Let $P$ be a polyhedron whose faces are colored black and white so that there are more black faces and no two black faces are adjacent. Show that $P$ is not circumscribed about a sphere.
Problem
Source: http://www.artofproblemsolving.com/Forum/viewtopic.php?f=270
Tags: geometry, 3D geometry, sphere, geometry proposed, Contest 8
04.05.2017 13:14
I'm interested in a solution to this problem. Anyone have one?
04.05.2017 21:10
If we cut the corners (8 black triangles) of a cube with 6 white faces, we obtain a polyhedron satisfying the coloring condition but the corners are a tad far from the center. l think this is representative of the dilemma between circumscribing around a sphere and having on average n>m for white n-gons and m-gons to satisfy the coloring condition. (This is because each edge is shared by a white and a black, but there are more black faces). Consider a characteristic c(n) of a n-gon = E/D, where E is the sum of all edges, and D is the sum of distances from vertices to an interior point P which is tangent to the sphere. Perhaps we could derive a cobtradiction between the range of c(n) and c(m), while the entire sets of black e's and d's are identical with those of white. I am driving and cannot really work this out now when a kid called me about this, but I will think about it later....
05.05.2017 00:19
The result is probably false. Consider the Herschel graph, https://en.wikipedia.org/wiki/Herschel_graph. It can be realized as the polyhedral graph of some polyhedron $Q$. We can assume $Q$ is inscribed in a sphere $S$: just take a large sphere and project $Q$ onto it. Now invert $Q$ with respect to the sphere $S$ to get a polyhedron $P$ circumscribed about $S$. From this construction, the adjacency relations among the faces of $P$ are captured by the Herschel graph. Because this graph has 11 vertices and it is bipartite, one can color 6 faces black such that no two black faces are adjacent.
05.05.2017 18:56
This seems to amount to asserting that every convex polyhedron had an inscribed and circumscribed topological equivalence. An immediate logical consequence is that the cube octahedron is also circumscribed -- just project its conjugate to the sphere and invert back. In fact I think the spherical inversion is only guaranteed to work for Platonic (plus a select set of) polyhedra; correct me if I am wrong.