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Determine all the permutations $\sigma \in S_5$ that commute with the permutation $\alpha = (1 \space 2)(3 \space 4 \space 5).$

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Hint: $\varphi\in S_n$ then $$\varphi(a_1,a_2,\cdots,a_k)\varphi^{-1}=(\varphi(a_1),\cdots,\varphi(a_k)).$$

Suppose $\varphi\in S_5$ such that $\varphi\sigma \varphi^{-1}=\sigma$ then $\varphi(1)=1,2$ and hence $\varphi(2)=2,1$.

Similarly for $(3,4,5)$.

Nirvanacs
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