Orientation of pieces in permutation puzzles

The goal is to provide a general statement about the orientations of pieces of a certain permutation puzzles. Since there are so many different kinds of permutation puzzles we will have to exactly describe the properties of the puzzles for which the statement will be valid. We also will have to explain how the orientation of pieces can be measured for these puzzles in way as general as possible.

Then we are able to show that under these assumptions for a given kind of pieces with k possible orientations the sum of the orientations of all these pieces module k is invariant under permutations.

We restrict our consideration to permutation puzzles which do not change their shape. We call the moving parts of the puzzle the pieces and the locations of the pieces the places of the pieces. For three-dimensional puzzles we assume that the pieces are polyhedrons which are bounded by faces, for two- dimensional puzzles we assume that the pieces are polygons which are bounded by a circuit of edges. Since our main goal is the examination of three-dimensional puzzles we only use the term "faces" in the following text, but it could be replaced by "edges" to handle the two-dimensional case.

We further assume that at least some pieces can occupy their places in different positions/orientations. Since we restrict to puzzles which do not change their shape, these pieces must have at least one axis of rotational symmetry. We restrict our consideration to puzzles where the different orientations of a piece are defined by exactly one axis of rotational symmetry of order k (which does not seem to be a significant restriction). In this case we pick an arbitrary face which is not fixed by the symmetry and name it f_0. A counter clockwise (as seen from outside the puzzle) rotation by 2Pi/k*j, 1<=j<k, then maps f_0 to another face, which is denoted by f_j. The faces f_0, f_1, f_2... may be visible or not. The faces f_0, f_1 and f_2 of a Rubik's cube corner are for example visible while for the centre pieces of a 4x4x4 cube the faces f_0, f_1, f_2 and f_3 are not visible.

To define the orientation of a given piece at any possible place in any possible position we define a reference frame for the orientations. The geometric positions of the faces f_0, f_1... of a piece at a certain place of the puzzle we call slots. It is important to emphasize that only the faces move when a permutation is applied, not the slots. Applying a permutation, the slots of the places just are "filled" with the faces f_0, f_1... of a different piece of the same kind. Now we arbitrarily choose exactly one slot of every place to and call it the reference slot. We say that the orientation of a piece in this place is i if the reference slot is filled with the face f_i of that piece.

A move M of the puzzle is a permutation of some of its pieces. We hardly can establish any statement about the orientations of the pieces if there is no restriction on the permutations. We call a place P M-unambiguous if applying move M one or several times we cannot have the same piece in different orientations in place P. A place which does not have this property we call M-ambiguous. For example, for all nxnxn Rubik's cubes all places are X- unambiguous for all conceivable moves X except for odd n in the places where the face diagonals meet.

A move M usually affects several pieces and places. If we choose a piece p at a place P_0 this piece p usually visits several places P_0,P_1,... if we apply the move M several times. All the pieces which occupy these places form the M-orbit of p. All these pieces have the same shape as p and the involved places P_0, P_1... are all M-unambiguous or all M-ambiguous. The orbits of any piece of nxnxn Rubik's cube except the centre piece for odd n consist of 4 pieces for example.

With these preliminary considerations we are now able to establish the following proposition:

Proposition 1: Let M be a move of a permutation puzzle which does not change its shape and p a piece with a single k-fold rotational symmetry in a M-unambiguous place P. Then the sum of the orientations of all pieces in the orbit of p does not change its value modulo k if M is applied.

Proof: We name the involved places and slots in a way that it is easy to keep track of the orientations when the move M is applied. The place of piece p is named by P_0, the slot which is filled by face f_i (0<=i<k) of p is referred to as slot_i. Let the size of the M-orbit of p be s. Then applying M j times (0<j<s) moves piece p to a place which we call P_j. The slots of P_j are denoted in the same way as for P_0, using the faces of p – now in place P_j - again as a reference to name the slots. Named in this way we can make the following observation in the table which describes the faces of all pieces of the orbit in the slots of their place:

The faces in the slots of P_0 are well defined by the definition of the slot names. For other places, place P_2 for example we neither know the piece in P_2 nor the face in slot_1 but since the faces of all pieces in the orbit are named in the same way counter clockwise we know that we have face_(a+1) mod k in slot_2 of P_2 and face_(a+2) mod k in slot_3 of P_2.

And most important, in the way the slots and places are named, if we apply move M all entries of the table are cyclically shifted one line down, the entries of line P_(s-1) move to line P_0 (this would not work if the places would be M-ambiguous).

With the following example we show that the sum of the orientations modulo k does not change when applying move M. The orientations are defined by the reference slots which can in principle be chosen arbitrarily, they are highlighted in the example below. Obviously there has to be one reference slot in a line, but for the number of reference slots in a column there are no restrictions.

Before applying move M:

According to the definition of the orientations the sum of the orientations in the orbit modulo k is
1 + (a+2) + b + (c+k-1) + (d+1) + (e+k-1) mod k = a+b+c+d+e+2k+2 mod k.

After applying move M:

The orientation sum is
(e+1) + 2 + a +(b+k-1) + (c+1) + (d+k-1) mod k = a+b+c+d+e+2k+2 mod k.

This is the same as before and the reason is quite clear: The shifting of the table entries just shifts the order in which the variables a, b, c ... are added and the "offsets" 0, 1, 2 ... k-1 stay the same.

This argument obviously does not depend on the choice of the orbit size s - which was 6 in this example- or the choice of the reference slots, so the proposition is proved.

Proposition 2: Let in addition to the conditions of proposition 1 be P a place which is unambiguous with respect to several moves M_1, M_2, ...M_n. Then the sum of all orientations modulo k of the pieces in the union U of all orbits of M_1, M_2,.. M_n does not change if any of the moves is applied.

Proof: Applying a move M_i for a fixed i does not change the the sum of the orientations of the pieces in the orbit O of M_i. Pieces in the complement U\O do not move at all and hence their orientations do not change. So the sum of all orientations of the pieces of U does not change modulo k.

Corollary : Given a permutation puzzle which does not change its shape, a place P that is unambiguous for all possible moves and a piece p in P. Then rotating p in place P by a sequence of moves without changing the orientation of other pieces is impossible. p