44
$\begingroup$

In three dimensions it is quite easy to prove that there exist at most five Platonic Solids. Each has to have at least three polygons meeting at each vertex, and the angles of these polygons have to add up to less than $2\pi$. This narrows down the possibilities to three, four or five triangles, three squares or three pentagons.

But the proof is not quite complete. One also has to show that each of these possibilities is actually realised. Of course it turns out that they all are. I've been wondering about how to prove this without having to construct each of them individually. After reading this answer, I've managed to reconstruct the following proof. I'm wondering if it's valid.

Suppose we want a polyhedron where $m$ $n$-gons meet at each vertex. Take any sphere. By Gauss-Bonnet we can draw a regular $n$-gon on the sphere with angles $2\pi/m$. Draw congruent $n$-gons along each edge of this one, and continue to extend the tiling in this way. Because of our choice of angle these polygons must join up locally. We want to verify that they join up locally.

Consider the topological space with one $n$-gon for each $n$-gon drawn on the sphere, joined along the edges whenever the corresponding $n$-gons share that edge. Then this topological space is a covering space of the sphere. But the sphere is already simply connected, so our covering space must be the sphere itself. So we do have a regular tiling of the sphere. Now create an actual regular polyhedron by taking the convex hull of the vertices.

If this argument does work, can it be simplified so that it could be understood by someone with no knowledge of algebraic topology?


Below this line is an attempt by David Speyer to restate the question. I like simplicial complexes better than CW complexes, so I am going to subdivide the polygons in the original question. Instead of a spherical $m$-gon with angles $2 \pi/n$, I'm going to place a vertex in the center of the polygon and connect it to all the vertices and the midpoints of all the edges. So I have $2m$ spherical triangles with angles $\pi/m$, $\pi/n$ and $\pi/2$.

So, here is my rephrasing. Let $(a,b,c)$ be positive integers with $1/a+1/b+1/c > 1$ (in our case, $(2,m,n)$). We form a two dimensional simplicial complex $\Delta$ whose vertices are colored amber, blue and crimson, with two triangles on each edge and $2a$, $2b$, $2c$ triangles around the amber, blue and crimson vertices respectively. One way to make this more precise is to define $W$ to be the group generated by $s_1$, $s_2$, $s_3$ subject to $s_1^2=s_2^2=s_3^2=(s_1 s_2)^a = (s_1 s_3)^b = (s_2 s_3)^c = 1$. Our vertices correspond to cosets of the subgroups $H_a:=\langle s_1, s_2 \rangle$, $H_b:=\langle s_1, s_3 \rangle$ and $H_c:=\langle s_2, s_3 \rangle$, with vertices in the same triangle if they are of the form $(w H_a, w H_b, w H_c)$.

Then $\Delta$ maps to the $2$-sphere, sending our base simplex to the spherical triangle $T$ with angles $(\pi/a, \pi/b, \pi/c)$, and choosing the images of all the other vertices by making $s_1$, $s_2$, $s_3$ act by reflections over the sides of $T$.

Anyone who has taught a course on Coxeter groups knows it is true, but a pain to prove, that the abstractly defined $\Delta$ maps isomorphically to the sphere $S^2$ and, in particular, $W$ is finite.

How much can we reduce the pain by knowing that $S^2$ is simply connected?

$\endgroup$
  • 7
    $\begingroup$ There are only five of them, what is wrong with constructing them individually? $\endgroup$ – Morgan Rodgers Apr 26 '18 at 20:50
  • 3
    $\begingroup$ @MorganRodgers All the regular polytopes also exist in higher dimensions. Also if the angles add to $2\pi$ or more, then I think we always get a valid tiling of flat or hyperbolic space. The fact that they all always work is very surprising to me, and it would be nice if there was a reason why. $\endgroup$ – Oscar Cunningham Apr 26 '18 at 20:55
  • 1
    $\begingroup$ @MorganRodgers As a test case for developing machinery for classifying them, or more general kinds of polytopes, in higher dimensions? $\endgroup$ – Noah Schweber Apr 26 '18 at 20:55
  • 1
    $\begingroup$ @LeeMosher I implicitly use it when I say "By Gauss-Bonnet we can draw a regular $n$-gon on the sphere with angles $2\pi/m$." The angles of the $n$-gon range between $\frac{2\pi}2-\frac{2\pi}n$ and $\frac{2\pi}2+\frac{2\pi}n$ depending on its area. So for $m\geq 3$, $2\pi/m$ lies in this range iff $1/m + 1/n \geq 1/2$. $\endgroup$ – Oscar Cunningham Apr 27 '18 at 8:30
  • 1
    $\begingroup$ Related math.stackexchange.com/questions/2136782 . Nice question! I think something like this should be able to be made to work. $\endgroup$ – David E Speyer Oct 18 '18 at 14:16

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Browse other questions tagged or ask your own question.