- #1
Mr Davis 97
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Homework Statement
Suppose that ##\langle S,*\rangle## has an identity e for *. If ##\phi : S \rightarrow S'## is an isomorphism of ##\langle S,*\rangle## with ##\langle S',*\rangle##, then ##\phi (e)## is an identity element for the binary operation ##*'## on S'.
Homework Equations
The Attempt at a Solution
We know that ##e*s = s*e = s##. Since ##\phi## is a function, we then have ##\phi (e*s) = \phi (s*e) = \phi(s)##. By definition of homomorphism, we have that ##\phi (e) *' \phi (s) = \phi (s) *' \phi (e) = \phi(s)##. Since ##\phi## is a surjection, we know that there exists an s in S s.t. for all s' in S' ##\phi(s) = s'##. Thus ##\phi (e) *' s' = s' *' \phi (e) = s'##, for all s' in S'.
This seems to be the correct proof. However, nowhere do I use the fact that phi is injective. Does this mean that I have done something wrong?