English

$(a,a)$-Carmichael numbers and greatest common divisors of $p-a$

Number Theory 2026-07-02 v1

Abstract

Define an (a,a)(a,a)-Carmichael number to be a squarefree natural number nn such that pnp\mid n implies panap-a\mid n-a. For such a number nn with prime factors p1,,pmp_1,\cdots,p_m, define K=GCD[p1a,,pma],K=GCD[p_1-a,\cdots,p_m-a], and let Cν(X,a)C_\nu(X,a) denote the number of (a,a)(a,a)-Carmichael numbers up to XX such that K=νK=\nu. Assuming a strong conjecture on the first prime in an arithmetic progression, we prove that for any integer aa and for any natural number ν\nu with (ν,a)=1(\nu,a)=1 and aa and ν\nu having opposite parity, Cν(X,a)X1(2+o(1))loglogloglogXlogloglogX.C_\nu(X,a)\geq X^{1-(2+o(1))\frac{\log\log\log \log X}{\log\log\log X}}. This is a departure from many traditional constructions of Carmichael numbers, which generally require KK to grow along with nn.

Keywords

Cite

@article{arxiv.2607.02738,
  title  = {$(a,a)$-Carmichael numbers and greatest common divisors of $p-a$},
  author = {Thomas Wright},
  journal= {arXiv preprint arXiv:2607.02738},
  year   = {2026}
}

Comments

This is a significant revision and generalization of arXiv:2409.16397, since that paper is now largely subsumed by Larsen's result on Carmichael numbers in arithmetic progressions