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Abelian Groups - László Fuchs [Springer]

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2 Maps and Diagrams 7<br />

A homomorphism ˛ with Im ˛ D 0 is referred to as a zero homomorphism; it<br />

will be denoted by 0. IfA B, then the map that assigns to every a 2 A itself may<br />

be regarded as a homomorphism of A into B; itiscalledtheinjection or inclusion<br />

map. The injection 0 ! A is the unique homomorphism of the group 0 into A. If<br />

˛ W A ! B and C A, then the restriction D ˛ C has the domain C and<br />

codomain B. In this case, ˛ is viewed as an extension of , written ˛.<br />

If ˛0 <

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