1. J. Peck, oral communication.
2. Graphs and Hypergraphs;Berge,1973
3. Sub modular functions, matroids and certain polyhedra;Edmonds,1970
4. On a combinatorial problem II;Erdös;Acta Math. Acad. Sci. Hungar.,1964
5. Maximum-minimum Sätze und verallgemeinerte Faktoren von Graphen;Gallai;Acta Math. Acad. Sci. Hungar.,1961