arXiv subjects

Damien Roy

Publications and source records attributed to Damien Roy.

43 records · Page 3Linked to original sources

arXiv2004

On two exponents of approximation related to a real number and its square

For any irrational real number xi, let lambda(xi) denote the supremum of all real numbers lambda such that, for each sufficiently large X, the inequalities |x_0| < X, |x_0*xi-x_1| < X^{-lambda} and |x_0*xi^2-x_2| < X^{-lambda} admit a solution in integers x_0, x_1 and x_2 not all zero, and let omega(xi) denote the supremum of all real numbers omega such that, for each sufficiently large X, the dual inequalities |x_0+x_1*xi+x_2*xi^2| < X^{-omega}, |x_1| < X and |x_2| < X admit a solution in integers x_0, x_1 and x_2 not all zero. Answering a question of Y. Bugeaud and M. Laurent, we show that the exponents lambda(xi) where xi ranges through all irrational non-quadratic real numbers form a dense subset of the interval [1/2, (sqrt{5}-1)/2] while, for the same values of xi, the dual exponents omega(xi) form a dense subset of [2, (sqrt{5}+3)/2]. Part of the proof rests on a result of V. Jarnik showing that lambda(xi) = 1-1/omega(xi) for these real numbers xi.

math.NT↗

arXiv2003

Approximation to real numbers by cubic algebraic integers I

In 1969, H. Davenport and W. M. Schmidt studied the problem of approximation to a real number ξby algebraic integers of degree at most three. They did so, using geometry of numbers, by resorting to the dual problem of finding simultaneous approximations to ξand ξ^2 by rational numbers with the same denominator. In this paper, we show that their measure of approximation for the dual problem is optimal and that it is realized for a countable set of real numbers ξ. We give several properties of these numbers including measures of approximation by rational numbers, by quadratic real numbers and by algebraic integers of degree at most three.

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arXiv2003

Diophantine approximation by conjugate algebraic integers

Building on work of Davenport and Schmidt, we mainly prove two results. The first one is a version of Gel'fond's transcendence criterion which provides a sufficient condition for a complex or $p$-adic number $ξ$ to be algebraic in terms of the existence of polynomials of bounded degree taking small values at $ξ$ together with most of their derivatives. The second one, which follows from this criterion by an argument of duality, is a result of simultaneous approximation by conjugate algebraic integers for a fixed number $ξ$ that is either transcendental or algebraic of sufficiently large degree. We also present several constructions showing that these results are essentially optimal.

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arXiv2003

Diophantine approximation in small degree

We discuss the problem of finding optimal exponents in Diophantine estimates involving one real number and, in some cases where such an exponent is known, present some properties of the corresponding extremal numbers.

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arXiv2002

A Gel'fond type criterion in degree two

We establish a criterion for a complex number to be algebraic over Q of degree at most two. It requires that, for any sufficiently large real number X, there exists a non-zero polynomial with integral coefficients, of degree at most two and height at most X, whose absolute value at that complex number is at most (1/4)X^{-(3+sqrt{5})/2}. We show that the exponent (3+sqrt{5})/2 in this condition is optimal, and deduce from this criterion a result of simultaneous approximation of a real number by conjugate algebraic numbers.

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arXiv2002

Approximation to real numbers by cubic algebraic integers II

It has been conjectured for some time that, for any integer n\ge 2, any real number ε>0 and any transcendental real number ξ, there would exist infinitely many algebraic integers αof degree at most n with the property that |ξ-α| < H(α)^{-n+ε}, where H(α) denotes the height of α. Although this is true for n=2, we show here that, for n=3, the optimal exponent of approximation is not 3 but (3+\sqrt{5})/2 = 2.618...

math.NT↗

arXiv2002

Approximation simultanée d'un nombre et de son carré [Simultaneous approximation to a real number and its square]

In 1969, H. Davenport and W. Schmidt established a measure of simultaneous approximation for a real number ξand its square by rational numbers with the same denominator, assuming only that ξis not rational nor quadratic over Q. Here, we show by an example, that this measure is optimal. We also indicate several properties of the numbers for which this measure is optimal, in particular with respect to approximation by algebraic integers of degree at most three.

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