1. M. Ajtai, V. Chvátal, M. Newborn, and E. Szemerédi: Crossing-free subgraphs,Ann. Discrete Math. 12 (1982), 9–12.
2. N. Alon and P. Erdős: Disjoint edges in geometric graphs,Discrete Comput. Geom. 4 (1989), 287–290.
3. S. Avital and H. Hanani: Graphs,Gilyonot Lematematika 3 (1966), 2–8 (in Hebrew).
4. Th. H. Cormen, Ch. E. Leiserson, and R. L. Rives:Introduction to Algorithms, MIT Press, Cambridge, MA, and McGraw-Hill, New York, 1990.
5. R. P. Dilworth: A decomposition theorem for partially ordered sets,Ann. of Math. 51 (1950), 161–166.