1. D. Cartwright, F. Harary, On Colorings of Signed Graphs Elem. Math. (to appear)
2. Planar Permutation Graphs;Chartrand;Ann. Inst. H. Poincaré,1967
3. Grötzsch's Theorem on 3-Colorings;Grünbaum;Michigan Math. J.,1963
4. Map Color Theorems;Heawood;Quart. J. Math.,1890