A commutative algebra over a field is a vector space
over
equipped with bilinear multiplication satisfying
for all
. The usual associative
algebra convention also requires multiplication
to be associative and to have a multiplicative
identity (Wisbauer 1991, p. 30). For example, the polynomial
ring
and the ring of dual numbers
are commutative
algebras over
.
More generally, for a commutative ring with multiplicative
identity, a unital commutative
-algebra is a commutative ring
with a specified identity-preserving
ring homomorphism
. This ring homomorphism
need not be injective. For instance,
is a commutative
-algebra via the map
. For a Lie algebra,
the term "commutative" conventionally means that the Lie
bracket is zero for every pair of elements.
The term "commutative algebra" also refers to the branch of abstract algebra that studies commutative rings. Commutative algebra is important in algebraic geometry.