The congruence speed formula

Marco Ripà
Notes on Number Theory and Discrete Mathematics
Print ISSN 1310–5132, Online ISSN 2367–8275
Volume 27, 2021, Number 4, Pages 43–61
DOI: 10.7546/nntdm.2021.27.4.43-61
Full paper (PDF, 227 Kb)

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Authors and affiliations

Marco Ripà
sPIqr Society, World Intelligence Network
Rome, Italy

Abstract

We solve a few open problems related to a peculiar property of the integer tetration ba, which is the constancy of its congruence speed for any sufficiently large b = b(a). Assuming radix-10 (the well known decimal numeral system), we provide an explicit formula for the congruence speed V(a) ∈ ℕ0 of any a ∈ ℕ − {0} that is not a multiple of 10. In particular, for any given n ∈ ℕ, we prove to be true Ripà’s conjecture on the smallest a such that V(a) = n. Moreover, for any a ≠ 1 ∶ a ≢ 0 (mod 10), we show the existence of infinitely many prime numbers, pj = pj(V(a)), such that V(pj) = V(a).

Keywords

  • Tetration
  • Decadic number
  • Exponentiation
  • Integer sequence
  • Congruence speed
  • Modular arithmetic
  • Radix-10
  • Dirichlet’s theorem
  • Arithmetic progression
  • Prime number

2020 Mathematics Subject Classification

  • 11A07
  • 11N13

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  2. Ripà, M. (2020). On the constant congruence speed of tetration, Notes on Number Theory and Discrete Mathematics, 26(3), 245–260.
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Cite this paper

Ripà, M. (2021). The congruence speed formula. Notes on Number Theory and Discrete Mathematics, 27(4), 43-61, DOI: 10.7546/nntdm.2021.27.4.43-61.

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