A cube has vertices The 12 edges of the cube are each coloured one of four colours subject to the following conditions:
A) Each colour appears exactly once around each face of the cube.
B) No two faces of the cube have the same cyclic ordering of the colors (taken clockwise).
How many different ways are there to colour the edges of the cube?
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Let the colours be 1 , 2 , 3 , 4 . Notice that there are 6 ways to cyclically order the numbers 1 , 2 , 3 , 4 , so each ordering must occur on one of the six faces of the cube. Consider the ordering 1 , 2 , 3 , 4 . There are 6 choices for which face this occurs on and 4 choices for which edge on that face has the colour 1. We label the other edges as in the figure of the cube below:
The edge a can be either 2 or 3 so as not to violate condition A , similarly the edge b can be either 3 or 4. If neither edge is 3, then we can not complete the top face without violating one of the two conditions. We also cannot have both being 3, since this violates A . If a = 3 , then b = 4 , c = 1 , d = 2 and the other edges are uniquely determined by rule A and all faces will satisfy rule B . Similarly, if b = 3 , then c = 4 , d = 1 , a = 2 and the rest of the edges are uniquely determined. This gives two colourings of the cube for each of the choices for our first face, for a total of 2 × 2 4 = 4 8 colourings.