Solution
C(a) is not empty since ∃e∈G such that a∗e=e∗a so e∈C(a) (where e is the identity element of G)
Since C(a)⊆G then C(a) is associative under the operation ∗ .
Let x,y∈C(a), then a∗x=x∗a and a∗y=y∗a
a∗x∗y=a∗(x∗y)=(x∗y)∗a (by definition) so (x∗y)∈C(a)
Since e∈C(a) we have a∗e=a∗(x∗x−1)=(a∗x)∗x−1 and e∗a=(x−1∗x)∗a=x−1∗(x∗a) which implies that x−1∈C(a)
Therefore (C(a),∗) is a subgroup of the group (G,∗) .
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