1. Garey MR, Johnson DS. Computers and Intractability: New York, WH Freeman, 1979.
2. Lawler EL, Lenstra JK, Rinnooy AHG, Shmoys DB. The Travelling Salesman Problem. Chichester: Wiley, 1985.
3. New Scientist, June 27 1992.
4. Lin S, Kernighan BW. Operat. Res. 1973; 21: 516.
5. Durbin R, Willshaw. Nature. 1987; 326: 689?691.