Solution

ID: past-exam-of-the-groups-and-representations-course-of-the-university-of-oxford/2025/1/c/solution

Put and . Direct matrix multiplication gives
Moving every to the right shows that every word in these generators of a group has the form , with and . The three are distinct diagonal matrices, and the three are distinct off-diagonal matrices. Hence
Since , this is a non-abelian group. Its group presentation identifies it with the dihedral group of an equilateral triangle, equivalently the symmetric group .
The conjugacy classes are
Indeed interchanges and by conjugation, while runs through all three reflections. No reflection is conjugate to a rotation, since conjugation preserves order of a group element.
There are three inequivalent complex irreducible representations, one for each conjugacy class. The sum of squares of irreducible representation dimensions gives . One representation is trivial, and the only possible positive integer dimensions are

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