1. P. Erdös, personal communication to P. Lemke, May 1987.
2. J. Selfridge, personal communication to P. Erdös, December 1987.
3. Two combinatorial problems in group theory;Eggleton;Acta Arith.,1972
4. An addition theorem modulo p;Olson;J. Combin. Theory,1968
5. On a conjecture of Erdös and Heilbronn;Szemeréd;Acta Arith.,1970