Talk:Four color theorem

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Former good article nomineeFour color theorem was a Mathematics good articles nominee, but did not meet the good article criteria at the time. There are suggestions below for improving the article. Once these issues have been addressed, the article can be renominated. Editors may also seek a reassessment of the decision if they believe there was a mistake.
Article milestones
April 7, 2009Peer reviewReviewed
October 29, 2009Good article nomineeNot listed
Current status: Former good article nominee

Contradiction in proof section?[edit]

The last paragraph in Four color theorem#Proof by computer: 'They responded that the rumors were due to a "misinterpretation of [Schmidt's] results" and obliged with a detailed article' - without further comment it looks like Schmidt was wrong. But then we learn that their book later " explained and corrected the error discovered by Schmidt [...]", so Schmidt was right? Related question: Why was it "a rumor" years after the master thesis was published? --mfb (talk) 07:22, 16 May 2019 (UTC)

Requested move 22 August 2020[edit]

The following is a closed discussion of a requested move. Please do not modify it. Subsequent comments should be made in a new section on the talk page. Editors desiring to contest the closing decision should consider a move review after discussing it on the closer's talk page. No further edits should be made to this discussion.

The result of the move request was: not moved. (closed by non-admin page mover) Jerm (talk) 03:18, 30 August 2020 (UTC)

Four color theoremFour-color theorem – Punctuation Electricmaster (talk) 08:39, 22 August 2020 (UTC)

  • Oppose on grounds given. Most of the scholarly, reliable sources don't punctuate this. See the references section, with titles like "The Four Color Theorem: History, Topological Foundations and Idea of Proof." There do appear to be a few sources that punctuate this, but we should follow the preponderance of the sources here. SnowFire (talk) 16:25, 22 August 2020 (UTC)
  • Oppose per argument above, but Move to Four colour theorem for harmonize the grammar. (talk) 16:48, 22 August 2020 (UTC)
That would involve switching from American English to English English. As Appel is American and Haken taught in America I think color is the most appropriate spelling to use.--Salix alba (talk): 16:57, 22 August 2020 (UTC)
Indeed, and since the article title has used the American spelling since its creation, MOS:RETAIN applies. Favonian (talk) 17:01, 22 August 2020 (UTC)
  • Oppose - it's a theorem about four colors, not a theorem being four-color. --16:55, 22 August 2020 (UTC) — Preceding unsigned comment added by CiaPan (talkcontribs) 16:55, 22 August 2020 (UTC)
    • I confirm, the above vote was mine. Thank you, David. --CiaPan (talk) 14:46, 24 August 2020 (UTC)
  • Oppose because this article tells about a theorm about four colors, which IMO the correct title should be Four colour theorm for more NPOV title. (talk) 16:59, 22 August 2020 (UTC)
Two IPs from Sumatra !voting for a switch to British spelling. Fascinating. Favonian (talk) 17:06, 22 August 2020 (UTC)
  • Oppose. Hyphen is not needed and therefore should be omitted. —David Eppstein (talk) 17:15, 22 August 2020 (UTC)
  • Oppose both hyphenation and Britishification per the arguments by SnowFire, Salix alba and others above. XOR'easter (talk) 22:33, 22 August 2020 (UTC)
  • Oppose hypernated is not needed for that article and thw title should be remained as this, thought IMO, the correct title of Four color theorem shouldn't be color but Colour. (talk) 23:54, 22 August 2020 (UTC)
  • Oppose. Hyphen is not needed and the correct title should be four colour theorem. (talk) 00:39, 23 August 2020 (UTC)
Now four IPs from Sumatra, with even more participating on the same request at Talk:Five color theorem. —David Eppstein (talk) 05:44, 23 August 2020 (UTC)
I've taken the liberty of striking out all but the first of the Sumatran IPs, on the principle that they're obvious puppetry of some sort (meat or socks, don't care which). —David Eppstein (talk) 22:16, 23 August 2020 (UTC)
  • Oppose hyphenation (and "colour"), per WP:COMMONNAME. Paul August 14:20, 23 August 2020 (UTC)
  • Move to four colour theorem hypernation is not necessary for the article. (talk) 00:07, 26 August 2020 (UTC)
    Stop replying to this discussion repeatedly with many IPs. It's annoying and doesn't strengthen your position at all. --mfb (talk) 01:06, 26 August 2020 (UTC)

OP here. While I personally disagree on keeping the title without hyphenation, I respect the counterarguments and the current democratic consensus. This reminds me of the debate regarding air superiority fighter vs air-superiority fighter. It seems to be a matter of stylistic preference. Thanks for your input. Electricmaster (talk) 10:34, 26 August 2020 (UTC)

The discussion above is closed. Please do not modify it. Subsequent comments should be made on the appropriate discussion page. No further edits should be made to this discussion.

Simple Proof[edit]

Discussion closed because it is not about improvements to the article based on reliably-published sources; see WP:TALK
The following discussion has been closed. Please do not modify it.

On a square grid start with a single square. Add a layer of squares around it so that it becomes a 3x3 square. Each square you add will touch at most 3 other squares, so will only require at most 4 colors to map it. Add layer after layer to infinity, similarly the criterion for 4 colors is met. Is this what was said to be the proof?GuildCompounder (talk) 03:14, 15 December 2020 (UTC)

You are only colouring a particular map. Most maps don't consist of layers like this. McKay (talk) 04:42, 15 December 2020 (UTC)
Fussball 1936.jpg
@GuildCompounder: Consider this ball. When you come to adding the last segment, it will touch five segments previously added. Does your method guarantee those five use no more than three colors? --CiaPan (talk) 15:47, 2 March 2021 (UTC)
The four color theorem only applies to planar maps/graphs. It's well-known (and discussed in the article) that other topologies can need more colors. I don't see the relevance to this section. --mfb (talk) 04:18, 3 March 2021 (UTC)
Sphere is equivalent to plane, actually. But McKay's original reply is correct -- building from layers of squares doesn't deal with any of the interesting maps. (And of course the observation that it sat unsolved for a century, and then required computer assistance, should be a good indication that any "simple" proof attempt is quite likely to be wrong!) Joule36e5 (talk) 05:47, 3 March 2021 (UTC)
A map of the ball's surface
@Mfb: Yes, other topologies can need more colors. But this is the same topology. If you exclude any point of a sphere (which may be an interior point of any "country" region, hence meaningless in map coloring), then the rest of the sphere is homeomorphic with a plane (see the Stereographic projection for an example of a continuous bijection between a punctured sphere and a plane), so any result of coloring a map on a sphere applies verbatim to a plane and vice versa. --CiaPan (talk) 10:46, 3 March 2021 (UTC)
@Mfb: I have added a planar map corresponding to the ball's surface structure. Hopefully it makes it clear how the 'planar' theorem applies to the ball. --CiaPan (talk) 18:54, 3 March 2021 (UTC)
I still don't see any relevance to the original question in this section. --mfb (talk) 04:51, 4 March 2021 (UTC)
So look closer. Can you apply the proposed algorithm to the ball? Can you color it the way described? If not, then the reasoning does not guarantee this particular map can be properly filled with four colors, hence it's not a proof of the theorem (as the theorem applies to all planar maps, to this particular one among them). --CiaPan (talk) 07:06, 4 March 2021 (UTC)
Of course you cannot, but why use a ball? The first reply already pointed out that the algorithm only works for specific maps - without needing to introduce a ball. --mfb (talk) 10:48, 5 March 2021 (UTC)
Nope, the first reply pointed out that User:GuildCompounder applied the algorithm to a specific map, but User:McKay did not prove "that the algorithm only works for specific maps", i.e. the algorithm can't be applied to other kinds of maps. And it actually can, for example it works perfectly well also for hexagonal tiling, and for rhombitrihexagonal tiling, too. So I gave the ball as an example of another simple map, so OP can explain how their method applies to it, or try to strengthen their intended 'proof' by expanding the presented method so that it handles the ball, too. --CiaPan (talk) 18:37, 6 March 2021 (UTC)

It recently occurred to me that layers can be added inwards instead of outwards. Start for example with a 9x9 layer, add a 7x7...3x3. All the squares in the layers touch at most 3 other squares making them 4 colorable. The exception is the 1x1 centre square which touches 4 squares. However, if opposite sides of the centre square touch each other, that would separate the other opposite sides of the centre square which could then be the same colour. That is why what works out for the 2 dimensional map does not work for the 3 dimensional map (which has no limit to the number of colours required).GuildCompounder (talk) 17:59, 13 March 2021 (UTC)

Doesn't work, either. Assume two-'layers' honey-comb pattern. The central piece touches six outer pieces. No part of your proposal guarantees those six use three colors only. And if they use more, you can't color the central one. --CiaPan (talk) 21:13, 13 March 2021 (UTC)
It seems to me that if the squares are relatively infinitesimals, they can cover patterns like the honey comb.GuildCompounder (talk) 20:43, 15 March 2021 (UTC)
While interesting, this discussion does not really conform to the purpose of a talk page, which is confined to discussions of how to improve the article. Paul August 12:25, 14 March 2021 (UTC)
Sorry but maybe there is some relevance? One more little thing...GuildCompounder (talk) 20:43, 15 March 2021 (UTC)

THe "opposites cutoff" theorem I mentioned above is valid for convex objects like spheres and cylinders, but is blown for toroids. We can visualize 6 colours for the toroid by applying a diagonal slash through the failed theorem rectangle. Then delete the original 5 colour region allowing a 2 colour loop to touch in 2 places when it only needs 1 connection. This allows a gap at the far end of the loop. Now the 3 five colour regions all touch each other, requiring 7 colours. GuildCompounder (talk) 20:43, 15 March 2021 (UTC)