1. A greedy heuristic for the set-covering problem;Chvátal;Math. Oper. Res.,1979
2. P. Crescenzi, V. Kann, 1995, A Compendium of NP Optimization Problems, Department of Computer Science, University of Rome “La Sapienza”
3. U. Feige, A thereshold of lnn, Proceedings of the ACM Symposium on Theory of Computing, 1996, 314, 318
4. Computers and Intractability: A Guide to the Theory of NP-Completeness;Garey,1979
5. M. M. Halldórsson, Approximating discrete collections via local improvements, Proceedings of the 6th Annual ACM-SIAM Symposium on Discrete Algorithms, 1995