Consider the different ways to colour a cube with given colours such that each face will be given a single colour and all the six colours will be used. Define two such colourings to be equivalent if one will get from another just by rotation. Then what would be the number of inequivalent colourings?

TIFR 2019 GS Part A, Problem 18

Combinatorics

Moderate

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For example, first let us choose the colours on the two opposite sides in ways, then another two opposite sides in ways, and the leftover sides in ways. So, in total, we get , but think about over counting.

you’re choosing three pairs of oppositely placed colours, but you are overcounting because the same division into three pairs can be chosen in

different orders, being counted erroneously as distinct by you while. At the same time, you consider one colouring and its mirror image non-distinct (swapping two opposite colours is erroneously seen as the same choice). This gives a total overcounting by a factor of .

different orders, being counted erroneously as distinct by you while. At the same time, you consider one colouring and its mirror image non-distinct (swapping two opposite colours is erroneously seen as the same choice). This gives a total overcounting by a factor of .

Another approach: There are different ways of colouring without taking rotations into consideration. There are different rotations, and for any colouring, no two rotations give identical result. Therefore these colourings may be divided into groups of essentially equal colourings. There must be such groups.

Another approach: In a circular permutation, arrangements which may be obtained from one another by rotating the cube are considered identical. Place a colour on the bottom. It does not matter which. The top colour may be chosen in five ways. Place another colour facing you. Again, it does not matter which. The remaining three colours may be arranged relative to the colour facing you in ways as we proceed clockwise around the cube. Hence, there are admissible colourings.

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