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• # Artykuł - szczegóły

## Acta Arithmetica

2015 | 168 | 3 | 269-287

## Approximation properties of β-expansions

EN

### Abstrakty

EN
Let β ∈ (1,2) and x ∈ [0,1/(β-1)]. We call a sequence $(ϵ_{i})_{i=1}^{∞} ∈ {0,1}^{ℕ}$ a β-expansion for x if $x=∑_{i=1}^{∞}ϵ_{i}β^{-i}$. We call a finite sequence $(ϵ_{i})_{i=1}^{n} ∈ {0,1}^{n}$ an n-prefix for x if it can be extended to form a β-expansion of x. In this paper we study how good an approximation is provided by the set of n-prefixes.
Given $Ψ: ℕ → ℝ_{≥ 0}$, we introduce the following subset of ℝ:
$W_{β}(Ψ) := ⋂ _{m=1}^{∞} ⋃ _{n=m}^{∞} ⋃ _{(ϵ_{i})_{i=1}^{n}∈{0,1}^{n}} [∑_{i=1}^{n} (ϵ_{i})/(β^{i}),∑_{i=1}^{n}(ϵ_{i})/(β^{i}) + Ψ(n)] In other words,$W_{β}(Ψ)$is the set of x ∈ ℝ for which there exist infinitely many solutions to the inequalities$0 ≤ x - ∑_{i=1}^{n} (ϵ_{i})/(β^{i}) ≤ Ψ(n)$. When$∑_{n=1}^{∞} 2^{n}Ψ(n) < ∞$, the Borel-Cantelli lemma tells us that the Lebesgue measure of$W_{β}(Ψ)$is zero. When$∑_{n=1}^{∞}2^{n}Ψ(n) = ∞$, determining the Lebesgue measure of$W_{β}(Ψ)$is less straightforward. Our main result is that whenever β is a Garsia number and$∑_{n=1}^{∞}2^{n}Ψ(n) = ∞$then$W_{β}(Ψ)\$ is a set of full measure within [0,1/(β-1)]. Our approach makes no assumptions on the monotonicity of Ψ, unlike in classical Diophantine approximation where it is often necessary to assume Ψ is decreasing.

269-287

wydano
2015

### Twórcy

autor
• School of Mathematics, The University of Manchester, Oxford Road, Manchester M13 9PL, United Kingdom