Symmetric systems of van der Pol equations. Balanov, Z., Farzamirad, M., & Krawcewicz, W. Topol. Methods Nonlinear Anal., 27(1):29--90, 2006.
bibtex   
@article{ MR2236411,
  author = {Balanov, Zalman and Farzamirad, Meymanat and Krawcewicz,
              Wies{ł}aw},
  title = {Symmetric systems of van der {P}ol equations},
  journal = {Topol. Methods Nonlinear Anal.},
  fjournal = {Topological Methods in Nonlinear Analysis},
  volume = {27},
  year = {2006},
  number = {1},
  pages = {29--90},
  issn = {1230-3429},
  mrclass = {34C25 (34C14 37C27 47H11)},
  mrnumber = {2236411 (2007b:34101)}
}

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