@article{journals/jacm/GottlobKS04,
  title = "Existential second-order logic over graphs: Charting the tractability frontier",
  author = "Georg Gottlob and Phokion G. Kolaitis and Thomas Schwentick",
  year = "2004",
  journal = "J. {ACM}",
  number = "2",
  pages = "312-362",
  volume = "51",
  doi = "10.1145/972639.972646",
}