The No-ghost Theorem for AdS_3 and the Stringy Exclusion Principle
  A complete proof of the No-ghost Theorem for bosonic and fermionic string theories on AdS_3, or the group manifold of SU(1,1), is given. It is then shown that the restriction on the spin (in terms of the level) that is necessary to obtain a ghost-free spectrum corresponds to the stringy exclusion principle of Maldacena and Strominger. 
J.M. Evans, M.R. Gaberdiel, M.J. Perry