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.[1][2] 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.


Let (A, +) 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 (f + g)(x) := f(x) + g(x). 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 (fg)(x) := f(g(x)). 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.[3] This set of endomorphisms is a canonical example of a near-ring which is not a ring.




  1. Fraleigh (1976, p. 211)
  2. Passman (1991, pp. 4–5)
  3. Dummit & Foote, p. 347)
  4. Jacobson 2009, p. 118.
  5. Jacobson 2009, p. 111, Prop. 3.1.
  6. Wisbauer 1991, p.163.
  7. Wisbauer 1991, p. 263.
  8. Camillo et al. Zhou.
  9. Abelian groups may also be viewed as modules over the ring of integers.
  10. Drozd & Kirichenko 1994, pp. 23–31.


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