I always tried to understand how the [*finite reflection groups*](https://en.wikipedia.org/wiki/Reflection_group) of $\Bbb R^d$ (of some fixed dimension $d$) relate to the [*point groups*](https://en.wikipedia.org/wiki/Point_group) of the same space $\smash{\Bbb R^d}$ (finite subgroup of the orthogonal group $\smash{\mathrm O(\Bbb R^d)}$).


Initially, I was under the impression that each point group is a subgroup of a finite reflection group. This turned out to be [wrong](https://math.stackexchange.com/a/916327/415941), which is obvious in hindsight. Many reflections groups have symmetries in the placement of their mirrors that can be used to enlarge the group.


So let's take these enlarged groups instead. From my geometric understanding, by that I mean the symmetry groups of the [uniform polytopes](https://en.wikipedia.org/wiki/Uniform_polytope). So I shall call them **uniform point groups**. Most (or all?) uniform polytopes can be generated from a reflection group, and then it has all the symmetries of this group, but might have more.



> 
> **Question:** Is every point group a subgroup of a uniform point group?
> 
> 
> 


Regardles of the answer to that question, I am open for any statement that sheds light on the placement of reflections groups (or easily derived groups thereof) inside the family of general point groups.




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**Update** Sep. 2019


There seems to exist a counterexample in dimension four, namely, a point group denoted $\pm[I\times C\_n]$ that is supposedly not the subgroup of a symmetry group of a uniform polytope.
This was mentioned in [this answer](https://mathoverflow.net/a/119420/108884) by Günter Rote.
Currently I am not able to verify the claim.
So, any hint is welcome.



How are reflection groups related to general point groups?
Here is a more worked-out and concrete version of the proposed counterexample mentioned in the "Update" of the post.


1. We start from a symmetric arrangement of 12 great circles $F\_1,\ldots,F\_{12}$ on the 3-sphere $\mathbb{S}^3$:
[12 circles from the Hopf fibration, shown in stereographic projection to $\mathbb{R}^3$](https://i.stack.imgur.com/uvoV0.png).
They are the inverse image of the 12 corners of the regular icosahedron under the Hopf map $\mathbb{S}^3\to\mathbb{S}^2$ (*fibers* of the *Hopf fibration*).
2. On each circle we place 70 equidistant points: [840 points on 12 circles](https://i.stack.imgur.com/DBFC2.png).
3. The supporting hyperplanes of the 3-sphere at these points form a 4-polytope $P$ with 840 equal facets: [A perspective view of a facet, a 3-polytope with 40 vertices and 22 sides](https://i.stack.imgur.com/UWWNH.png). (In order to ensure that all faces are equal, the regular 70-gons on the different circles cannot just be placed arbitrarily in Step 2. The points form the orbit of a specially chosen starting point under the group $\pm[I\times C\_7]$.)


The facets are thin plates of roughly pentagonal shape centered at points of a circle $F\_i$ and lying perpendicular to $F\_i$. The plates stack up to form a twisted pentagonal tube that surrounds $F\_i$. Such a tube makes one full turn of $360^\circ$ as it winds around the circle. The 12 tubes fill the space around the 3-sphere and enclose it completely.


The symmetry group $G$ of $P$ has 8400 elements: a given plate can be mapped to any of the 840 plates in 10 different ways. There are only rotations (determinant $+1$), no reflections (determinant $-1$). (Because of the special choice of the starting point of the orbit, the group $G$ is larger than the group $\pm[I\times C\_7]$ by which the orbit was generated. I guess, with a generic orbit, the symmetry group will reduce to size 840, but the picture will be messier.)


The polytope $P$ is clearly not a uniform polytope: none of its 2-faces is a regular polygon.
To definitely answer the question, one would have to argue why $G$ is not a subgroup of the symmetries of a different, uniform, polytope $P'$. One could use the classification of the point groups from the book of Conway and Smith, but maybe there is a more direct argument. The group must contain all rotations of a regular 70-gon.


This example and the programs (in Sage) that were used to produce the images were prepared with the help of my student Laith Rastanawi.


