An approximation of the Collatz map and a lower bound for the average total stopping time

Manuel Inselmann

Published 2024-02-05, updated 2024-02-08Version 2

Define the (accelerated) Collatz map on the positive integers by $\mathsf{Col}_2(n)=\frac{n}{2}$ if $n$ is even and $\mathsf{Col}_2(n)=\frac{3n+1}{2}$ if $n$ is odd. We show that $\mathsf{Col}_2$ can be approximated by multiplication with $\frac{3^{\frac{1}{2}}}{2}$ in the sense that the set of $n$ for which $(\frac{3^{\frac{1}{2}}}{2})^kn^{1-\epsilon}\leq \mathsf{Col}_2^k(n)\leq (\frac{3^{\frac{1}{2}}}{2})^kn^{1+\epsilon}$ for all $0\leq k\leq 2(\log\frac{4}{3})^{-1}\log n\approx 6.952\log n$ has natural density $1$ for every $\epsilon>0$. Let $\tau(n)$ be the minimal $k\in\mathbb{N}$ for which $\mathsf{Col}_2^k(n)=1$ if there exist such a $k$ and set $\tau(n)=\infty$ otherwise. As an application of the above we show that $\liminf_{x\rightarrow\infty}\frac{1}{x\log x}\sum_{m=1}^{\lfloor x\rfloor}\tau(m)\geq 2(\log\frac{4}{3})^{-1}$, partially answering a question of Crandall and Shanks. We show also that assuming the Collatz Conjecture is true in the strong sense that $\tau(n)\in O(\log n)$, then $\lim_{x\rightarrow\infty}\frac{1}{x\log x}\sum_{m=1}^{\lfloor x\rfloor}\tau(m)= 2(\log\frac{4}{3})^{-1}$.