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Finite-State Dimension and the Davenport-Erdős Theorem

Authors: Joe Clanin and Matthew Rayman

Published in: LIPIcs, Volume 386, 51st International Symposium on Mathematical Foundations of Computer Science (MFCS 2026)


Abstract
A 1952 result of Davenport and Erdős states that if p is an integer-valued polynomial, then the real number 0.p(1)p(2)p(3)… is Borel normal in base ten. A later result of Nakai and Shiokawa extends this result to polynomials with arbitrary real coefficients and all bases b ≥ 2. It is well-known that finite-state dimension, a finite-state effectivization of the classical Hausdorff dimension, characterizes the Borel normal sequences as precisely those sequences of finite-state dimension 1. For an infinite set A of natural numbers, and a base b ≥ 2, the base-b Copeland-Erdős sequence of A, CE_b(A), is the infinite sequence obtained by concatenating the base-b expansions of the numbers in A in increasing order. In this work we investigate the possible relationships between the finite-state dimensions of CE_b(A) and CE_b(p(A)) where p is a polynomial. We show that, if the polynomial is permitted to have arbitrary real coefficients, then for any s,s^′ in the unit interval, there is a set A of natural numbers and a linear polynomial p so that the finite-state dimensions of CE_b(A) and CE_b(p(A)) are s and s^′ respectively. The corresponding result for strong finite-state dimension is also shown. We demonstrate that linear polynomials with rational coefficients do not change the finite-state dimension of any Copeland-Erdős sequence, but there exist polynomials with rational coefficients of every larger integer degree that change the finite-state dimension of some sequence. We also prove the surprising fact that there exist sets A and integer-valued monomials p such that CE_b(A) is normal, but CE_b(p(A)) has finite-state dimension strictly less than one.

Cite as

Joe Clanin and Matthew Rayman. Finite-State Dimension and the Davenport-Erdős Theorem. In 51st International Symposium on Mathematical Foundations of Computer Science (MFCS 2026). Leibniz International Proceedings in Informatics (LIPIcs), Volume 386, pp. 38:1-38:15, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2026)


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@InProceedings{clanin_et_al:LIPIcs.MFCS.2026.38,
  author =	{Clanin, Joe and Rayman, Matthew},
  title =	{{Finite-State Dimension and the Davenport-Erd\H{o}s Theorem}},
  booktitle =	{51st International Symposium on Mathematical Foundations of Computer Science (MFCS 2026)},
  pages =	{38:1--38:15},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-442-0},
  ISSN =	{1868-8969},
  year =	{2026},
  volume =	{386},
  editor =	{Kouck\'{y}, Michal and Petrișan, Daniela},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.MFCS.2026.38},
  URN =		{urn:nbn:de:0030-drops-274195},
  doi =		{10.4230/LIPIcs.MFCS.2026.38},
  annote =	{Keywords: Normal numbers, finite-state dimension, polynomials}
}
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