Nonlinear recursions on the reals and a problem of Graham

Abstract

We study sequences $(x_n)_{n=1}^{\infty}$ of reals given by $x_{n+1} = f(x)$ where $$f(x) = x - \sum_{i=1}^{m} \frac{α_i}{x - β_i},$$ where $α_1, \dots, α_m \in \mathbb{R}_{>0}$ and $β_1, \dots, β_m \in \mathbb{R}$ are arbitrary. A special case is $x_{n+1} = x_n - 1/x_n$ 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 $β_i$ relatively quickly. More precisely, for a positive proportion of initial values $x \in \mathbb{R}$, the sequence gets very close (distance $\lesssim |x|^{-1}$) to one of the poles $β_i$ within a relatively small ($\lesssim x^2$) number of iteration steps.

Figures (2)

• Figure 1: $f(x) = x-1/x$ and $f^{(8)}(x)$, both on $[-3,3]$.
• Figure 2: A sketch of iterating $f(x) = x-1/x$. Points bigger than 1 get sent to point in $[0,1]$ which then moves to a point $\leq -1$ which moves to $[-1,0]$ which is sent to a point $\geq 1$ and so on.

Theorems & Definitions (9)

• Theorem : Chamberland & Martelli chamberland
• Theorem
• Lemma 1
• proof
• Lemma 2
• proof
• Theorem : Glasser's Master Theorem
• Lemma 3
• proof