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Suppose that ##\phi : G \to H## is an isomorphism. Let ##a,b \in G##. Then ##ab = ba \implies \phi (a) \phi (b) = \phi (b) \phi (a)##. Here is where my question lies. To show that ##H## is abelian, I need to show that any two arbitrary elements commute. Why does ##\phi (a) \phi (b) = \phi (b) \phi (a)## tell me that any two arbitrary elements of ##H## commute? Does it have something to with ##\phi## being a bijection?