# Ring homomorphism

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 $f:R\to S$ that preserves addition, multiplication and multiplicative identity; that is,
    ${\begin{aligned}f(a+b)&=f(a)+f(b),\\f(ab)&=f(a)f(b),\\f(1_{R})&=1_{S},\end{aligned}}$
for all $a,b$ in $R.$
These conditions imply that additive inverses and the additive identity are preserved too.
If in addition f is a bijection, then its inverse f$^{−1}$ is also a ring homomorphism. In this case, f 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(1$_R$) = 1$_S$. 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 f : R → S be a ring homomorphism. Then, directly from these definitions, one can deduce:
* f(0$_R$) = 0$_S$.
* f(−a) = −f(a) for all a in R.
* For any unit a in R, f(a) is a unit element such that f(a)$^{−1}$ = f(a$^{−1}$) . In particular, f induces a group homomorphism from the (multiplicative) group of units of R to the (multiplicative) group of units of S (or of im(f)).
* The image of f, denoted im(f), is a subring of S.
* The kernel of f, defined as ker(f) = {a in R | f(a) = 0$_S$}, is a two-sided ideal in R. Every two-sided ideal in a ring R is the kernel of some ring homomorphism.
* A homomorphism is injective if and only if kernel is the zero ideal.
* The characteristic of S divides the characteristic of R. This can sometimes be used to show that between certain rings R and S, no ring homomorphism R → S exists.
* If R$_p$ is the smallest subring contained in R and S$_p$ is the smallest subring contained in S, then every ring homomorphism f : R → S induces a ring homomorphism f$_p$ : R$_p$ → S$_p$.
* If R is a field (or more generally a skew-field) and S is not the zero ring, then f is injective.
* If both R and S are fields, then im(f) is a subfield of S, so S can be viewed as a field extension of R.
* If I is an ideal of S then f$^{−1}$(I) is an ideal of R.
* If R and S are commutative and P is a prime ideal of S then f$^{−1}$(P) is a prime ideal of R.
* If R and S are commutative, M is a maximal ideal of S, and f is surjective, then f$^{−1}$(M) is a maximal ideal of R.
* If R and S are commutative and S is an integral domain, then ker(f) is a prime ideal of R.
* If R and S are commutative, S is a field, and f is surjective, then ker(f) is a maximal ideal of R.
* If f is surjective, P is prime (maximal) ideal in R and ker(f) ⊆ P, then f(P) is prime (maximal) ideal in S.
Moreover,
* The composition of ring homomorphisms S → T and R → S is a ring homomorphism R → T.
* For each ring R, the identity map R → R is a ring homomorphism.
* Therefore, the class of all rings together with ring homomorphisms forms a category, the category of rings.
* The zero map R → S that sends every element of R to 0 is a ring homomorphism only if S is the zero ring (the ring whose only element is zero).
* For every ring R, there is a unique ring homomorphism Z → R. This says that the ring of integers is an initial object in the category of rings.
* For every ring R, there is a unique ring homomorphism from R to the zero ring. This says that the zero ring is a terminal object in the category of rings.
* As the initial object is not isomorphic to the terminal object, there is no zero object in the category of rings; in particular, the zero ring is not a zero object in the category of rings.

## Examples

* The function f : Z → Z/nZ, defined by f(a) = [a]$_n$ = a mod n is a surjective ring homomorphism with kernel nZ (see modular arithmetic).
* The complex conjugation C → C is a ring homomorphism (this is an example of a ring automorphism).
* For a ring R of prime characteristic p, R → R, x → x$^p$ is a ring endomorphism called the Frobenius endomorphism.
* If R and S are rings, the zero function from R to S is a ring homomorphism if and only if S is the zero ring (otherwise it fails to map 1$_R$ to 1$_S$). On the other hand, the zero function is always a rng homomorphism.
* If R[X] denotes the ring of all polynomials in the variable X with coefficients in the real numbers R, and C denotes the complex numbers, then the function f : R[X] → C defined by f(p) = p(i) (substitute the imaginary unit i for the variable X in the polynomial p) is a surjective ring homomorphism. The kernel of f consists of all polynomials in R[X] that are divisible by X$^2$ + 1.
* If f : R → S is a ring homomorphism between the rings R and S, then f induces a ring homomorphism between the matrix rings M$_n$(R) → M$_n$(S).
* Let V be a vector space over a field k. Then the map ρ : k → End(V) given by ρ(a)v = av is a ring homomorphism. More generally, given an abelian group M, a module structure on M over a ring R is equivalent to giving a ring homomorphism R → End(M).
* A unital algebra homomorphism between unital associative algebras over a commutative ring R is a ring homomorphism that is also R-linear.

## Non-examples

* The function f : Z/6Z → Z/6Z defined by f([a]$_6$) = [4a]$_6$ is a ring homomorphism (and ring endomorphism), with kernel 3Z/6Z and image 2Z/6Z (which is isomorphic to Z/3Z).
* There is no ring homomorphism Z/nZ → Z for any n ≥ 1.
* If R and S are rings, the inclusion R → R × S that sends each r to (r,0) is a rng homomorphism, but not a ring homomorphism (if S is not the zero ring), since it does not map the multiplicative identity 1 of R to the multiplicative identity (1,1) of R × S.

## Category of rings
### Endomorphisms, isomorphisms, and automorphisms

* A ring endomorphism is a ring homomorphism from a ring to itself.
* A ring isomorphism is a ring homomorphism having a 2-sided inverse that is also a ring homomorphism. One can prove that a ring homomorphism is an isomorphism if and only if it is bijective as a function on the underlying sets. If there exists a ring isomorphism between two rings R and S, then R and S are called isomorphic. Isomorphic rings differ only by a relabeling of elements. Example: Up to isomorphism, there are four rings of order 4. (This means that there are four pairwise non-isomorphic rings of order 4 such that every other ring of order 4 is isomorphic to one of them.) On the other hand, up to isomorphism, there are eleven rngs of order 4.
* A ring automorphism is a ring isomorphism from a ring to itself.

### Monomorphisms and epimorphisms

Injective ring homomorphisms are identical to monomorphisms in the category of rings: If f : R → S is a monomorphism that is not injective, then it sends some r$_1$ and r$_2$ to the same element of S. Consider the two maps g$_1$ and g$_2$ from Z[x] to R that map x to r$_1$ and r$_2$, respectively; f ∘ g$_1$ and f ∘ g$_2$ are identical, but since f is a monomorphism this is impossible.
However, surjective ring homomorphisms are vastly different from epimorphisms in the category of rings. For example, the inclusion Z ⊆ Q is a ring epimorphism, but not a surjection. However, they are exactly the same as the strong epimorphisms.