On the complete integrability of a nonlinear oscillator from group   theoretical perspective
  In this paper, we investigate the integrability aspects of a physically important nonlinear oscillator which lacks sufficient number of Lie point symmetries but can be integrated by quadrature. We explore the hidden symmetry, construct a second integral and derive the general solution of this oscillator by employing the recently introduced $\lambda$-symmetry approach and thereby establish the complete integrability of this nonlinear oscillator equation from a group theoretical perspective. 
A. Bhuvaneswari, V. K. Chandrasekar, M. Senthilvelan and M. Lakshmanan
nlin.SI