also known as the Exponential Local-Global Principle) and the $p$-adic Schanuel Conjecture. Preliminary experiments with an implementation of this algorithm within the tool \textsc{Skolem} point to the practical applicability of this method.

cs.LO↗
arXiv2021

Trinomials, singular moduli and Riffaut's conjecture

Riffaut (2019) conjectured that a singular modulus of degree $h\ge 3$ cannot be a root of a trinomial with rational coefficients. We show that this conjecture follows from the GRH, and obtain partial unconditional results.

math.NT↗
arXiv2021

On the Shorey-Tijdeman Diophantine equation involving terms of Lucas sequences

Let $r\ge 1$ be an integer and ${\bf U}:=\{U_n\}_{n\ge 0}$ be the Lucas sequence given by $U_0=0,~U_1=1$, and $U_{n+2}=rU_{n+1}+U_n$ for $n\ge 0$. In this paper, we explain how to find all the solutions of the Diophantine equation, $AU_{n}+BU_{m}=CU_{n_1}+DU_{m_1}$, in integers $r\ge 1$, $0\le m<n,~0\le m_1<n_1$, $AU_n\ne CU_{n_1}$, where $A,B,C,D$ are given integers with $A\ne 0,~B\ne 0$, $m,n,m_1,n_1$ are nonnegative integer unknowns and $r$ is also unknown.

math.NT↗