Date: 22 March 1981 0829-EST (Sunday)
From: Dan Hoey at CMU-10A
To: Cube-Lovers at mit-mc
Subject: No short relations and a new local maximum
Message-Id: <22Mar81 082919 DH51@CMU-10A>
Well, the gigabyte (well, 300Mb) came in, and brute force is
having its day. I have a little program that generates all positions
accessible from a given position in a given number of quarter-twists.
With the increased storage available here, I was able to run it to five
quarter-twists.
The first important fact to emerge is that there are exactly
105046 different positions at a distance of at most 5 qtw from START.
This has two consequences to the argument given in my message on the
Supergroup, part 2 (9 January 1981 0629-EST). Note that the results
here pertain to the usual group of the cube, rather than the
Supergroup, since the program does not keep track of face-center
orientations.
The first consequence is that there are exactly 93840 positions
exactly 5 qtw from START. The message cited above proved the
inequality P[5] <= 93840; this is now known to be an equality.
The second consequence is that there are no relations
(sequences that lead back to START) of length 10, with the exception of
those that follow from the relations FFFF = FBF'B' = I (and their
M-conjugates). This is because relations of length 10 would reduce
P[5], which is not the case. There are, however, relations of length
12; the only known ones are FR'F'R UF'U'F RU'R'U [given in Singmaster]
and its M-conjugates.
These results can be extended to the Supergroup, by noting that
the set of observed positions places a lower bound on the number of
Supergroup positions at a distance of 5 qtw, while the upper bound
given in the cited message relies on the relations FFFF = FBF'B' = I,
which are relations in the Supergroup.
A particular result which may be of greater interest to readers
of this list concerns the relation between symmetry and local maxima.
In our message on the subject (14 December 1980 1916-EST) Jim Saxe and
I mentioned that the six-spot pattern is not a local maximum, as
verified by computer. [The same program was used, but only four-qtw
searches were needed.]
With five-qtw searches, it became possible to check another
conjecture, using an approach that Jim suggested. The four-spot
pattern
U U U
U U U
U U U
R R R B B B L L L F F F
R L R B F B L R L F B F
R R R B B B L L L F F F
D D D
D D D
D D D
is solvable in twelve qtw, either by (FFBB)(UD')(LLRR)(UD') or by its
inverse, (DU')(LLRR)(DU')(FFBB). It is immediate that a twelve qtw
path from this pattern to START can begin with a twist of any face in
either direction. The program was used to verify that there are no ten
qtw paths. (It generated the set of positions attainable at most five
qtw from START and the set of positions obtainable from the four-spot
in at most five qtw, and verified that the intersection of the two sets
is empty.) Thus the four-spot is exactly twelve qtw from START and all
its neighbors are exactly eleven qtw from START, proving that the
four-spot is a local maximum. (Worried that there might be an eleven
qtw solution to the four-spot? Send me a note.)
This is the first example of a local maximum which cannot be
shown to be a local maximum on the basis of its symmetry. To be more
precise, let us define a "Q-symmetric" position to be a position whose
symmetry group is Q-transitive. This extends the terminology developed
in "Symmetry and Local Maxima". In that message, we showed that all
Q-symmetric positions, except the identity, are local maxima. Until
now, these were the only local maxima known. The four-spot, however,
is not Q-symmetric; the position obtained by twisting the U or D face
of the four-spot is not M-conjugate to the position obtained by
twisting any of the other faces. This lays to rest the old speculation
that one might find all local maxima, and thereby bound the maximum
distance from START, by examining Q-symmetric positions.