Prove Tait’s theorem about planar cubic bridgeless graph being 3-edge-colorable

coloringgraph theory

How can be proved, that

The four-color theorem is equivalent to the claim that every planar cubic bridgeless graph is 3-edge-colorable.

I can't figure out or find any prove of this theorem.

Thanks.

Best Answer

The four colors can be represented by bit-vectors of length 2.

We can define $3$ transformations, $L, R, B$ on these bit-vectors, where $L$ inverts the left-bit, $R$ inverts the right-bit, and $B$ inverts both bits. Each of these is bijective with no fixed point.

Note that composing all three of these gives the identity, and composing any two of them gives the third.

Now consider a maximal planar graph. If the vertices can be $4$-colored then each edge determines one of these transformations and the edges of a triangle must have all $3$ transformations in order to compose to the identity transformation.

Conversely, if the edges can be $3$-colored such that the $3$ edges of each triangle have different colors, then $L,R,B$ can be used for the colors, and the transformations can be used to extend a color on a starting vertex to the whole graph.