Carmichael numbers and least common multiples of $p-1$

Abstract

For a Carmichael number $n$ with prime factors $p_1,\cdots,p_m$, define $$K=GCD[p_1-1,\cdots,p_m-1],$$ and let $C_ν(X)$ denote the number of Carmichael numbers up to $X$ such that $K=ν$. Assuming a strong conjecture on the first prime in an arithmetic progression, we prove that for any even natural number $ν$, $$C_ν(X)\geq X^{1-(2+o(1))\frac{\log\log\log \log X}{\log\log\log X}}.$$ This is a departure from standard constructions of Carmichael numbers, which generally require $K$ to grow along with $n$.