For the vertices of a finite 4-colored planar graph, regular of degree three, certain orientation indices are defined and studied. A connection is shown some elementary combinatorial results in fixed point theory. © 1966 Academic Press Inc.
Baxter, G. (1966). A necessary condition for four-coloring a planar graph. Journal of Combinatorial Theory, 1(3), 375–384. https://doi.org/10.1016/S0021-9800(66)80058-3