8 Sep
1992
8 Sep
'92
2:43 p.m.
Please also note "A categorical characterization of the four colour theorem"by Barry Fawcett, Canad. Math. Bull. Vol. 29 (4), 1986, perhaps the main result of which is the equivalence of the four colour theorem with the statement that epimorphisms are surjective in the category of planar graphs. Wood ==============================================================================