In collaboration with W. Haken. An unavoidable set of configurations in planar triangulations. Every planar map is four colorable. Illinois J. Math. Every planar map is four colorable.
Every planar map is four colorable.
A proof of the four color theorem. Every planar map is four colorable. The existence of unavoidable sets of geographically good configurations. Every planar map is four colorable. With the collaboration of J. Koch.