@article{bianchi1992ode,
  title={An ODE approach to the equation, in R n, in R n},
  author={Bianchi, G. and Egnell, H.},
  journal={Mathematische Zeitschrift},
  volume={210},
  number={1},
  pages={137--166},
  year={1992},
  publisher={Springer}
}