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Journal of Number Theory
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Journal of Number Theory
Article . 2007
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On approximation of real numbers by algebraic numbers of bounded degree

Authors: Tsishchanka, Kiryl I.;

On approximation of real numbers by algebraic numbers of bounded degree

Abstract

Dirichlet proved that, for any real irrational number \(\xi\), there exist infinitely many rational numbers \(\frac{p}{q}\) such that \(|\xi-\frac{p}{q}|2\). Let \(\mathbf A_n, \;n>2\) denote the set of algebraic numbers of degree \(\leq n\). Let \(\alpha\in \mathbf A_n\) and \(H(\alpha)\) the height of \(\alpha\), that is the largest absolute value of the coefficients of its minimal polynomial. In 1961, \textit{E. Wirsing} [J. Reine Angew. Math. 206, 67--77 (1961; Zbl 0097.03503)] made the conjecture that, for any real number \(\xi\not\in \mathbf A_n\) and any real number \(\varepsilon>0\), there exist infinitely many algebraic numbers \(\alpha\in\mathbf A_n\) such that \[ |\xi-\alpha|\ll H(\alpha)^{-n-1-\varepsilon},\tag{1} \] where \(\ll\) is for the Vinogradov symbol and the implicit constant should depend on \(\xi,n\) and \(\varepsilon\) only. Later, \textit{W. M. Schmidt} [Diophantine Approximation. Berlin etc.: Springer (1980; Zbl 0421.10019)] conjectured the optimal exponent \(-n-1\) in (1). \textit{V. G. Sprindzhuk} [Izv. Akad. Nauk SSSR, Ser. Mat. 29, 379--436 (1965; Zbl 0156.05405)] showed that the conjecture of Wirsing is true at for almost all real numbers. In [loc. cit.] Wirsing proved that for any real number \(\xi\not\in \mathbf A_n\) there exist infinitely many algebraic numbers \(\alpha\in \mathbf A_n\) such that \[ |\xi-\alpha|\ll H(\alpha)^{-C(n)} \] where \(\lim_{n\to \infty}(C(n)-n/2)=2\) and the implicit constant depends only on \(\xi\) and \(n\). In 1993 V. I. Bernik and the author proved in [Dokl. Akad. Nauk Belarusi 37, No. 5, 9--11 (1993; Zbl 0811.11048)] that for any real number \(\xi\not\in \mathbf A_n\) there exist infinitely many algebraic numbers \(\alpha\in \mathbf A_n\) such that \[ |\xi-\alpha|\ll H(\alpha)^{-B(n)} \] where \(\lim_{n\to \infty}(B(n)-n/2)=3\) and the implicit constant depends only on \(\xi\) and \(n\). In this paper, the author improves all the previous estimates concerning the real case for \(n>2\) in the following theorem. Let \(n\) be an integer at least \(3\). Then for any real number \(\xi\not\in \mathbf A_n\) there exist infinitely many algebraic numbers \(\alpha\in\mathbf A_n\) such that \[ |\xi-\alpha|\ll H(\alpha)^{-A(n)}, \] where \(A(n)\) is the largest real root of the polynomial \[ \begin{multlined} T(x)=4x^5-(4n+18)x^4+(n^2+11n+30)x^3\\ -(2n^2+10n+22)x^2+(2n^2+7n+4)x+n^2-5n+2,\end{multlined} \] if \(n=3,4,5\) and \[ T(x)=2x^5-(n+12)x^4+(2n+30)x^3+(2n-41)x^2-(3n-29)x+2n-10 \] if \(n>5\). The implicit constant depends on \(\xi\) and \(n\) only. This table gives an example of comparison of \(C(n), B(n)\) and \(A(n)\) for some values of the integer \(n\). \[ \begin{matrix} n& C(n)& B(n)& A(n)\\ 3& 3.28&3.5&3.73\\ 4& 3.82& 4.12& 4.45\\ 5& 4.35&4.71&5.14\\ 10& 6.92&7.47& 8.06\\ 100& 51.99& 52.92& 53.84\\ \end{matrix} \]

Related Organizations
Keywords

Approximation by numbers from a fixed field, approximation of real numbers by algebraic numbers, Algebra and Number Theory, Homogeneous approximation to one number

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selected citations
These citations are derived from selected sources.
This is an alternative to the "Influence" indicator, which also reflects the overall/total impact of an article in the research community at large, based on the underlying citation network (diachronically).
BIP!Citations provided by BIP!
popularity
This indicator reflects the "current" impact/attention (the "hype") of an article in the research community at large, based on the underlying citation network.
BIP!Popularity provided by BIP!
influence
This indicator reflects the overall/total impact of an article in the research community at large, based on the underlying citation network (diachronically).
BIP!Influence provided by BIP!
impulse
This indicator reflects the initial momentum of an article directly after its publication, based on the underlying citation network.
BIP!Impulse provided by BIP!
6
Average
Top 10%
Average
hybrid