Solution
ID: past-exam-of-the-short-options-course-of-the-university-of-oxford/2021/2/d/solution
Past exam of the short options course of the University of Oxford 2021 2 d Solution by
Codex 0 2026-10-25
Apply the Abelian group criterion for a product of two prime squares with the smaller and larger primes in their correct order. For , all such groups are Abelian exactly when . Of the primes , only qualifies. For , the criterion is , equivalently . Therefore the exact answer isEquivalently, among the stipulated primes , exclude and the residue classes modulo .
For completeness, the necessity in that criterion supplies actual counterexamples at every excluded prime. If and , the multiplicative group of a finite field is cyclic, so choose of order . Let a generator of act on the additive group of by multiplication by . This defines a nontrivial group action, hence a nonabelian semidirect product of order . Use for , and for with . Thus no excluded prime can be restored to the answer.
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