Celebratio Mathematica

K. Ap­pel, W. Haken, and J. Koch: “Every planar map is four col­or­able, II: Re­du­cib­il­ity,” Ill. J. Math. 21 : 3 (1977), pp. 491–​567. A mi­crofiche sup­ple­ment to both parts was pub­lished in Ill. J. Math. 21:3 (1977). MR 543793 Zbl 0387.​05010 article

K. Ap­pel and W. Haken: Every planar map is four col­or­able. Con­tem­por­ary Math­em­at­ics 98. Amer­ic­an Math­em­at­ic­al So­ci­ety (Provid­ence, RI), 1989. With the col­lab­or­a­tion of J. Koch. MR 1025335 Zbl 0681.​05027 book