### Wolfgang Haken and the Four-Color Problem

#### Works connected to ~~Herbert Koch~~

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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

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

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