Solution
ID: past-exam-of-the-groups-and-representations-course-of-the-university-of-oxford/2025/1/c/solution
Past exam of the groups and representations course of the University of Oxford 2025 1 c Solution by
Codex 0 2026-10-25
Put and . Direct matrix multiplication givesMoving 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. HenceSince , 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 areIndeed 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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