We extend the classical Belitskii normal form theorem uniformly for planar nilpotent foci and limit cycles. Then by using fractal analysis, isochronous properties near such invariant sets are well investigated. © 2010 Elsevier Masson SAS.
Wu, H., & Li, W. (2010). Isochronous properties in fractal analysis of some planar vector fields. Bulletin Des Sciences Mathematiques, 134(8), 857–873. https://doi.org/10.1016/j.bulsci.2010.03.006