頂点被覆
↑ Gallai, Tibor "Über extreme Punkt- und Kantenmengen." Ann. Univ. Sci. Budapest, Eotvos Sect. Math. 2, 133-138, 1959.
↑ Vazirani, Vijay V. (2001). Approximation Algorithms. Springer-Verlag. pp. pp.122-123. ISBN 3-540-65367-8
↑ Garey, M. R.; Johnson, D. S.; Stockmeyer, L. (1974), “Some simplified NP-complete problems”, Proceedings of the sixth annual ACM symposium on Theory of computing, pp. 47–63, http://portal.acm.org/citation.cfm?id=803884
↑ Garey, M. R.; D. S. Johnson (1977). “The rectilinear Steiner tree problem is NP-complete”.