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Isochronous centres and foci via commutators and normal forms

Published online by Cambridge University Press:  05 February 2008

A. Algaba
Affiliation:
Departamento de Matemáticas, Facultad Ciencias Experimentales, Universidad de Huelva, Avenida de las Fuerzas Armadas, 21071 Huelva, Spain ([email protected]; [email protected])
M. Reyes
Affiliation:
Departamento de Matemáticas, Facultad Ciencias Experimentales, Universidad de Huelva, Avenida de las Fuerzas Armadas, 21071 Huelva, Spain ([email protected]; [email protected])

Abstract

Consider the two-dimensional autonomous systems of differential equations

$$ \dot{x}=-y+\lambda x+P(x,y),\qquad\dot{y}=x+\lambda y+Q(x,y), $$

where $\lambda$ is a real constant and $P$ and $Q$ are $\mathcal{C}^{\infty}$-functions of order greater than or equal to two. These systems, so-called centre-focus-type systems, have either a centre or a focus at the origin. In this work, we give necessary and sufficient conditions of isochronicity using normal forms. We characterize the systems which have either an isochronous centre or an isochronous focus at the origin by means of the existence of a commutator of the field. Moreover, we prove that the maximum order of a weak isochronous focus for quadratic systems is two, and that for systems with cubic nonlinearities is three.

Type
Research Article
Copyright
2008 Royal Society of Edinburgh

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