Solution

ID: past-exam-of-the-mathematics-course-of-the-university-of-oxford/2015/prelims/mathematics-i/6/a/i/solution

Let , with a prime number, and choose . By Lagrange's theorem, the order of a group element divides and is greater than one. It is therefore , and . Hence a group of prime cardinality is a cyclic group.
The converse is false: the cyclic group has four elements, and is not a prime number.

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