1. 1.. F. Allaire and E. R. Swart, A systematic approach to the determination of reducible configurations, J. Combinatorial Theory Ser. B (to appear).
2. The existence of unavoidable sets of geographically good configurations;Appel, K.;Illinois J. Math.,1976
3. B. I. Hochschulskripten;Heesch, Heinrich,1969
4. Kempe chains and the four colour problem;Whitney, Hassler;Utilitas Math.,1972