# Representing Ordinal Numbers with Arithmetically Interesting Sets of Real Numbers

• D. D. Blair, J. D. Hamkins, and K. O’Bryant, “Representing Ordinal Numbers with Arithmetically Interesting Sets of Real Numbers,” Mathematics arXiv, 2019. (under review)
@ARTICLE{BlairHamkinsOBryant:Representing-ordinal-numbers-with-arithmetically-interesting-sets-of-real-numbers,
author = {D. Dakota Blair and Joel David Hamkins and Kevin O'Bryant},
title = {Representing Ordinal Numbers with Arithmetically Interesting Sets of Real Numbers},
journal = {Mathematics arXiv},
year = {2019},
volume = {},
number = {},
pages = {},
month = {},
note = {under review},
abstract = {},
keywords = {under-review},
source = {},
doi = {},
url = {https://wp.me/p5M0LV-1Tg},
eprint = {1905.13123},
archivePrefix = {arXiv},
primaryClass = {math.NT},
}

Abstract. For a real number $x$ and set of natural numbers $A$, define $x∗A = \{xa \mod 1 \mid a \in A\}\subseteq [0,1)$. We consider relationships between $x$, $A$, and the order-type of $x∗A$. For example, for every irrational $x$ and order-type $\alpha$, there is an $A$ with $x ∗ A \simeq\alpha$, but if $\alpha$ is an ordinal, then $A$ must be a thin set. If, however, $A$ is restricted to be a subset of the powers of $2$, then not every order type is possible, although arbitrarily large countable well orders arise.