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An icosahedron has 20 faces which are equilateral triangles.

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Tom wants to create a very hard mechanical puzzle. With his brand new 3D printer in mind he designs 20 puzzle pieces with identical shapes which in principle nicely fit together to build the icosahedron:

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The face that is visible after assembling the icosahedron is on the back.

To hold the parts together he uses magnets which he glues into the 5 holes that he had prepared quite devilishly.

There are exactly 10 ways to orient the magnets such that two magnets are oriented with north poles facing outwards. These 10 parts he wants to print in yellow. Here is one example:
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And of course there are also exactly 10 ways to orient the magnets with two south poles facing outwards. These 10 parts he wants to print in blue. Example:

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Hey, we got a complete set of exactly 20 different pieces! There are $19!*3^{19}> 10^{26}$ ways to assemble the icosahedron if we ignore the magnets.

  1. Is at least one way for the magnets to match everywhere (north to south)?
  2. Are there especially nice looking solutions with 5-fold symmetry? Obviously, only the colors of the pieces are interesting for this part.
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2 Answers 2

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Partial Solution:
The answer to part 1 is:

Yes. I wrote a program to brute force solutions.
While it is true that there are 19! * 3^19 possible arrangements of triangles on an icosahedron, the magnets severely limit tilings. My brute force search is limited to solutions that match the orientation of magnets in the solution below, which restrains the rotation of each triangle and so has a mere 19! solutions to comb through.

Numbering the possible triangles like so:
The possible triangles, numbered 0-19 One possible solution is:
A numbered net showing the solution

For part 2:

I was able to find a more symmetrical solution by hand.
I started by filling in the top and bottom 5 triangles on the net. I tried to use up as many of the pieces with two of the same pole on the sides. I also made sure to use the inverse of each piece on the opposite end:
Partial Net with top and bottom five triangles selected

Once I had these two ends, I worked on the center. I started by connecting piece 10 to piece 0, and piece 11 to piece 1, since those were the only possible connections left for those sets of edges. This part was the most trial-and-error, getting a row that connected and had the correct pattern of poles on the top and bottom. While I wanted to keep things symmetrical here, that didn't seem possible. I eventually found this pattern that works, though the top end cap would need to be shifted to line up:
Partial net, with the center 10 triangles selected

From here, I was able to get a complete net that looked like this:
Completed net
Just for fun, I started shifting the net around, and it ended up in this configuration. I noticed that the diamond formed by pieces 7 and 16 could be rotated 180 degrees, and this is the final net I've settled upon: Completed net with symmetry

While this construction does not have 5-fold symmetry, it does have a single axis of rotational symmetry, shown in the animation below:
Animation of the net folding into an icosahedron

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  • $\begingroup$ @DqwertyC: You definitely answered part 1! You mentioned you found a solution also without computer search first. With my physical puzzle I did not manage to find a solution by hand, the puzzle is really hard. $\endgroup$ Commented Aug 6 at 10:05
  • $\begingroup$ I always like your animations. Would like to be able to do so too. There are many solutions with two fold symmetry though. There also exist solutions with 3-fold symmetry and also solutions with mirror symmetry btw. $\endgroup$ Commented Aug 7 at 12:20
  • $\begingroup$ Since you got the most votes I accepted your answer of part 1, though answering part 2 was also worth to be accepted. But I cannot split. $\endgroup$ Commented Aug 18 at 18:02
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Part 2:

enter image description here
There are only two ways to arrange the 10 single-magnet edges (shown with thicker lines), and only one of these arrangements leads to a solution.
Also, this is the only color pattern that works.

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  • $\begingroup$ Wow this looks like a nice "inversion" symmetry. Just to clarify, there are two ways to do what? And what is the other way? $\endgroup$ Commented Aug 6 at 5:38
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    $\begingroup$ Two ways to pick edges on an icosahedron such that each face has exactly 1 picked edge. The other way is picking edges such that they form 2 parallel pentagons (like in DqwertyC's part 1 solution). $\endgroup$ Commented Aug 6 at 5:50
  • $\begingroup$ Yes, there is only 1 solution with 5-fold symmetry and it has additionally an inversion symmetry (exchanging colors does not change the pattern). $\endgroup$ Commented Aug 6 at 10:12
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    $\begingroup$ Inversion symmetry in this context means that you flip all magnets (and hence also the color) of all puzzle pieces and after some rotation/reflection you get the old configuration again. $\endgroup$ Commented Aug 6 at 16:26
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    $\begingroup$ Addition: Old configuration is a bit ambiguous. It may refer only to the colors, or if you want to distinguish the different solutions which lead to the same color scheme you can also include the magnets. $\endgroup$ Commented Aug 6 at 16:37

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