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DIRECTIONS SETS: A GENERALISATION OF RATIO SETS

Directions sets: a generalisation of ratio sets
Authors: PAOLO LEONETTI; CARLO SANNA;

DIRECTIONS SETS: A GENERALISATION OF RATIO SETS

Abstract

For every integer$k\geq 2$and every$A\subseteq \mathbb{N}$, we define the$k$-directions setsof$A$as$D^{k}(A):=\{\boldsymbol{a}/\Vert \boldsymbol{a}\Vert :\boldsymbol{a}\in A^{k}\}$and$D^{\text{}\underline{k}}(A):=\{\boldsymbol{a}/\Vert \boldsymbol{a}\Vert :\boldsymbol{a}\in A^{\text{}\underline{k}}\}$, where$\Vert \cdot \Vert$is the Euclidean norm and$A^{\text{}\underline{k}}:=\{\boldsymbol{a}\in A^{k}:a_{i}\neq a_{j}\text{ for all }i\neq j\}$. Via an appropriate homeomorphism,$D^{k}(A)$is a generalisation of theratio set$R(A):=\{a/b:a,b\in A\}$. We study$D^{k}(A)$and$D^{\text{}\underline{k}}(A)$as subspaces of$S^{k-1}:=\{\boldsymbol{x}\in [0,1]^{k}:\Vert \boldsymbol{x}\Vert =1\}$. In particular, generalising a result of Bukor and Tóth, we provide a characterisation of the sets$X\subseteq S^{k-1}$such that there exists$A\subseteq \mathbb{N}$satisfying$D^{\text{}\underline{k}}(A)^{\prime }=X$, where$Y^{\prime }$denotes the set of accumulation points of$Y$. Moreover, we provide a simple sufficient condition for$D^{k}(A)$to be dense in$S^{k-1}$. We conclude with questions for further research.

Keywords

accumulation points, Mathematics - Number Theory, Density, gaps, topology, FOS: Mathematics, ratio sets, closure, Number Theory (math.NT), Elementary number theory, 11B05, 11A99, 2010 Mathematics subject classification; primary 11B05; secondary 11A99

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selected citations
These citations are derived from selected sources.
This is an alternative to the "Influence" indicator, which also reflects the overall/total impact of an article in the research community at large, based on the underlying citation network (diachronically).
BIP!Citations provided by BIP!
popularity
This indicator reflects the "current" impact/attention (the "hype") of an article in the research community at large, based on the underlying citation network.
BIP!Popularity provided by BIP!
influence
This indicator reflects the overall/total impact of an article in the research community at large, based on the underlying citation network (diachronically).
BIP!Influence provided by BIP!
impulse
This indicator reflects the initial momentum of an article directly after its publication, based on the underlying citation network.
BIP!Impulse provided by BIP!
2
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