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
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
in
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:
- f(0R) = 0S.
- f(−a) = −f(a) for all a in R.
- For any unit a in R, f(a) is a unit element such that . 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, is a two-sided ideal in R. Every two-sided ideal in a ring R is the kernel of some ring homomorphism.
- An 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 exists.
- If Rp is the smallest subring contained in R and Sp is the smallest subring contained in S, then every ring homomorphism induces a ring homomorphism .
- If R is a field (or more generally a skew-field) and S is not the zero ring, then 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 −1(I) is an ideal of R.
- If R and S are commutative and P is a prime ideal of S then −1(P) is a prime ideal of R.
- If R and S are commutative, M is a maximal ideal of S, and is surjective, then −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 is surjective, then ker(f) is a maximal ideal of R.
- If is surjective, P is prime (maximal) ideal in R and, then f(P) is prime (maximal) ideal in S.
Moreover,
- The composition of ring homomorphisms and is a ring homomorphism .
- For each ring R, the identity map is a ring homomorphism.
- Therefore, the class of all rings together with ring homomorphisms forms a category, the category of rings.
- The zero map 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 . 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, defined by is a surjective ring homomorphism with kernel nZ (see modular arithmetic).
- The complex conjugation is a ring homomorphism (this is an example of a ring automorphism).
- For a ring R of prime characteristic 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 1R to 1S). 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 defined by (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 .
- If is a ring homomorphism between the rings R and S, then f induces a ring homomorphism between the matrix rings .
- Let V be a vector space over a field k. Then the map given by 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 .
- 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 defined by is a rng homomorphism (and rng endomorphism), with kernel 3Z/6Z and image 2Z/6Z (which is isomorphic to Z/3Z).
- There is no ring homomorphism for any .
- If R and S are rings, the inclusion 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 .
Category of rings
See main article: 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 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
- Book: Artin, Michael . Michael Artin . 1991 . Algebra . Englewood Cliffs, N.J. . Prentice Hall.
- Book: Bourbaki . N. . Nicolas Bourbaki . 1998 . Algebra I, Chapters 1–3 . Springer.
- Book: Eisenbud, David . David Eisenbud . 1995 . Commutative algebra with a view toward algebraic geometry . New York . . . 150 . 1322960 . 0-387-94268-8 . true . xvi+785.
- Book: Hazewinkel . Michiel . Michiel Hazewinkel . 2004 . Algebras, rings and modules . . 1-4020-2690-0.
- Book: Jacobson . Nathan . Nathan Jacobson . 1985 . Basic algebra I . 2nd . 9780486471891 .