Twist number and order properties of periodic orbits

TL;DR

This paper investigates the twist number of periodic orbits in area-preserving twist maps, providing new definitions, an algorithm for exact computation, and insights into their order properties and bifurcations.

Contribution

It introduces a new algorithm to determine the twist number, relates it to orbit ordering, and presents a novel 1-cone function to analyze symmetric periodic orbits.

Findings

Deduced the exact or approximate twist number for periodic orbits.

Established that period-doubling bifurcations increase the twist number.

Identified limitations of residue-based characterization for large twist orbits.

Abstract

A less studied numerical characteristic of periodic orbits of area preserving twist maps of the annulus is the twist or torsion number, called initially the amount of rotation. It measures the average rotation of tangent vectors under the action of the derivative of the map along that orbit, and characterizes the degree of complexity of the dynamics. The aim of this paper is to give new insights into the definition and properties of the twist number, and to relate its range to the order properties of periodic orbits. We derive an algorithm to deduce the exact value or a demi--unit interval containing the exact value of the twist number. We prove that at a period-doubling bifurcation threshold of a mini-maximizing periodic orbit, the new born doubly periodic orbit has the absolute twist number larger than the absolute twist of the original orbit after bifurcation. We give examples of periodic orbits having large absolute twist number, that are badly ordered, and illustrate how characterization of these orbits only by their residue can lead to incorrect results. In connection to the study of the twist number of periodic orbits of standard--like maps we introduce a new tool, called 1-cone function. We prove that the location of minima of this function with respect to the vertical symmetry lines of a standard-like map encodes a valuable information on the symmetric periodic orbits and their twist number.