On the constant congruence speed of tetration

Marco Ripà
Notes on Number Theory and Discrete Mathematics
Print ISSN 1310–5132, Online ISSN 2367–8275
Volume 26, 2020, Number 3, Pages 245—260
DOI: 10.7546/nntdm.2020.26.3.245-260
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Marco Ripà
sPIqr Society, World Intelligence Network
Rome, Italy

Abstract

Integer tetration, the iterated exponentiation ba for a ∈ ℕ − {0, 1}, is characterized by fascinating periodicity properties involving its rightmost figures, in any numeral system. Taking into account a radix-10 number system, in the book “La strana coda della serie n ^ n ^ … ^ n” (2011), the author analyzed how many new stable digits are generated by every unitary increment of the hyperexponent b, and he indicated this value as V(a) or “congruence speed” of a ≢ 0 (mod 10). A few conjectures about V(a) arose. If b is sufficiently large, the congruence speed does not depend on b, taking on a (strictly positive) unique value. We derive the formula that describes V(a) for every a ending in 5. Moreover, we claim that V(a) = 1 for any a (mod 25) ∈ {2, 3, 4, 6, 8, 9, 11, 12, 13, 14, 16, 17, 19, 21, 22, 23} and V(a) ≥ 2 otherwise. Finally, we show the size of the fundamental period P for any of the remaining values of the congruence speed: if V(a) ≥ 2, then P(V(a)) = 10V(a)+1.

Keywords

  • Number theory
  • Power tower
  • Tetration
  • Hyperoperation
  • Charmichael function
  • Euler’s totient function
  • Primitive root
  • Exponentiation
  • Integer sequence
  • Congruence speed
  • Modular arithmetic
  • Stable digit
  • Rightmost digit
  • Cycle
  • Periodicity

2010 Mathematics Subject Classification

  • 11A07
  • 11F33

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Cite this paper

Ripà, M. (2020). On the constant congruence speed of tetration. Notes on Number Theory and Discrete Mathematics, 26 (3), 245-260, doi: 10.7546/nntdm.2020.26.3.245-260.

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