Does the $3+4$ coloring $\chi$ determine the homomorphism $\rho_{\chi}$?
Consider the seed:
$$ \mathcal S = (\mathcal I, \Gamma, \Pi) $$where $\mathcal I$ is a geometric realization, $\Gamma \subset \mathcal I$ is the distinguished $1$-skeleton, and $\Pi$ is the piecewise isometry group generated by "Rubik style moves" on the instrument $\mathscr I= (\mathcal I, \Gamma).$
In the undecorated setting I learned that there is no natural nontrivial homomorphism from the abstract $2 \times 2$ Rubik's cube group to $\mathrm{PGL}(3,2)$. I realized that the correct source of projective symmetry is not the bare group, rather a decorated realization of $\mathcal S$, which is what I was implicitly assuming, and should have made clear in that post.
A Fano decorated seed is a tuple:
$$ \mathcal S_\chi=(\mathcal I,\Gamma,\Pi,\chi), $$where \(\chi\) is a \(3+4\) partition of seven distinguished cells of \((\mathcal I,\Gamma)\), written $$ \mathcal C=\lbrace m_x,m_y,m_z,s_{xy},s_{xz},s_{yz},s_{xyz} \rbrace. $$
Here $m_x,m_y,m_z$ denote the three meridian cells, and $s_{xy},s_{xz},s_{yz},s_{xyz}$ denote the remaining four cells.
We identify these seven cells with the seven nonzero vectors of $\mathbf F_2^3$ by the rule $$ m_x=e_1,\qquad m_y=e_2,\qquad m_z=e_3, $$ $$ s_{xy}=e_1+e_2,\qquad s_{xz}=e_1+e_3,\qquad s_{yz}=e_2+e_3,\qquad s_{xyz}=e_1+e_2+e_3. $$
This determines a canonical Fano plane incidence structure on \(\mathcal C\).
The line set \(\mathcal L\) associated to \(\mathcal C\) is the collection of seven triples $$ \mathcal L= \Big\lbrace \lbrace m_x,m_y,s_{xy}\rbrace, \lbrace m_x,m_z,s_{xz} \rbrace, \lbrace m_y,m_z,s_{yz} \rbrace, \lbrace s_{xy},s_{xz},s_{yz} \rbrace, $$ $$ \lbrace m_x,s_{yz},s_{xyz} \rbrace, \lbrace m_y,s_{xz},s_{xyz} \rbrace, \lbrace m_z,s_{xy},s_{xyz} \rbrace \Big \rbrace. $$ Equivalently, under the identification \(\mathcal C\cong \mathbf F_2^3\setminus\{0\}\), the lines are exactly the zero-sum triples $$ {a,b,a+b}, $$ or equivalently the triples \(\{a,b,c\}\) satisfying $$ a+b+c=0. $$
Thus $(\mathcal C,\mathcal L)$ is canonically isomorphic to the Fano plane. There is a canonical homomorphism $$ \rho_\chi:\Pi\longrightarrow \mathrm{PGL}(3,2). $$
It is important to state that the projective symmetry is a property of the decorated realization, not of the undecorated group in isolation. One could easily define a different color partition $\chi$ allowing for the manifestation of a different symmetry group than the Fano plane symmetry.
I created this depiction of the Fano decorated seed $ \mathcal S_\chi=(\mathcal I,\Gamma,\Pi,\chi) $:
Question. Does the $3+4$ coloring $\chi$ determine the homomorphism $$ \rho_\chi:\Pi\longrightarrow \mathrm{PGL}(3,2) $$ intrinsically, uniquely up to conjugacy, as the induced incidence-preserving action of $\Pi$ on the associated Fano plane?
1 answer
As said in the comment I could make very little sense of the question.
Still, because other users might be interested in knowing whether there is a "natural" way to describe the permutations in $GF(3,2)$ using the $2×2×2$ Rubik's cube—there is, in some sense! (Though it's sufficiently complicated that I very much doubt its utility, pædagogical or otherwise.)
First of all, we can compute the group of transformations of the Rubik's cube that preserve the vertical faces1:
cube := Group( (2,14,23,9)(3,6,22,17)(7,15,16,8), (3,11,24,16)(4,19,23,8)(9,10,18,17), (20,14,16,18)(19,13,15,17)(21,22,23,24), (1,2,3,4)(5,7,9,11)(6,8,10,12), (1,7,22,20)(2,15,21,12)(5,6,14,13), (1,10,24,13)(4,18,21,5)(11,19,20,12) );
StructureDescription(Stabilizer(cube,[1,2,3,4,21,22,23,24],OnSets));
And it turns out to be $S_8$ exactly!
Then we can compute the stabilizer of this group on an arbitrary point (which we shall take to be the origin in $\mathbb{Z}_2^3$):
StructureDescription(Stabilizer(Stabilizer(cube,[1,2,3,4,21,22,23,24],OnSets),[1],OnSets));
And this is $S_7$. To complete our mapping from the cubelets of the Rubik's cube to $\mathbb{Z}_2^3$ (now that we have selected an arbitrary point to be our origin), we can just define the three cubelets touching the "origin" cubelet to be the points $(1,0,0)$, $(0,1,0)$, and $(0,0,1)$ (in any order) in the cube. And this gives us a full coordinate structure mapping the points of the Rubik's cube to the points of $\mathbb{Z}_2^3$; of course $GF(3,2)$ is the set of automorphisms of $\mathbb{Z}_2^3$ so this completes our construction.
Nevertheless, this construction is relatively trivial and unenlightening as we are making no use of the facelet structure of the Rubik's cube (just permuting the cubelets) and we have to use the geometry of the cube (and an arbitrarily selected origin!) to uniquely specify an embedding of $GF(3,2)$ into the group of permutations of the cubelets.
One might hope it possible to make a less arbitrary construction by employing the action of $GF(3,2) \sim GF(2,7)$ on eight points and identifying those eight points with the eight cubelets. Unfortunately the Sylow subgroup of $GF(2,7)$ is the dihedral group and there is no non-arbitrary way to choose a dihedral group that permutes the eight cubelets, so this construction can be ruled out immediately.
1 Again, this isn't original to me; it's from Jim Belk's answer to a similar question.

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