Solution

ID: past-exam-of-the-probability-statistics-and-options-i-course-of-the-university-of-oxford/2010/r/1/ii/b/solution

The modified graph is planar. Place and , and place the other vertices on the vertical line between them in the bottom-to-top order . Draw each edge as a straight segment. Each half of the drawing is a fan sharing one endpoint, so its edges do not cross. The added edges and are consecutive vertical segments, meeting other edges only at their endpoints. The following original drawing exhibits the embedding.
Figure 1.
A planar embedding and a three-colouring of the modified complete bipartite graph
. Details.

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