1. Reducibility among combinatorial problems;Karp,1972
2. Computers and Intractability: A Guide to the Theory of NP-Completeness;Garey,1979
3. Graph Algorithms;Even,1979
4. A greedy-heuristic for the set-covering problem;Chvátal;Math. Operations Res.,1979
5. Approximation algorithm for the weighted set covering and node cover problems;Hochbaum,1980