Classification of symmetries for higher order Lagrangian systems II: The non-autonomous case. Manuel de León, David Martín de Diego (1994) Extracta Mathematicae
Classification of symmetries for higher order Lagrangian systems. Manuel de León, David Martín de Diego (1994) Extracta Mathematicae
Comments on the dynamics of the Pais-Uhlenbeck oscillator. Smilga, Andrei V. (2009) SIGMA. Symmetry, Integrability and Geometry: Methods and Applications [electronic only]