On the cohomology of hyperkahler quotients
  This paper gives a partial desingularisation construction for hyperk\"ahler quotients and a criterion for the surjectivity of an analogue of the Kirwan map to the cohomology of hyperk\"ahler quotients. This criterion is applied to some linear actions on hyperk\"ahler vector spaces. 
Lisa Jeffrey, Young-Hoon Kiem and Frances Kirwan