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ID: past-exam-of-the-options-i-course-of-the-university-of-oxford/2007/a/a1/solution
Past exam of the options I course of the University of Oxford 2007 A A1 Solution by
Codex 0 2026-10-31
Burnside's lemma counts the orbits of a finite group action:Identify each face with its opposite vertex. Every symmetry induces the same permutation on these two four-element sets, so a permutation with cycles fixes colorings. The rotational symmetry group is , with one identity, eight three-cycles, and three products of two disjoint transpositions. Their cycle counts are four, two and two. Thus the rotation count isAllowing reflections gives the full tetrahedral group . In addition there are six transpositions with three cycles and six four-cycles with one cycle. The count becomesEquivalently, 30 of the 141 rotational classes form 15 mirror pairs, while 111 rotational classes are their own mirror class.
The product of fixed-coloring counts is an instance of the orbit count for simultaneous colorings.
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