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Dynamics of the $ w$ function and the Green-Tao theorem on arithmetic progressions in the primes
Authors:Yong-Gao Chen  Ying Shi
Institution:Department of Mathematics, Nanjing Normal University, Nanjing 210097, People's Republic of China ; Department of Mathematics, Nanjing Normal University, Nanjing 210097, People's Republic of China
Abstract:Let $ A_3$ be the set of all positive integers $ pqr$, where $ p,q,r$ are primes and possibly two, but not all three of them are equal. For any $ n=pqr\in A_{3} $, define a function $ w$ by $ w(n)=P(p+q)P(p+r)P(q+r),$ where $ P(m)$ is the largest prime factor of $ m$. It is clear that if $ n=pqr\in A_{3} $, then $ w(n) \in A_3$. For any $ n\in A_{3}$, define $ w^{0}(n)=n$, $ w^{i}(n)=w(w^{i-1}(n))$ for $ i=1,~2,~\ldots $. An element $ n\in A_{3}$ is semi-periodic if there exists a nonnegative integer $ s$ and a positive integer $ t$ such that $ w^{s + t}(n)= w^{s}(n)$. We use ind$ (n)$ to denote the least such nonnegative integer $ s$. Wushi Goldring Dynamics of the $ w$ function and primes, J. Number Theory 119(2006), 86-98] proved that any element $ n\in A_{3}$ is semi-periodic. He showed that there exists $ i$ such that $ w^{i}(n)\in\{20,98,63,75\}$, ind$ (n)\leqslant 4(\pi(P(n))-3)$, and conjectured that ind$ (n)$ can be arbitrarily large.

In this paper, it is proved that for any $ n\in A_{3}$ we have ind$ (n)=$ $ O((\log P(n))^2)$, and the Green-Tao Theorem on arithmetic progressions in the primes is employed to confirm Goldring's above conjecture.

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