algebra Algebra is a branch of mathematics that deals with abstract systems, known as algebraic structures, and the manipulation of expressions within those systems. It is a generalization of arithmetic that introduces variables and algebraic ope ...

, an augmentation of an associative

''A'' over a

commutative ring In mathematics, a commutative ring is a Ring (mathematics), ring in which the multiplication operation is commutative. The study of commutative rings is called commutative algebra. Complementarily, noncommutative algebra is the study of ring prope ...

''k'' is a ''k''- algebra homomorphism

A \to k

, typically denoted by ε. An algebra together with an augmentation is called an augmented algebra. The kernel of the augmentation is a two-sided ideal called the augmentation ideal of ''A''. For example, if

A =k /math> is the group algebra of a

finite group In abstract algebra, a finite group is a group whose underlying set is finite. Finite groups often arise when considering symmetry of mathematical or physical objects, when those objects admit just a finite number of structure-preserving tra ...

''G'', then :

A \to k,\, \sum a_i x_i \mapsto \sum a_i

is an augmentation. If ''A'' is a

graded algebra In mathematics, in particular abstract algebra, a graded ring is a ring such that the underlying additive group is a direct sum of abelian groups R_i such that . The index set is usually the set of nonnegative integers or the set of integers, ...

which is connected, i.e.

A_0=k

, then the homomorphism

A\to k

which maps an element to its homogeneous component of degree 0 is an augmentation. For example, :

to k, \sum a_ix^i \mapsto a_0

is an augmentation on the

polynomial ring In mathematics, especially in the field of algebra, a polynomial ring or polynomial algebra is a ring formed from the set of polynomials in one or more indeterminates (traditionally also called variables) with coefficients in another ring, ...

k /math>.

References

* Algebras {{algebra-stub