Ring homomorphism explained

In mathematics, a ring homomorphism is a structure-preserving function between two rings. More explicitly, if R and S are rings, then a ring homomorphism is a function

f:R\toS

that preserves addition, multiplication and multiplicative identity; that is,

\begin{align} f(a+b)&=f(a)+f(b),\\ f(ab)&=f(a)f(b),\\ f(1R)&=1S, \end{align}

for all

a,b

in

R.

These conditions imply that additive inverses and the additive identity are preserved too.

If in addition is a bijection, then its inverse −1 is also a ring homomorphism. In this case, is called a ring isomorphism, and the rings R and S are called isomorphic. From the standpoint of ring theory, isomorphic rings have exactly the same properties.

If R and S are rngs, then the corresponding notion is that of a rng homomorphism, defined as above except without the third condition f(1R) = 1S. A rng homomorphism between (unital) rings need not be a ring homomorphism.

The composition of two ring homomorphisms is a ring homomorphism. It follows that the rings forms a category with ring homomorphisms as morphisms (see Category of rings).In particular, one obtains the notions of ring endomorphism, ring isomorphism, and ring automorphism.

Properties

Let be a ring homomorphism. Then, directly from these definitions, one can deduce:

Moreover,

Examples

Non-examples

Category of rings

See main article: Category of rings.

Endomorphisms, isomorphisms, and automorphisms

Monomorphisms and epimorphisms

Injective ring homomorphisms are identical to monomorphisms in the category of rings: If is a monomorphism that is not injective, then it sends some r1 and r2 to the same element of S. Consider the two maps g1 and g2 from Z[''x''] to R that map x to r1 and r2, respectively; and are identical, but since is a monomorphism this is impossible.

However, surjective ring homomorphisms are vastly different from epimorphisms in the category of rings. For example, the inclusion is a ring epimorphism, but not a surjection. However, they are exactly the same as the strong epimorphisms.

See also

References