http://theoryofeverything.org/theToE/2017/10/21/the-concentric-hulls-of-e8-projected-to-h4/

]]>It’s probably questionable to say these are the only interesting appearances of 17. See also http://mathworld.wolfram.com/LuckyNumberofEuler.html Lucky primes are related to Heegner numbers .

]]>Certainly the vertex-edge-face flags are more fundamental—or in general, for any polytope, the ‘complete’ flags.

I don’t know the meaning of the number of these flags. But there’s an excellent group that acts on the complete flags regardless of whether the polytope is regular or not! It’s called the **cartographic group**. It has one generator for each dimension.

In the case at hand, when we’re starting with a polyhedron, these generators could be called and Their meaning is ‘change the vertex’, ‘change the edge’ and ‘change the face’.

You see, if you have a vertex-edge-face flag, there’s exactly one way to change the vertex and get a new such flag. Similarly there’s one way to change the edge, and one way to change the face.

In the cartographic group, these generators obey only the relations that hold *regardless* of which polyhedron we’re dealing with:

This presentation defines the cartographic group.

This story extends to vertex-edge flags, or edge-face flags, or vertex-face flags. Various quotient groups of the cartographic group act on these.

All this generalizes to arbitrary dimensions. In general, each generator of the cartographic group. squares to 1, and generators that aren’t of neighboring dimensions commute.

]]>It must be more general than the order of the rotation group if it applies to all polyhedra. Perhaps it is the vertex-edge-face flags that count. There would be twice as many of those. This would generalise to higher dimensions. Is that something from cohomology?

]]>For the regular polyhedra someone on G+, probably Layra Idarani, pointed out that these numbers are called “the order of the rotational symmetry group of the polyhedron”. There’s a nice generalization to the polytopes associated to Coxeter groups.

For other, non-regular polyhedra these numbers seem a bit more mysterious to me.

]]>I haven’t been following the discussion in G+ so excuse me if this has already been discussed. You can count vertex-face flags, vertex-edge flags or face-edge flags. All three give the same number for any polyhedron. They are also the same for dual polyhedra. Is there a name for this number? The numbers are 12,24,60 for tetrahedron, cube/octahedron and dodecahedron/icosahedron. These numbers come up a lot in exceptional structures, especially 24 of course. For example https://en.wikipedia.org/wiki/Complex_reflection_group

]]>Impressive analysis! This is one of the few interesting appearances of the number 17 in mathematics… and it’s seemingly unrelated to the others.

One is that Gauss showed it’s possible to construct a regular 17-gon using a ruler and compass, because

and 17 is prime. The other is that there are 17 wallpaper groups.

Plutarch wrote that the Pythagoreans “utterly abominate” the number 17.

]]>This looks a little bit like the math problem for periodic patterns in peptides

https://johncarlosbaez.wordpress.com/2017/04/06/periodic-patterns-in-peptide-masses/

except that it seems to come down to counting unordered sums instead of ordered sums.

Two actions of a finite group on a finite set are isomorphic if and only if they have the same number of orbits of the same length. The length of an orbit is the index of a point stabilizer in the full group.

The subgroups of have orders 1,2,3,4,12 so the orbits can have lengths 12, 6, 4, 3, 1. The number of equivalence classes of actions of on a set of 12 elements comes down to the number of ways 12 can be written as an unordered sum of 1, 3, 4, 6, 12. I wrote the cases out as Young diagrams here:

There appear to be 17 equivalence classes of actions of on a 12 element set.

In a solid tetrahedron there are no points with orbits of length 3 under the action of . So it appears that for any 12 point subset of a solid tetrahedron there are only 7 equivalence classes of actions by .

But if acts on collections of subsets of the tetrahedron then we can get an orbit of length 3. For instance there are 3 pairs of opposite edges.

We can reduce this to finite sets. Denote the vertices by 1,2,3,4. acts transitively on the 3 element “class of collections of finite subsets”:

{{{1,2},{3,4}}, {{1,3},{2,4}}, {{1,4},{2,3}}}

Other orbits of length 3 could be obtained this way by selecting other points from the tetrahedron’s medians. So it seems that all possible equivalence classes of actions of on a 12 element set would be possible with a regular tetrahedron.

]]>Puzzle 1: There are 12 unique directions along the edges. Each can be traversed only once in making a complete circuit.

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