@article{10272/25457,
year = {2008},
month = {2},
url = {https://hdl.handle.net/10272/25457},
abstract = {Consider the two-dimensional autonomous systems of differential equations ẋ = -y + λx + P(x,y), ẏ = x + λy + Q(x,y), where λ is a real constant and P and Q are C∞-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. © 2008 The Royal Society of Edinburgh.},
organization = {This work has been supported by the Ministerio de Ciencia y Tecnología,
fondos FEDER (project MTM2004-04066) and by the Conserjería de E\-ducación y Ciencia de la Junta de
Andalucia (project FQM 276).},
publisher = {Cambridge University Press},
title = {Isochronous centres and foci via commutators and normal forms},
doi = {10.1017/S0308210506000382},
author = {Algaba Durán, Antonio and Reyes Columé, Manuel},
}