Made me dig up this old guy: https://www.google.com/search?q=5+%2B+(-sqrt(1-x%5E2-(y-abs(...
You never need more than four colors to color every country
on a map a different color from its neighbours. This was
proved in the 20th century — but nobody knows why it is true.
I think that this is a reasonable philosophical position; given the heavily computer-based nature of the proof, even mathematicians who might accept that (say) the proof of the solubility of odd-order groups explains 'why' they are soluble are reluctant to accept this (the 4-colour proof) as a 'true' explanation.
Even if you have no problem with computer-generated proofs, I think that there is a big gap, for professionals and amateurs alike, between proof and explanation.
Is that true in today's geography? I know of one almost [1] counterexample (of that specific statement, not of the four-color theorem): the North Sea, Belgium, the French Republic, Germany and the Kingdom of the Netherlands all border each other, so you need five different colors to color them on a map.
[1] Almost because the Kingdom of the Netherlands isn't a country.
Confused? Read http://en.m.wikipedia.org/wiki/Kingdom_of_the_Netherlands, in particular the part about overseas territories, and notice that Sint Maarten borders Saint-Martin.
[on an even more sideways track: the US dollar is an official currency in part of the _country_ of the Netherlands (in Bonaire, Saba and St Eustatius)]
Back to my original question: does anybody know of a valid counterexample for the statement on countries?
Neither is the North Sea! Nonetheless, this is a neat example.
> Back to my original question: does anybody know of a valid counterexample for the statement on countries?
A standard counterexample to the hypotheses of the 4-colour theorem (though not to the conclusion, as consulting a map easily verifies) is Michigan, which is not connected.
The 4-colour theorem's hypotheses also rules out the possibility of 4 countries meeting at a corner (or, rather, declare that they don't meet in that case). If there were such an arrangement—and I'd be surprised if there isn't; for 3 countries, one has the example of Finland, Sweden, and Norway—then it would be easy to juice it up to a counterexample.
Maybe the guy who established an island with a bizarre currency, including one coin that had a denomination of π (I can't remember who—I thought Dean Kamen, but his Wikipedia page doesn't mention it), could be induced to subdivide his island in such a way as to create a counterexample. :-)
> I know of one almost [1] counterexample (of that specific statement, not of the four-color theorem): the North Sea, Belgium, the French Republic, Germany and the Kingdom of the Netherlands all border each other, so you need five different colors to color them on a map.
On Google Maps, Belgium seems to interpose entirely between France and the Netherlands. Am I deceived by appearances, or is there some subtle geopolitical point?
Exhaustion doesn't qualify as a real argument, in my opinion.
There are infinitely-many possible planar graphs, so you of course need to find some invariants that allow you to bring the necessary number of graphs to check to be finite. Beyond that, the techniques in the 4CT which make it actually possible to reduce the number of cases to a feasibly enumerable number are very clever and require some insight.
So, why make the cut-off point of when exhaustiong is an explanation at some finite number of cases?
Y'all did not invent a d20!
This is just a really nifty site. Much deeper than I know how to express. One image per page, but it's really good. ("Bookmarked", so to speak.)
This is a game? It takes maybe 2 seconds to find a solution on this thing.