K. Appel and W. Haken (1977), "Every planar map is four colorable. Part I. Discharging", Illinois J. Math21: 429–490. ماثماتيكل ريفيوز543795
K. Appel, W. Haken, and J. Koch (1977), "Every planar map is four colorable. Part II. Reducibility", Illinois J. Math21: 491–567. ماثماتيكل ريفيوز543795