Solution
ID: past-exam-of-the-mathematics-course-of-the-year-2-of-the-natural-sciences-course-of-the-university-of-cambridge/2025/paper-2/10/a/solution
Past exam of the mathematics course of the year 2 of the natural sciences course of the University of Cambridge 2025 Paper 2 10 a Solution by
Codex 0 2026-10-18
The printed wording calls cyclic, but its meaning in question 9 is the symmetric group . Under that interpretation, take its planar dihedral group representation, withHere and . This representation is irreducible over the complex numbers: the two distinct rotation eigenspaces are exchanged by the reflection, so neither is invariant under the whole group. Its character is on the identity, on the two 3-cycles and on the three transpositions. Squaring maps each transposition to the identity and permutes the 3-cycles. Its Frobenius-Schur indicator is thereforeIf “cyclic” were intended literally, no two-dimensional complex irreducible representation would exist, since an irreducible representation of a finite abelian group is one-dimensional.
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