# Algebra homomorphism

In mathematics, an algebra homomorphism is an homomorphism between two associative algebras. More precisely, if A and B are algebras over a field (or commutative ring) K, it is a function $F\colon A\to B$ such that for all k in K and x, y in A,

• $F(kx)=kF(x)$ • $F(x+y)=F(x)+F(y)$ • $F(xy)=F(x)F(y)$ The first two conditions say that F is a K-linear map (or K-module homomorphism if K is a commutative ring), and the last condition says that F is a (non-unital) ring homomorphism.

If F admits an inverse homomorphism, or equivalently if it is bijective, F is said to be an isomorphism between A and B.

## Unital algebra homomorphisms

If A and B are two unital algebras, then an algebra homomorphism $F:A\rightarrow B$  is said to be unital if it maps the unity of A to the unity of B. Often the words "algebra homomorphism" are actually used to mean "unital algebra homomorphism", in which case non-unital algebra homomorphisms are excluded.

A unital algebra homomorphism is a (unital) ring homomorphism.

## Examples

• Every ring is a $\mathbb {Z}$ -algebra since there always exists a unique homomorphism $\mathbb {Z} \to R$ . See Associative algebra#Examples for the explanation.
• Any homomorphism of commutative rings $R\to S$  gives $S$  the structure of a commutative R-algebra. Conversely, if S is a commutative R-algebra, the map $r\mapsto r\cdot 1_{S}$  is a homomorphism of commutative rings. It is straightforward to deduce that the overcategory of the commutative rings over R is the same as the category of commutative $R$ -algebras.
• If A is a subalgebra of B, then for every invertible b in B the function that takes every a in A to b−1 a b is an algebra homomorphism (in case $A=B$ , this is called an inner automorphism of B). If A is also simple and B is a central simple algebra, then every homomorphism from A to B is given in this way by some b in B; this is the Skolem–Noether theorem.