Ideal (ring theory)
In ring theory, a branch of abstract algebra, an ideal of a ring R is a subset I of R which is closed under R-linear combinations, in a sense made precise below. The concept of an order ideal that is known in order theory is derived from this notion is discussed in its dedicated article.An ideal can be used to construct a factor ring in a similar way as a normal subgroup in group theory can be used to construct a factor group.
Table of contents |
2 Examples 3 Further properties of ideals 4 Types of ideals 5 Factor rings (quotient rings) and kernels 6 Ideal operations 7 Ideals as "ideal numbers" |
Definitions
To accommodate non-commutative rings, we must distinguish three cases: left ideals, right ideals, and two-sided ideals.
A subset I of the ring R is a left ideal of R if
- 1: the zero element 0 of R belongs to I
- 2: for any a,b in I, we have a + b in I, and
- 3L: for any a in I and r in R, we have ra in I
- 3R: for any a in I and r in R, we have ar in I
If the ring R is commutative, then all three sorts of ideals are the same. If the ring is noncommutative, however, then they may be different.
Examples
- The even integers form an ideal in the ring Z of all integers; it is usually denoted by 2Z.
- The set of all polynomials with real coefficients which are divisible by the polynomial x^{2} + 1 is an ideal in the ring of all polynomials.
- The set of all n-by-n matrices whose last column is zero forms a left ideal in the ring of all n-by-n matrices. It is not a right ideal. The set of all n-by-n matrices whose last row is zero forms a right ideal but not a left ideal.
- The ring C(R) of all continuous functions f from R to R contains the ideal of all continuous functions f such that f(1) = 0. Another ideal in C(R) is given by those functions which vanish for large enough arguments, i.e. those continuous functions f for which there exists a number L > 0 such that f(x) = 0 whenever |x| > L.
- {0} and R are ideals in every ring R. If R is commutative, then R is a field iff it has precisely two ideals, {0} and R.
Further properties of ideals
Because zero belongs to it, any ideal is nonempty. In fact, property 1 in the definition can be replaced with simply the requirement that I be nonempty.
Any left, right or two-sided ideal is a subgroup of the additive group (R,+).
The ring R can be considered as a left module over itself, and the left ideals of R are then seen as the submodules of this module. Similarly, the right ideals are submodules of R as a right module over itself, and the two-sided ideals are submodules of R as a bimodule over itself. If R is commutative, then all three sorts of module are the same, just as all three sorts of ideal are the same.
Types of ideals
An ideal I is called proper if I is not equal to R. An ideal is proper if and only if it doesn't contain 1. A proper ideal is called maximal if the only proper ideal it is contained in is itself. Every ideal is contained in a maximal ideal, a consequence of Zorn's lemma. A proper ideal I is called prime if, whenever ab belongs to I, then so does a or b (or both). Every maximal ideal is prime.
Factor rings (quotient rings) and kernels
Recall that a function f from R to S is a ring homomorphism iff f(a + b) = f(a) + f(b) and f(ab) = f(a) f(b) for all a, b in R and f(1) = 1. Then the kernel of f is defined as
- ker(f) := {a in R : f(a) = 0}.
Conversely, if we start with a two-sided ideal I of R, then we may define a congruence relation ~ on R as follows: a ~ b if and only if b - a is in I. In case a ~ b, we say that a and b are congruent modulo I. The equivalence class of the element a in R is given by
- [a] = a + I := {a + r : r in I}.
- (a + I) + (b + I) = (a + b) + I;
- (a + I) * (b + I) = (ab) + I.
The map p from R to R/I defined by p(a) = a + I is a surjective ring homomorphism (or regular epimorphism) whose kernel is the original ideal I. In summary, we see that ideals are precisely the kernels of ring homomorphisms.
If R is commutative and I is a maximal ideal, then the factor ring R/I is a field; if I is only a prime ideal, then R/I is only an integral domain.
The most extreme examples of factor rings are provided by modding out by the most extreme ideals, {0} and R itself. R/{0} is naturally isomorphic to R, and R/R is the trivial ring {0}.
Ideal operations
The sum and the intersection of ideals is again an ideal; with these two operations as join and meet, the set of all ideals of a given ring forms a lattice.
If A is any subset of the ring R, then we can define the ideal generated by A to be the smallest ideal of R containing A; it is denoted by <A> or (A) and contains all finite sums of the form
- r_{1}a_{1}s_{1} + ··· + r_{n}a_{n}s_{n}
The product of two ideals I and J is defined to be the ideal IJ generated by all products of the form ab with a in I and b in J. It is contained in the intersection of I and J.
Important properties of these ideal operations are recorded in the Noether isomorphism theorems.
Ideals as "ideal numbers"
The term "ideal" comes from the notion of ideal number: ideals were seen as a generalization of the concept of number. In the ring Z of integers, every ideal can be generated by a single number (so Z is a principal ideal domain), and the ideal determines the number up to its sign. The concepts of "ideal" and "number" are therefore almost identical in Z (and in any principal ideal domain). In other rings, it turned out that the concept of "ideal" allows one to generalize several properties of numbers. For instance, in general rings one studies prime ideals instead of prime numbers, one defines coprime ideals as a generalization of coprime numbers, and one can prove a generalized Chinese remainder theorem about ideals. In a certain class of rings important in number theory, the Dedekind domains, one can even recover a version of the fundamental theorem of arithmetic: in these rings, every nonzero ideal can be uniquely written as a product of prime ideals.
See also: