1. The complexity of theorem-proving procedures;Cook,1971
2. Computers and Intractibility, A Guide to the Theory of NP-Completeness;Garey,1979
3. Reducibility among combinatorial problems;Karp,1972
4. On cliques in graphs;Moon;Israël J. Math.,1965
5. A property of finitely generated submonoids;Schützenberger,1976