English

Descriptive complexity in Cantor series

Logic 2020-10-28 v1 Number Theory

Abstract

A Cantor series expansion for a real number xx with respect to a basic sequence Q=(q1,q2,)Q=(q_1,q_2,\dots), where qi2q_i \geq 2, is a representation of the form x=a0+i=1aiq1q2qix=a_0 + \sum_{i=1}^\infty \frac{a_i}{q_1q_2\cdots q_i} where 0ai<qi0 \leq a_i<q_i. These generalize ordinary base bb expansions where qi=bq_i=b. Ki and Linton showed that for ordinary base bb expansions the set of normal numbers is a Π30\boldsymbol{\Pi}^0_3-complete set, establishing the exact complexity of this set. In the case of Cantor series there are three natural notions of normality: normality, ratio normality,and distribution normality (these notions are equivalent for base bb expansions). We show that for any QQ the set DN(Q)\mathscr{DN}(Q) of distribution normal number is Π30\boldsymbol{\Pi}^0_3-complete, and if QQ is 11-divergent (i.e., i=11qi\sum_{i=1}^\infty \frac{1}{q_i} diverges) then the sets N(Q)\mathscr{N}(Q) and RN(Q)\mathscr{RN}(Q) of normal and ratio normal numbers are Π30\boldsymbol{\Pi}^0_3-complete. We further show that all five non-trivial differences of these sets are D2(Π30)D_2(\boldsymbol{\Pi}^0_3)-complete if limiqi=\lim_i q_i=\infty and QQ is 11-divergent (the trivial case is N(Q)RN(Q)=\mathscr{N}(Q)\setminus \mathscr{RN}(Q)=\emptyset). This shows that except for the containment N(Q)RN(Q)\mathscr{N}(Q)\subseteq \mathscr{RN}(Q), these three notions are as independent as possible.

Keywords

Cite

@article{arxiv.2010.13947,
  title  = {Descriptive complexity in Cantor series},
  author = {Dylain Airey and Steve Jackson and Bill Mance},
  journal= {arXiv preprint arXiv:2010.13947},
  year   = {2020}
}

Comments

22 pages

R2 v1 2026-06-23T19:40:11.827Z