How are the snub polyhedra uniform?

Discussion of tapertopes, uniform polytopes, and other shapes with flat hypercells.

How are the snub polyhedra uniform?

Postby ubersketch » Sun Dec 01, 2019 8:27 pm

The snub cuboctahedron and snub icosidodecahedron have triangles on the edges of the squares or pentagons which don't seem to be vertex-transitive, as there are three separate edges connecting to a square or pentagon, a triangle of the same type, and a triangle of a different type.
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Re: How are the snub polyhedra uniform?

Postby Mecejide » Mon Dec 02, 2019 6:18 am

Elements of a uniform polytope don't have to be vertex-transitive under the symmetry of the polytope, only their own.
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Re: How are the snub polyhedra uniform?

Postby Marek14 » Tue Dec 03, 2019 6:34 pm

Mecejide wrote:Elements of a uniform polytope don't have to be vertex-transitive under the symmetry of the polytope, only their own.


Don't you mean the other way around?
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Re: How are the snub polyhedra uniform?

Postby Mecejide » Wed Dec 04, 2019 1:19 am

When I said their own, i meant on their own, as in not part of the polytope. So no.
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Re: How are the snub polyhedra uniform?

Postby Mercurial, the Spectre » Wed Dec 04, 2019 8:08 pm

In three dimensions there are three variables that represent the three different edges of the girco or grid's verf (a scalene triangle in general).
When alternating into a snic or snid, the vertex figure becomes a face of the polyhedron, and since you have to solve three equations for three edges, there is a guaranteed solution to get an equilateral result.

This is not the case in general in 4D where the solution involves solving four equations for six edges, meaning that there are cases in which no solution can be found for uniformity.
SárHwās Hnáras táygʲasi Hr̥táy kʲa waĉatás samHatás kʲa ĵn̥Hyántay. Táy kʲitˢtáH mánasaH kʲa dádHn̥tay Hántaray práti bʰraHtrásya pn̥tHáH kriHánt.

All human beings are born free and equal in dignity and rights. They are endowed with reason and conscience and should act towards one another in a spirit of brotherhood.
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Re: How are the snub polyhedra uniform?

Postby Klitzing » Wed Dec 04, 2019 10:35 pm

Snubbing as such happens to be a sequence of 2 different operations!
  • One is the to be applied alternation (e.g. vertex alternation applied onto omnitruncates), seen as a mere faceting of the seed polytope. - This one clearly is possible always.
  • The other one is a topolgical variation, which tries to head for all unit edges only (thus bringing back to uniformity). - This is the one, which is not always possible in general.

A short intro into snubbing theory can be found on my Incmats webpage here.
A detailed article on snubbing theory is also available, cf. "Snubs, Alternated Facetings, and Stott-Coxeter-Dynkin Diagrams", by Dr. R. Klitzing, in "Symmetry: Culture and Science", vol. 21, no.4, 329-344, 2010 - or downloadable from Research Gate or even from my website too.

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Re: How are the snub polyhedra uniform?

Postby Mecejide » Thu Dec 05, 2019 1:34 am

Wait it actually was the other way around.
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Re: How are the snub polyhedra uniform?

Postby Klitzing » Thu Dec 05, 2019 9:55 am

Mecejide wrote:Wait it actually was the other way around.

What are you refering by "it"? - I'm assuming you are refering to the 2 processes involved by snubbing.

Snubbing indeed in the past was considered to be a variation (of some Wythoffian polytope) first and applying a faceting thereafter.
But even then the same point remains: the process of (appropriate) variation is not generally possible as that one has to bow to an according degree of freedom. Whereas the mere faceting is clearly applicable in every case.

And in fact this was the ingenious idea here, simply to switch the applications of those 2 operations. So that the faceting part becomes applicable to ANY Wythoffian seed polytope, and it is up to a subordinate consideration, whether a further variation towards uniformity can be achieved or not.

And infact, as I worked out in that cited paper, the decorations of the Dynkin diagram with node marks "o" (unringed node), "x" (ringed node), and "s" (snub node) comes out to rather describe the mere faceting process, as it is applicable generally, and so has nothing to do with the post-poned process of topological variation. I.e. it not truely describes the WHOLE snubbing process (even if such symbols like s3s4s usually are being considered to describe the uniform variant of the snub cube).

The genuine idea, which was also outlined in that paper, was that alternation needs not be restricted to vertices only. Even other polytopal elements, like edges, faces, cells, whatever, could be alternatedly replaced by the according sectioning facet underneath ("sefa"), thereby generalysing the vertex figure ("verf"). Keeping this in mind, it becomes obvious that indeed ANY decoration of the Dynkin symbol with any "powdering" by "s"-, "o"-, and "x"-nodes perfectly makes sense! (Then describing the mere faceted figure.)

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Re: How are the snub polyhedra uniform?

Postby Mecejide » Thu Dec 05, 2019 8:19 pm

Klitzing wrote:
Mecejide wrote:Wait it actually was the other way around.

What are you refering by “it”?
--- rk

I was referring to my answer to Marek14’s question.
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Re: How are the snub polyhedra uniform?

Postby mr_e_man » Wed Dec 11, 2019 6:53 pm

ubersketch wrote:The snub cuboctahedron and snub icosidodecahedron have triangles on the edges of the squares or pentagons which don't seem to be vertex-transitive, as there are three separate edges connecting to a square or pentagon, a triangle of the same type, and a triangle of a different type.


Vertex-transitivity means that any vertex of the object can be sent to any other vertex by a symmetry of the same object. A regular triangle is vertex-transitive by itself; the surrounding faces are irrelevant. The snub cube is vertex-transitive; this makes no reference to its triangular faces.
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Re: How are the snub polyhedra uniform?

Postby Mercurial, the Spectre » Wed Dec 11, 2019 7:36 pm

mr_e_man wrote:
ubersketch wrote:The snub cuboctahedron and snub icosidodecahedron have triangles on the edges of the squares or pentagons which don't seem to be vertex-transitive, as there are three separate edges connecting to a square or pentagon, a triangle of the same type, and a triangle of a different type.


Vertex-transitivity means that any vertex of the object can be sent to any other vertex by a symmetry of the same object. A regular triangle is vertex-transitive by itself; the surrounding faces are irrelevant. The snub cube is vertex-transitive; this makes no reference to its triangular faces.

He means the triangles that share an edge with the squares or pentagons, which are topologically scalene. Scalene triangles obviously are not vertex transitive. The fact is that the snub cube/dodecahedron has two different triangles: one always equilateral, and the other scalene, owing to them being s{3,4} and s{3,5}. The snub {4,5}, for example, has squares, pentagons, and scalene triangles. Of course it can be made into unit edges, but the topology does not change, unless it is of the form s{p,p}, which is just an alternated t{p,4}.
SárHwās Hnáras táygʲasi Hr̥táy kʲa waĉatás samHatás kʲa ĵn̥Hyántay. Táy kʲitˢtáH mánasaH kʲa dádHn̥tay Hántaray práti bʰraHtrásya pn̥tHáH kriHánt.

All human beings are born free and equal in dignity and rights. They are endowed with reason and conscience and should act towards one another in a spirit of brotherhood.
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Re: How are the snub polyhedra uniform?

Postby mr_e_man » Thu Dec 12, 2019 5:56 am

The triangle is "topologically scalene" only when we consider it as a part of the snub cube; its vertices cannot be sent to each other by symmetries of the snub cube. They can be sent to each other by symmetries of the triangle: reflections and rotations that map the triangle onto itself, regardless of what they do to the snub cube.

Uniformity of the snub cube only requires the triangle to have this "self-symmetry".
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