Date: Mon, 23 Aug 82 16:23:00 -0400 (EDT)
From: Dan Hoey <Hoey@CMU-10A >
~~~ Subject: Hypermove lower bounds

There is an easy lower bound on the number of hypermoves needed to
solve Rubik's Revenge. If we distinguish like-colored face
centers, let us fix the BL center of the D face, permitting only
the shallow moves B1, F1, U1, U3, L1, R1, and their inverses, and
the deep moves F2, U2, R2, and their inverses. Let us compute the
number of hypermoves needed to solve just the face centers of
Rubik's Revenge. A shallow hypermove can achieve SF = 4^6 = 4096
different face center positions. A deep hypermove can achieve
DF = 7! 3^6 = 3674160 different face center positions. So in four
hypermoves, at most
1 + (SF + DF) + 2 SF DF + (SF + DF) SF DF + 2 SF DF SF DF
= 453,021,789,719,303,692,337
face center positions can be achieved. Since this is fewer than
the
23! = 25,852,016,738,884,976,640,000
face center positions of Rubik's Revenge, some face-center
positions will require at least five hypermoves.

If like-colored face centers are not distinguished, the best lower
bound I can find using this method is three hypermoves. If stomach
cubies are considered, I think both bounds increase by one, since
only deep moves can touch them.

It seems strange that this method relies only on the face center
solution. Similar arguments about edges are not as good, because
so many edge positions are achievable using shallow hypermoves.
Corners are practically irrelevant, since they can be fixed using
only shallow hypermoves.

With respect to the question of odd sequences of hypermoves, Jim
Saxe mentions that ``it is plausible that sequences of the form
SDSDS may be sufficient while sequences of the form DSDSD may
not.'' I would like to add the further plausibility that both
types may be sufficient, while neither may suffice alone.