(sub)semigroup with an endomorphism

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SUMMARY

The discussion centers on the properties of a semigroup S and its proper sub-semigroup B, particularly in relation to an endomorphism f: S → S. It is established that the image set B' = {f(b) | b ∈ B} remains a sub-semigroup of S, as it is closed under the semigroup operation. Specifically, for any elements x and y in B', the product xy can be expressed as f(ab), where ab is in B, thus confirming that B' satisfies the closure property required for sub-semigroups.

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mnb96
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Let's have a semigroup S and a proper sub-semigroup B of S.
If we have also an endomorphism [tex]f:S\rightarrow S[/tex], is it possible that the subset [tex]B'=\{f(b)|b\in B \}[/tex] is not a sub-semigroup anymore?
 
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The only requirement in order that a set, A, be a "sub-semigroup" of a semigroup S is that it be closed under the group operation. If x and y are in B', then x= f(a) and y= f(b) for some a, b in B'. But xy= f(a)f(b)= f(ab) is in B' because ab is in B.
 

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