Does a Fixed Sequence of Moves Solve Any Rubik’s Cube Configuration When Repeated Enough Times

algorithmscombinatoricsgroup-theorypuzzlerubiks-cube

Question: Can a fixed sequence of moves restore any Rubik's Cube configuration to its solved state when repeated enough times?

I’m wondering if it's possible to use a fixed sequence of moves (a "set of actions") to always return a Rubik's Cube to its solved state, no matter the starting position, by repeating this sequence multiple times. Here’s what I mean by a "set of actions":

A set of actions is a specific, unchanging sequence of moves. The rules are:

  • The sequence must be followed exactly as given, without any changes.
  • You can repeat the sequence as many times as needed.
  • You cannot add, remove, or change moves in the sequence during its application.
  • You must complete the entire sequence before starting over.

For instance, a method that requires varying numbers of moves wouldn’t count because it doesn’t follow a fixed, repeatable sequence.

The question is whether such a sequence exists that, when applied repeatedly, will always solve the cube, regardless of the initial configuration. The sequence doesn’t have to be the shortest or most efficient solution, just a reliable way to restore the solved state.

Does such a fixed sequence exist?

Best Answer

No. According to Wikipedia it's known that the largest order of an element of the Rubik's cube group is $1260$; what this means is that any fixed sequence of moves must start repeating states after at most $1260$ iterations, so it's not possible for any fixed sequence of moves to reach more than $1260$ states from a given state, and the Rubik's cube has many more than $1260$ possible states.

(I'm assuming intermediate states reached before the sequence of moves is finished don't count, because if they did, you could just perform, once, an extremely long sequence of moves that is guaranteed to hit every state, but I assume that isn't what you intended.)

Edit: The reason I went to all this trouble is that I didn't understand whether the Rubik's cube group acted freely on the set of reachable states, but it does; given that, the question is equivalent to asking whether the Rubik's cube group is cyclic, and then other easier arguments are available, e.g. one can show that it's nonabelian.

The discussion in the linked question also gives us a self-contained way to bound the order of an element of the Rubik's cube group $G$, which is no longer necessary here but is interesting. Namely, we can embed $G$ into the "illegal Rubik's cube group" where we allow ourselves to freely disassemble and reassemble the cube; this group is

$$G_I \cong (C_3 \wr S_8) \times (C_2 \wr S_{12})$$

where the first factor corresponds to permuting and reorienting the corner cubies and the second factor corresponds to permuting and reorienting the edge cubies. Write $e(G)$ for the exponent of a group (the smallest $n$ such that $g^n = e$ for all $g \in G$). Then

  • $e(G \times H) = \text{lcm}(e(G), e(H))$,
  • $e(C_k \wr S_n) = k \, e(S_n)$, and
  • $e(S_n) = \text{lcm}(1, 2, \dots n)$, which is A003418 on the OEIS. We have $e(S_8) = 840, e(S_{12}) = 27720$.

Putting this all together gives

$$e(G_I) = \text{lcm}(3 \times 840, 2 \times 27720) = 55440$$

so the order of an element of $G$ divides this. Then we can prove in various ways that the Rubik's cube has many more states than this. A more careful version of this argument could find the largest order of an element of $G_I$; I think it's $2520$, which is twice the above.