Complete classification of second-order symmetric spacetimes

@inproceedings{Blanco2010CompleteCO,
  title={Complete classification of second-order symmetric spacetimes},
  author={Oihane F. Blanco and Miguel S{\'a}nchez and Jos'e M. M. Senovilla},
  year={2010}
}
We present the explicit local form of the metric of the second-order symmetric non-symmetric 4-dimensional Lorentzian manifolds. They turn out to be a specific subclass of plane waves.