Solution
"C(a)" is not empty since "\\exist e \\in G" such that "a *e=e*a" so "e\\in C(a)" (where e is the identity element of G)
Since "C(a)\\subseteq G" then "C(a)" is associative under the operation "*" .
Let "x,y\\in 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)\\in C(a)"
Since "e \\in 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}\\in C(a)"
Therefore "(C(a),*)" is a subgroup of the group "(G,*)" .
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