English

Nonlinear recursions on the reals and a problem of Graham

Dynamical Systems 2024-01-09 v1

Abstract

We study sequences (xn)n=1(x_n)_{n=1}^{\infty} of reals given by xn+1=f(x)x_{n+1} = f(x) where f(x)=xi=1mαixβi,f(x) = x - \sum_{i=1}^{m} \frac{\alpha_i}{x - \beta_i}, where α1,,αmR>0\alpha_1, \dots, \alpha_m \in \mathbb{R}_{>0} and β1,,βmR\beta_1, \dots, \beta_m \in \mathbb{R} are arbitrary. A special case is xn+1=xn1/xnx_{n+1} = x_n - 1/x_n due to Ronald Graham for which Chamberland \& Martelli showed that the dynamics is chaotic (topologically conjugate to the doubling map). We prove that the general nonlinear recursion, despite being potentially chaotic, is effective at ensuring that most iterates end up close to one of the poles βi\beta_i relatively quickly. More precisely, for a positive proportion of initial values xRx \in \mathbb{R}, the sequence gets very close (distance x1\lesssim |x|^{-1}) to one of the poles βi\beta_i within a relatively small (x2\lesssim x^2) number of iteration steps.

Keywords

Cite

@article{arxiv.2401.04058,
  title  = {Nonlinear recursions on the reals and a problem of Graham},
  author = {Stefan Steinerberger},
  journal= {arXiv preprint arXiv:2401.04058},
  year   = {2024}
}