Solution

ID: past-exam-of-the-options-ii-course-of-the-university-of-oxford/2013/b2/d/ii/solution

The symmetric group has subgroups of orders . There is one trivial subgroup, three conjugate subgroups generated by transpositions, one order-three subgroup , and the whole group. These are all subgroups by Lagrange's theorem. Thus again there is one conjugacy class for each index .
The coset action classification in part (c) supplies exactly one transitive orbit type of each of these sizes. The same orbit-multiplicity equation as in part (i) applies, giving
The equality of the two counts comes from their identical lists of transitive orbit types, rather than from the groups being isomorphic.

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