The cyclic spectrum of a Boolean flow. Kennison, John F. (2002) Theory and Applications of Categories [electronic only]
The period function near a polycycle with two semi-hyperbolic vertices Angélica Mansilla, Mariana Saavedra (2001) Annales mathématiques Blaise Pascal
Travaux de Moser sur la stabilité des mouvements périodiques René Thom (1962/1964) Séminaire Bourbaki