English

On small bases for which $1$ has countably many expansions

Number Theory 2016-01-27 v1

Abstract

Let q(1,2)q\in(1,2). A qq-expansion of a number xx in [0,1q1][0,\frac{1}{q-1}] is a sequence (δi)i=1{0,1}N(\delta_i)_{i=1}^\infty\in\{0,1\}^{\mathbb{N}} satisfying x=i=1δiqi. x=\sum_{i=1}^\infty\frac{\delta_i}{q^i}. Let B0\mathcal{B}_{\aleph_0} denote the set of qq for which there exists xx with a countable number of qq-expansions, and let B1,0\mathcal{B}_{1, \aleph_0} denote the set of qq for which 11 has a countable number of qq-expansions. In \cite{Sidorov6} it was shown that minB0=minB1,0=1+52,\min\mathcal{B}_{\aleph_0}=\min\mathcal{B}_{1,\aleph_0}=\frac{1+\sqrt{5}}{2}, and in \cite{Baker} it was shown that B0(1+52,q1]={q1}\mathcal{B}_{\aleph_0}\cap(\frac{1+\sqrt{5}}{2}, q_1]=\{ q_1\}, where q1(1.64541)q_1(\approx1.64541) is the positive root of x6x4x32x2x1=0x^6-x^4-x^3-2x^2-x-1=0. In this paper we show that the second smallest point of B1,0\mathcal{B}_{1,\aleph_0} is q3(1.68042)q_3(\approx1.68042), the positive root of x5x4x3x+1=0x^5-x^4-x^3-x+1=0. Enroute to proving this result we show that B0(q1,q3]={q2,q3}\mathcal{B}_{\aleph_0}\cap(q_1, q_3]=\{ q_2, q_3\}, where q2(1.65462)q_2(\approx1.65462) is the positive root of x62x4x31=0x^6-2x^4-x^3-1=0.

Keywords

Cite

@article{arxiv.1502.07212,
  title  = {On small bases for which $1$ has countably many expansions},
  author = {Yuru Zou and Lijin Wang and Jian Lu and Simon Baker},
  journal= {arXiv preprint arXiv:1502.07212},
  year   = {2016}
}

Comments

14 pages, 2 figures

R2 v1 2026-06-22T08:37:48.363Z