Re: [代數] homomorphism
Given two elements a,b in S.
Since f is onto, there exists a',b' in R, such that f(a')=a, f(b')=b.
Hence ab=f(a')f(b')=f(a'b')=f(b'a')=f(b')f(a')=ba.
(The third equation follows from R is commutative.)
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