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arXiv 1002.4614math.DSmath.CO
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二次差逼近数的精细结构

The Fine Structure of Dyadically Badly Approximable Numbers

Johan Nilsson

摘要:

我们考虑在并元丢番图逼近的情况下,可差逼近的数。对于单位圆$\mathbb{S}$和到整数$\|\cdot\|$的最小距离,我们给出了集合$F(c) = \{x \in \mathbb{S}: \|2^nx\| \geq c, n\geq 0\}$是豪斯多夫维数连续依赖于$c$的分形集,在构成Lebesgue测度集1的区间上是常数,并且是自相似的初等证明.因此它有一个分形图。此外,$F(c)$的维数为零当且仅当$c\geq 1-2τ$,其中$τ$是Thue-Morse常数。在维数保持不变的区间上,我们将其完全展开。因此,我们可以完整地描述$ c\mapsto \dim_H \{x\in[0,1]: \|x-\frac{m}{2^n}\|< \frac{c}{2^n} \textnormal{finitely often}\}$的图形。

英文摘要:

We consider badly approximable numbers in the case of dyadic diophantine approximation. For the unit circle $\mathbb{S}$ and the smallest distance to an integer $\|\cdot\|$ we give elementary proofs that the set $F(c) = \{x \in \mathbb{S}: \|2^nx\| \geq c, n\geq 0\}$ is a fractal set whose Hausdorff dimension depends continuously on $c$, is constant on intervals which form a set of Lebesgue measure 1 and is self-similar. Hence it has a fractal graph. Moreover, the dimension of $F(c)$ is zero if and only if $c\geq 1-2τ$, where $τ$ is the Thue-Morse constant. We completely characterise the intervals where the dimension remains unchanged. As a consequence we can completely describe the graph of $ c\mapsto \dim_H \{x\in[0,1]: \|x-\frac{m}{2^n}\|< \frac{c}{2^n} \textnormal{finitely often}\}$.

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