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endomorphism ring
In abstract algebra, the endomorphism ring of an abelian group ''X'', denoted by ''End''(''X''), is the set of all homomorphisms of ''X'' into itself. The addition operation is defined by pointwise addition of functions and the multiplication operation is defined by function composition.
The functions involved are restricted to what is defined as a homomorphism in the context, which depends upon the category of the object under consideration. The endomorphism ring consequently encodes several internal properties of the object. As the resulting object is often an algebra over some ring ''R,'' this may also be called the endomorphism algebra.
==Description==
Let be an abelian group and we consider the group homomorphisms from ''A'' into ''A''. Then addition of two such homomorphisms may be defined pointwise to produce another group homomorphism. Explicitly, given two such homomorphisms ''f'' and ''g'', the sum of ''f'' and ''g'' is the homomorphism . Under this operation ''End''(''A'') is an abelian group. With the additional operation of composition of homomorphisms, ''End''(''A'') is a ring with multiplicative identity. This composition is explicitly . The multiplicative identity is the identity homomorphism on ''A''.
If the set ''A'' does not form an ''abelian'' group, then the above construction is not necessarily additive, as then the sum of two homomorphisms need not be a homomorphism. This set of endomorphisms is a canonical example of a near-ring which is not a ring.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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