David C. Plummer (31 Dec 1980 1210-EST) gave a preliminary
analysis of the 5x5x5 cube. I complete it here. Let a move consist
of twisting any of the six faces, at a depth of 1 or 2. It will be
necessary to consider the two depths as distinct; M1P will refer to
the number of depth 1 moves (mod 2), while M2P will refer to the
number of depth 2 moves (mod two). It is important to realize that
the two parities vary independently. The tabs on each face are
assigned types
C L E R C
R D A D L
E A X A E
L D A D R
C R E L C
as in Plummer's analysis.

Let COP ("C" Orientation Parity) and CPP ("C" Permutation)
parity be defined as before. As before, COP=0 (mod 3). We must be
explicit about the CPP this time: Since either kind of move is an
odd permutation of the "C" faces, CPP=M1P+M2P.

As in the 4x4x4 case, "R"'s may be ignored and "L"'s have
no orientation. The permutation parity (LPP) is important, however.
Depth 1 moves are an even permutation of the "L"'s (two 4-cycles),
so they do not affect the LPP, but Depth 2 moves are an odd
permutation of the "L"'s (three 4-cycles). Therefore LPP=M2P. Note
that while LPP and CPP may vary independently, they together
determine both M1P(=LPP+CPP) and M2P(=LPP).

The "E" faces act as in the 3x3x3 case, with orientation
and permutation parity. Orientation changes on four "E"'s with
every move, so EOP=0 (mod 2). Permutation parity changes with every
move, so EPP=M1P+M2P. This has already been determined by CPP, so
only half of the "E" permutations are possible.

Every move is an odd permutation of the "D" faces, so
DPP=M1P+M2P. Since M1P+M2P=CPP is determined, only half of the "D"
face permutations are possible.

Moves work differently on "A" faces depending on depth:
Depth 1 moves are odd permutations of the "A"'s, and depth 2 moves
are even. Thus APP=M1P, which is determined by CPP+LPP, so only
half of the "A" permutations are possible.

Finally, the "X" faces have orientation which changes on
every move, so XOP=M1P+M2P, and only half of the "X" orientations
are possible.

Thus there are 96 orbits, corresponding to COP (mod 3) and
EOP, EPP+CPP, DPP+CPP, APP+CPP+LPP, and XOP+CPP (mod 2). The basic
combinatoric is as Plummer described:
8! C Permutations
3^8 C Orientations
24! L Permutations
1 R Permutation
12! E Permutations
2^12 E Orientations
24! D Permutations
24! A Permutations
4^6 X Orientations
which when multiplied together and divided by 96 yields about
5.289*10^93. [This differs from Plummer's result by a factor of
4096 because (4^6/2) he didn't count X Orientations, and (2) he did
not realize that LPP and CPP are independent.] My implementation of
Furst's algorithm claims that all of these are reachable.

To count the number of reachable color patterns, divide this note
that there are by (4!)^6/2 invisible D permutations, (4!)^6/2
invisible A permutations, and 4^6/2 invisible X orientations that
satisfy the invariants. While there are pairs of L/R edges that
look the same, they cannot be interchanged, for that would entail
putting an L tab into an R position. So there are 2.829*10^74
different color patterns achievable.

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