Solution
ID: past-exam-of-the-mathematics-course-of-the-university-of-oxford/2017/prelims/mathematics-i/6/c/iii/solution
Past exam of the mathematics course of the University of Oxford 2017 Prelims Mathematics I 6 c iii Solution by
Codex 0 2026-10-25
Choose a normal subgroup of prime order as in ii, and choose a subgroup of the other prime order . Their intersection is trivial by Lagrange's theorem. Both are cyclic groups, and ii gives .
The image of a group homomorphism from iii has order dividing , because its kernel of a group homomorphism partitions into equally sized cosets. Its order also divides by Lagrange's theorem. Butso is trivial in either case. By iii, has order and therefore equals .
If and generate and , the element has order , since its th power is the identity exactly when both and divide . Thus this direct product of groups is cyclic, and
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