@article{DBLP:journals/logcom/HalpernR07,
  author    = {Joseph Y. Halpern and
               Leandro Chaves R{\^e}go},
  title     = {Characterizing the NP-PSPACE Gap in the Satisfiability Problem
               for Modal Logic},
  journal   = {J. Log. Comput.},
  volume    = {17},
  number    = {4},
  year      = {2007},
  pages     = {795-806},
  ee        = {http://dx.doi.org/10.1093/logcom/exm029},
  bibsource = {DBLP, http://dblp.uni-trier.de}
}