@article{OehbergSurkovTittonenetal.1997,
  author    = {P. {\"O}hberg and E. L. Surkov and I. Tittonen and Stig Stenholm and Martin Wilkens and G. V. Shlyapnikov},
  title     = {Low-energy elementary excitations of a trapped Bose-condensed gas},
  year      = {1997},
}