page1880 Rings Matthias Lorentzen...mattegrisenforlag.com
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Question
Given a homomorphism `phi : A rightarrow B`, we define an equivalence
relation ~ on A by:
`x ~ y Leftrightarrow phi(x) = phi(y)`
`Rightarrow phi(x) - phi(y) = 0 Rightarrow
phi(x - y) = 0`
`Rightarrow x - y in ker(phi)`.
Then it follows that `y in x + ker(phi)`
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