Preprint Article Version 1 This version is not peer-reviewed

# An E-Squence Approach to the 3x + 1 Problem

Version 1 : Received: 4 August 2019 / Approved: 6 August 2019 / Online: 6 August 2019 (08:37:48 CEST)

How to cite: Wang, S. An E-Squence Approach to the 3x + 1 Problem. Preprints 2019, 2019080071 (doi: 10.20944/preprints201908.0071.v1). Wang, S. An E-Squence Approach to the 3x + 1 Problem. Preprints 2019, 2019080071 (doi: 10.20944/preprints201908.0071.v1).

## Abstract

For any odd positive integer $x$, define $(x_n)_{n\geqslant 0}$ and $(a_n )_{n\geqslant 1}$ by setting $x_{0}=x, \,\, x_n =\cfrac{3x_{n-1} +1}{2^{a_n }}$ such that all $x_n$ are odd. The 3x+1 problem asserts that there is an $x_n =1$ for all $x$. Usually, $(x_n )_{n\geqslant 0}$ is called the trajectory of $x$. In this paper, we concentrate on $(a_n )_{n\geqslant 1}$ and call it the E-sequence of $x$. The idea is that, we consider any infinite sequence $(a_n )_{n\geqslant 1}$ of positive integers and call it an E-sequence. We then define $(a_n )_{n\geqslant 1}$ to be $\Omega-$convergent to $x$ if it is the E-sequence of $x$ and to be $\Omega-$divergent if it is not the E-sequence of any odd positive integer. We prove a remarkable fact that the $\Omega-$divergence of all non-periodic E-sequences implies the periodicity of $(x_n )_{n\geqslant 0}$ for all $x_0$. The principal results of this paper are to prove the $\Omega-$divergence of several classes of non-periodic E-sequences. Especially, we prove that all non-periodic E-sequences $(a_n )_{n\geqslant 1}$ with $\mathop {\overline {\lim } }\limits_{n\to \infty } \cfrac{b_n }{n}>\log _23$ are $\Omega-$divergent by using the Wendel's inequality and the Matthews and Watts's formula $x_n =\cfrac{3^n x_0 }{2^{b_n }}\prod\limits_{k=0}^{n-1} {(1+\cfrac{1}{3x_k })}$, where $b_n =\sum\limits_{k=1}^n {a_k }$. These results present a possible way to prove the periodicity of trajectories of all positive integers in the 3x + 1 problem and we call it the E-sequence approach.

## Subject Areas

3x+1 problem; E-sequence approach; Ω−Divergence of non-periodic E-sequences; the Wendel’s inequality