  A formal proof has not been found for the four color theorem since 1852 when
Francis Guthrie first conjectured the four color theorem. Why? A bad idea, we
think, directed people to a rough road. Using a similar method to that for the
formal proof of the five color theorem, a formal proof is proposed in this
paper of the four color theorem, namely, every planar graph is four-colorable.
The formal proof proposed can also be regarded as an algorithm to color a
planar graph using four colors so that no two adjacent vertices receive the
same color.
