G-actions and the Fundamental Group. William Browder, Wu-Chung Hsiang (1981/1982) Inventiones mathematicae
Generalized Hénon difference equations with delay. Kennedy, Judy A., Yorke, James A. (2003) Zeszyty Naukowe Uniwersytetu Jagiellońskiego. Universitatis Iagellonicae Acta Mathematica