The best approximation of algebraic numbers by multidimensional continued fractions
V. G. Zhuravlev Vladimir State University
Abstract:
A karyon-modular algorithm (
$\mathcal {KM}$-algorithm) is proposed for decomposition of algebraic numbers
$\alpha = (\alpha_1, \ldots, \alpha_d)$ from
$\mathbb {R}^{d}$ to multidimensional continued fractions, that are a sequence of rational numbers
$$ \frac{P_{a}}{Q_{a}}=\Bigl( \frac{P^{a}_1}{Q^{a}},\ldots,\frac{P^{a}_d}{Q^{a}}\Bigr), a=1,2,3,\ldots, $$
from
$\mathbb{Q}^d$ with numerators
$P^{a}_1,\ldots,P^{a}_d \in \mathbb{Z}$ and the common denominator
$Q^{a}=1,2,3,\ldots$ The
$ \mathcal{KM}$-algorithm belongs to a class of tuning algorithms. It is based on the construction of localized Pisot units
$\zeta>1$, for which the moduli of all conjugates
$\zeta^{(i)}\ne \zeta $ are contained in the
$ \theta $-neighbourhood of the number
$ \zeta^{- 1 /d}$, where the parameter
$ \theta> 0 $ can take an arbitrary fixed value. It is proved that if
$ \alpha $ is a real algebraic point of degree
$ \mathrm {deg} (\alpha) = d + 1 $, then
$ \mathcal {KM} $ - algorithm allows to obtain the following approximation
$$ \Bigl | \alpha - \frac {P_{a}}{Q_{a}} \Bigr | \leq \frac {c} {Q^{1+ \frac{1}{d} - \theta}_{a}} $$
for all
$ a \geq a_{\alpha, \theta} $, where the constants
$ a_{\alpha, \theta}> 0 $ and
$ c = c_{\alpha, \theta}> 0 $ do not depend on
$ a = 1,2,3, \ldots $ and the convergent fractions
$ \frac {P_{a}} {Q_{a}} $ are calculated by means of some recurrence relation with constant coefficients determined by the choice of the localized units
$ \zeta $.
Key words and phrases:
multidimensional continued fractions, best approximations, localized Pisot units.
UDC:
511.3
Received: 18.04.2019