The Universe of Disco

3-coloring the vertices of an icosahedron

I don't know that I have a point about this, other than that it makes me sad.

A recent Math SE post (since deleted) asked:

How many different ways are there to color the vertices of the icosahedron with 3 colors such that no two adjacent vertices have the same color?

I would love to know what was going on here. Is this homework? Just someone idly wondering?

Because the interesting thing about this question is (assuming that the person knows what an icosahedron is, etc.) it should be solvable in sixty seconds by anyone who makes the least effort. If you don't already see it, you should try. Try what? Just take an icosahedron, color the vertices a little, see what happens. Here, I'll help you out, here's a view of part of the end of an icosahedron, although I left out most of it. Try to color it with 3 colors so that no two adjacent vertices have the same color, surely that will be no harder than coloring the whole icosahedron.

The explanation below is a little belabored, it's what OP would have discovered in seconds if they had actually tried the exercise.

Let's color the middle vertex, say blue.

The five vertices around the edge can't be blue, they must be the other two colors, say red and green, and the two colors must alternate:

Ooops, there's no color left for the fifth vertex.

The phrasing of the question, “how many” makes the problem sound harder than it is: the answer is zero because we can't even color half the icosahedron.

If OP had even tried, even a little bit, they could have discovered this. They didn't need to have had the bright idea of looking at a a partial icosahedron. They could have grabbed one of the pictures from Wikipedia and started coloring the vertices. They would have gotten stuck the same way. They didn't have to try starting in the middle of my diagram, starting at the edge works too: if the top vertex is blue, the three below it must be green-red-green, and then the bottom two are forced to be blue, which isn't allowed. If you just try it, you win immediately. The only way to lose is not to play.

Before the post was deleted I suggested in a comment “Give it a try, see what happens”. I genuinely hoped this might be helpful. I'll probably never know if it was.

Like I said, I would love to know what was going on here. I think maybe this person could have used a dose of Lower Mathematics.

Just now I wondered for the first time: what would it look like if I were to try to list the principles of Lower Mathematics? “Try it and see” is definitely in the list.

Buy How to Solve It from Bookshop.org (with kickback) (without kickback)

Then I thought: How To Solve It has that sort of list and something like “try it and see” is probably on it. So I took it off the shelf and found: “Draw a figure”, “If you cannot solve the proposed problem”, “Is it possible to satisfy the condition?”. I didn't find anything called “fuck around with it and see what you learn” but it is probably in there under a different name, I haven't read the book in a long time. To this important principle I would like to add “fuck around with it and maybe you will stumble across the answer by accident” as happened here.

Mathematics education is too much method, not enough heuristic.

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