The entire chromatic number of a normal graph is at most seven
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A multigraph is said to be normal if it is embedded in the plane such that each vertex is adjacent to exactly three edges and three regions. In [2], G. Ringel showed that the vertices and regions of a normal multigraph can be colored with six colors such that adjacent elements are colored differently. In this note we consider the problem of coloring vertices, regions, and edges of normal multigraphs. Formally, the entire chromatic number of a plane multigraph G is the fewest number of colors required to color the vertices, regions, and edges of G so that adjacent elements are colored differently. Here a region is adjacent to the vertices and edges which are on its boundary. Also a vertex is adjacent to its incident edges. In [1], H. Izbicki reported that by assuming the four color conjecture M. Neuberger has proved
[1] G. Ringel. Ein Sechsfarbenproblem auf der Kugel , 1965 .