arXiv subjects

Bakir Farhi

Publications and source records attributed to Bakir Farhi.

43 records · Page 3Linked to original sources

arXiv2008

A simple proof that any additive basis has only finitely many essential subsets

Let $A$ be an additive basis. We call ``essential subset'' of $A$ any finite subset $P$ of $A$ such that $A \setminus P$ is not an additive basis and that $P$ is minimal (for the inclusion order) to have this property. A recent theorem due to B. Deschamps and the author states that any additive basis has only finitely many essential subsets (see ``Essentialité dans les bases additives, J. Number Theory, 123 (2007), p. 170-192''). The aim of this note is to give a simple proof of this theorem.

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arXiv2008

A curious result related to Kempner's series

It is well known since A. J. Kempner's work that the series of the reciprocals of the positive integers whose the decimal representation does not contain any digit 9, is convergent. This result was extended by F. Irwin and others to deal with the series of the reciprocals of the positive integers whose the decimal representation contains only a limited quantity of each digit of a given nonempty set of digits. Actually, such series are known to be all convergent. Here, letting $S^{(r)}$ $(r \in \mathbb{N})$ denote the series of the reciprocal of the positive integers whose the decimal representation contains the digit 9 exactly $r$ times, the impressive obtained result is that $S^{(r)}$ tends to $10 \log{10}$ as $r$ tends to infinity!

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arXiv2008

Nontrivial lower bounds for the least common multiple of some finite sequences of integers

We present here a method which allows to derive a nontrivial lower bounds for the least common multiple of some finite sequences of integers. We obtain efficient lower bounds (which in a way are optimal) for the arithmetic progressions and lower bounds less efficient (but nontrivial) for quadratic sequences whose general term has the form $u_n = a n (n + t) + b$ with $(a, t, b) \in {\mathbb{Z}}^3, a \geq 5, t \geq 0, \rm{gcd}(a, b) = 1$. From this, we deduce for instance the lower bound: $\mathrm{lcm}\{1^2 + 1, 2^2 + 1, ..., n^2 + 1\} \geq 0,32 (1,442)^n$ (for all $n \geq 1$). In the last part of this article, we study the integer $\mathrm{lcm}(n, n + 1, ..., n + k)$ $(k \in \mathbb{N}, n \in {\mathbb{N}}^*)$. We show that it has a divisor $d_{n, k}$ simple in its dependence on $n$ and $k$, and a multiple $m_{n, k}$ also simple in its dependence on $n$. In addition, we prove that both equalities: $\mathrm{lcm}(n, n + 1, ..., n + k) = d_{n, k}$ and $\mathrm{lcm}(n, n + 1, ..., n + k) = m_{n, k}$ hold for an infinitely many pairs $(n, k)$.

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arXiv2008

Essentialité dans les bases additives

In this article we study the notion of essential subset of an additive basis, that is to say the minimal finite subsets $P$ of a basis $A$ such that $A \setminus P$ doesn't remains a basis. The existence of an essential subset for a basis is equivalent for this basis to be included, for almost all elements, in an arithmetic non-trivial progression. We show that for every basis $A$ there exists an arithmetic progression with a biggest common difference containing $A$. Having this common difference $a$ we are able to give an upper bound to the number of essential subsets of $A$: this is the radical's length of $a$ (in particular there is always many finite essential subsets in a basis). In the case of essential subsets of cardinality 1 (essential elements) we introduce a way to "dessentialize" a basis. As an application, we definitively improve the earlier result of Deschamps and Grekos giving an upper bound of the number of the essential elements of a basis. More precisely, we show that for all basis $A$ of order $h$, the number $s$ of essential elements of $A$ satisfy $s\leq c\sqrt{\frac{h}{\log h}}$ where $c=30\sqrt{\frac{\log 1564}{1564}}\simeq 2,05728$, and we show that this inequality is best possible.

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arXiv2006

Formulas giving prime numbers under Cramér's conjecture

Under Cramér's conjecture concerning the prime numbers, we prove that for any $x>1$, there exists a real $A=A(x)>1$ for which the formula $[A^{n^x}]$ (where $[]$ denotes the integer part) gives a prime number for any positive integer $n$. Under the same conjecture, we also prove that for any $ε>0$, there exists a positive real number $B$ for which the formula $[B.{n!}^{2+ε}]$ gives a prime number for any sufficiently large positive integer $n$.

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arXiv2006

About the fractional parts of the powers of the rational numbers

Let $p/q$ ($p, q \in \mathbb{N}^*$) be a positive rational number such that $p > q^2$. We show that for any $ε> 0$, there exists a set $A(ε) \subset [0, 1[$, with finite border and with Lebesgue measure $< ε$, for which the set of positive real numbers $λ$ satisfying $<λ(p / q)^n> \in A(ε)$ $(\forall n \in \mathbb{N})$ is uncountable.

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arXiv2006

Approximation diophantienne sur une courbe elliptique

We present here quantitative versions in 1 dimension of Faltings'theorem according to which the set of the K-rational points (where K is a given number field) of an abelian variety A definied over K, which are close (with respect to a v-adic distance on K) to some K-subvariety X of A, but not belonging to X, is finite. We treat more exactly the case where A is an elliptic curve and X is reduced to a point of A and we give (in this case) explicit bounds for the cardinal of the exceptional finite set. We consider also, more generally, instead of only one place v of K, a finite set S of places of K and the distance from the point of A to X taking into account of all places of S.

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