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- From: alopez-o@neumann.uwaterloo.ca (Alex Lopez-Ortiz)
- Newsgroups: sci.math,news.answers,sci.answers
- Subject: sci.math FAQ: The Four Colour Theorem
- Followup-To: sci.math
- Date: 17 Feb 2000 22:52:04 GMT
- Organization: University of Waterloo
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- Summary: Part 17 of 31, New version
- Originator: alopez-o@neumann.uwaterloo.ca
- Originator: alopez-o@daisy.uwaterloo.ca
- Xref: senator-bedfellow.mit.edu sci.math:347398 news.answers:177518 sci.answers:11226
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- Archive-name: sci-math-faq/fourcolour
- Last-modified: February 20, 1998
- Version: 7.5
-
-
-
- The Four Colour Theorem
-
- Theorem 2 [Four Colour Theorem] Every planar map with regions of
- simple borders can be coloured with 4 colours in such a way that no
- two regions sharing a non-zero length border have the same colour.
-
- An equivalent combinatorial interpretation is
-
- Theorem 3 [Four Colour Theorem] Every loopless planar graph admits a
- vertex-colouring with at most four different colours.
-
- This theorem was proved with the aid of a computer in 1976. The proof
- shows that if aprox. 1,936 basic forms of maps can be coloured with
- four colours, then any given map can be coloured with four colours. A
- computer program coloured these basic forms. So far nobody has been
- able to prove it without using a computer. In principle it is possible
- to emulate the computer proof by hand computations.
-
- The known proofs work by way of contradiction. The basic thrust of the
- proof is to assume that there are counterexamples, thus there must be
- minimal counterexamples in the sense that any subset of the graphic is
- four colourable. Then it is shown that all such minimal
- counterexamples must contain a subgraph from a set basic
- configurations.
-
- But it turns out that any one of those basic counterexamples can be
- replaced by something smaller, while preserving the need for five
- colours, thus contradicting minimality.
-
- The number of basic forms, or configurations has been reduced to 633.
-
- A recent simplification of the Four Colour Theorem proof, by
- Robertson, Sanders, Seymour and Thomas, has removed the cloud of doubt
- hanging over the complex original proof of Appel and Haken.
-
- The programs can now be obtained by ftp and easily checked over for
- correctness. Also the paper part of the proof is easier to verify.
- This new proof, if correct, should dispel all reasonable criticisms of
- the validity of the proof of this theorem.
-
- References
-
- K. Appel and W. Haken. Every planar map is four colorable. Bulletin of
- the American Mathematical Society, vol. 82, 1976 pp.711-712.
-
- K. Appel and W. Haken. Every planar map is four colorable. Illinois
- Journal of Mathematics, vol. 21, 1977, pp. 429-567.
-
- N. Robertson, D. Sanders, P. Seymour, R. Thomas The Four Colour
- Theorem Preprint, February 1994. Available by anonymous ftp from
- ftp.math.gatech.edu, in directory /pub/users/thomas/fcdir/npfc.ps.
-
- The Four Color Theorem: Assault and Conquest T. Saaty and Paul Kainen.
- McGraw-Hill, 1977. Reprinted by Dover Publications 1986.
- --
- Alex Lopez-Ortiz alopez-o@unb.ca
- http://www.cs.unb.ca/~alopez-o Assistant Professor
- Faculty of Computer Science University of New Brunswick
-