Originally posted by vulture
Consider the good old icosohedron. The vertices, where the points of five (equilateral) triangles meet is decidedly non-planar. As shown by the fact that the sum of all the angles meeting there is 300 degrees. Certainly a sphere is approximately planar at any given point, but that's not true for an icosohedron - the space is non-analytic along edges and at vertices.
Consider the good old icosohedron. The vertices, where the points of five (equilateral) triangles meet is decidedly non-planar. As shown by the fact that the sum of all the angles meeting there is 300 degrees. Certainly a sphere is approximately planar at any given point, but that's not true for an icosohedron - the space is non-analytic along edges and at vertices.
The Icosahedron is certainly planar as a graph.
What we care about is the underlying graph.
I can draw tiles on a plane such that each tile corresponds to a vertex of the icosahedron, and two tiles are adjacent if and only if they are adjacent vertices on the icosahedron...
The same is true for any platonic solid...
(Careful, of course my tiles will not be regular poygons, they won't even all be the same. They will have the right "adjacency" property though.)
Let's go back a bit.
What do we mean by a "tiling" in this case.
Why do we say we can tile with squares and hexagons but not with octogons.
Because we can draw tiles in the planes (for example squares and hexagons) such that each of them is adjacent to 4 and 6 other tiles respectively but not 8.
Now suppose I draw a square lattice on a plane and define "adjacency" to be all squares touching plus the diagonal one. Then each square is adjacent to 8 and you call this an octogonal tiling. This is what civ actually does.
But we can't actually draw it so that the "adjacent" tiles share part of an edge so we don't consider this a "true" solution.
For example going to a sphere (or even a space of negative curvature) doesn't help in this respect.
For example if you cut up the surface of a sphere in any different pieces, you can cut up the plane in pieces such that there is an isomorphism between the adjacency relation so that lowering the curvature (going from sphere to plane) did not help at all.
My point is that unless you take something like a torus with more and more holes as the graph grows this is the best you will be able to do.
Here is my claim :
if you have a surface with no holes, whatever curvature you like, you will NOT be able to draw a finite number of regions on it such that each region touches 6 or more other regions (touching in the usual sense, sharing part of an edge).
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