1 Introduction
The interest for isochrons of limit cycles appeared first in relation with mathematics of biosciences. Their use was particularly emphazised by Winfree [1] and latter appeared repeatedly in many classical references related with biological rhythms [2–5]. Under the influence of Winfree's articles, Guckenheimer [6] identified the isochrons with the stable manifold of a point on an attractive hyperbolic limit cycle. Another proof of their existence and a discussion of the case of non-hyperbolic limit cycles appeared in [7]. M. Sabatini [8,9] proved that the existence of isochron sections for a limit cycle of a vector field X is equivalent to the existence of a vector field Y such that the bracket
2 Definition of isochrons and of asymptotic phase
Existence of isochrons can be deduced from [13]. Let
3 One-parameter family of integrating factor and approximation to limit cycle
Given a one-form ω, a function H and a transverse section Σ, consider the continuous family of regular ovals
Define the ramified primitive
Define the polynomial D by
Define finally the function
We have thus obtained that any polynomial one-form ω can be written
This construction yields (recursively) a construction of a 1-parameter family of integrating factor (cf. [12]). We stick here to the first-order approximation, which displays
4 Van der Pol oscillator
Consider the van der Pol oscillator:
The associated foliation of the plane can be defined by the 1-form
5 Isochronous forms to a vector field
Assume that
Definition 1
A 1-form
For instance, if
Consider now a 1-parameter perturbation:
Definition 2
A 1-form
Construction of a 1-isochronous form proceeds in the same way as above. The unknown form
6 Isochrons for the van der Pol system
In the case of the van der Pol system, we obtain that:
Acknowledgements
This article was motivated by discussions with P. Baconnier in Saint-Flour (France). Financial support of the ACI ‘Nouvelles interactions des mathématiques instantbio’, which made possible such interactions, is gratefully acknowledged.
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