Biasco, L., &; Di Gregorio, L. (2009). A Birkhoff–Lewis Type Theorem for the Nonlinear Wave Equation. Archive for Rational Mechanics and Analysis, 303–362.https://doi.org/10.1007/s00205-009-0240-y 

Melbourne, I., Dellnitz, M., &; Golubitsky, M. (1993). The structure of symmetric attractors. Archive for Rational Mechanics and Analysis, 75–98.https://doi.org/10.1007/bf00386369