Integral element


In commutative algebra, an element b of a commutative ring B is said to be integral over a subring A of B if b is a root of some monic polynomial over A.
If A, B are fields, then the notions of "integral over" and of an "integral extension" are precisely "algebraic over" and "algebraic extensions" in field theory.
The case of greatest interest in number theory is that of complex numbers integral over Z ; in this context, the integral elements are usually called algebraic integers. The algebraic integers in a finite extension field k of the rationals Q form a subring of k, called the ring of integers of k, a central object of study in algebraic number theory.
In this article, the term ring will be understood to mean commutative ring with a multiplicative identity.

Definition

Let be a ring and let be a subring of
An element of is said to be integral over if for some there exists in such that
The set of elements of that are integral over is called the integral closure of in The integral closure of any subring in is, itself, a subring of and contains If every element of is integral over then we say that is integral over, or equivalently is an integral extension of

Examples

Integral closure in algebraic number theory

There are many examples of integral closure which can be found in algebraic number theory since it is fundamental for defining the ring of integers for an algebraic field extension .

Integral closure of integers in rationals

s are the only elements of Q that are integral over Z. In other words, Z is the integral closure of Z in Q.

Quadratic extensions

The Gaussian integers are the complex numbers of the form, and are integral over Z. is then the integral closure of Z in. Typically this ring is denoted.
The integral closure of Z in is the ring
This example and the previous one are examples of quadratic integers. The integral closure of a quadratic extension can be found by constructing the minimal polynomial of an arbitrary element and finding number-theoretic criterion for the polynomial to have integral coefficients. This analysis can be found in the quadratic extensions article.

Roots of unity

Let ζ be a root of unity. Then the integral closure of Z in the cyclotomic field Q is Z. This can be found by using the minimal polynomial and using Eisenstein's criterion.

Ring of algebraic integers

The integral closure of Z in the field of complex numbers C, or the algebraic closure is called the ring of algebraic integers.

Other

The roots of unity, nilpotent elements and idempotent elements in any ring are integral over Z.

Integral closure in algebraic geometry

In geometry, integral closure is closely related with normalization and normal schemes. It is the first step in resolution of singularities since it gives a process for resolving singularities of codimension 1.
  • For example, the integral closure of is the ring since geometrically, the first ring corresponds to the -plane unioned with the -plane. They have a codimension 1 singularity along the -axis where they intersect.
  • Let a finite group G act on a ring A. Then A is integral over AG, the set of elements fixed by G; see Ring of invariants.
  • Let R be a ring and u a unit in a ring containing R. Then
  1. u−1 is integral over R if and only if u−1 ∈ R.
  2. is integral over R.
  3. The integral closure of the homogeneous coordinate ring of a normal projective variety X is the ring of sections

    Integrality in algebra

  • If is an algebraic closure of a field k, then is integral over
  • The integral closure of Cx in a finite extension of C) is of the form

    Equivalent definitions

Let B be a ring, and let A be a subring of B. Given an element b in B, the following conditions are equivalent:
The usual proof of this uses the following variant of the Cayley–Hamilton theorem on determinants:
This theorem gives ⇒ and the rest is easy. Coincidentally, Nakayama's lemma is also an immediate consequence of this theorem.

Elementary properties

Integral closure forms a ring

It follows from the above four equivalent statements that the set of elements of that are integral over forms a subring of ' containing. This ring is called the integral closure''' of in.

Transitivity of integrality

Another consequence of the above equivalence is that "integrality" is transitive, in the following sense. Let be a ring containing and. If is integral over ' and ' integral over, then is integral over. In particular, if is itself integral over ' and ' is integral over, then is also integral over.

Integral closed in fraction field

If happens to be the integral closure of in ', then A is said to be integrally closed in '. If is the total ring of fractions of,, then one sometimes drops the qualification "in and simply says "integral closure of " and " is integrally closed." For example, the ring of integers is integrally closed in the field.

Transitivity of integral closure with integrally closed domains

Let A be an integral domain with the field of fractions K and A' the integral closure of A in an algebraic field extension L of K. Then the field of fractions of A' is L. In particular, A' is an integrally closed domain.
Transitivity in algebraic number theory
This situation is applicable in algebraic number theory when relating the ring of integers and a field extension. In particular, given a field extension the integral closure of in is the ring of integers.

Remarks

Note that transitivity of integrality above implies that if is integral over, then is a union of subrings that are finitely generated -modules.
If is noetherian, transitivity of integrality can be weakened to the statement:

Relation with finiteness conditions

Finally, the assumption that be a subring of can be modified a bit. If is a ring homomorphism, then one says is integral if is integral over. In the same way one says is finite or of finite type. In this viewpoint, one has that
Or more explicitly,

Integral extensions

Cohen-Seidenberg theorems

An integral extension A ⊆ B has the going-up property, the lying over property, and the incomparability property. Explicitly, given a chain of prime ideals in A there exists a in B with and two distinct prime ideals with inclusion relation cannot contract to the same prime ideal. In particular, the Krull dimensions of A and B are the same. Furthermore, if A is an integrally closed domain, then the going-down holds.
In general, the going-up implies the lying-over. Thus, in the below, we simply say the "going-up" to mean "going-up" and "lying-over".
When A, B are domains such that B is integral over A, A is a field if and only if B is a field. As a corollary, one has: given a prime ideal of B, is a maximal ideal of B if and only if is a maximal ideal of A. Another corollary: if L/''K is an algebraic extension, then any subring of L'' containing K is a field.

Applications

Let B be a ring that is integral over a subring A and k an algebraically closed field. If is a homomorphism, then f extends to a homomorphism B → k. This follows from the going-up.

Geometric interpretation of going-up

Let be an integral extension of rings. Then the induced map
is a closed map; in fact, for any ideal I and is surjective if f is injective. This is a geometric interpretation of the going-up.

Geometric interpretation of integral extensions

Let B be a ring and A a subring that is a noetherian integrally closed domain. If B is integral over A, then is submersive; i.e., the topology of is the quotient topology. The proof uses the notion of constructible sets.

Integrality, base-change, universally-closed, and geometry

If is integral over, then is integral over R for any A-algebra R. In particular, is closed; i.e., the integral extension induces a "universally closed" map. This leads to a geometric characterization of integral extension. Namely, let B be a ring with only finitely many minimal prime ideals. Then B is integral over a A if and only if is closed for any A-algebra R. In particular, every proper map is universally closed.

Galois actions on integral extensions of integrally closed domains

Proof. Suppose for any in G. Then, by prime avoidance, there is an element x in such that for any. G fixes the element and thus y is purely inseparable over K. Then some power belongs to K; since A is integrally closed we have: Thus, we found is in but not in ; i.e.,.

Application to algebraic number theory

The Galois group then acts on all of the prime ideals lying over a fixed prime ideal. That is, if
then there is a Galois action on the set. This is called the Splitting of prime ideals in Galois extensions.

Remarks

The same idea in the proof shows that if is a purely inseparable extension, then is bijective.
Let A, K, etc. as before but assume L is only a finite field extension of K. Then
Indeed, in both statements, by enlarging L, we can assume L is a normal extension. Then is immediate. As for, by the going-up, we can find a chain that contracts to. By transitivity, there is such that and then are the desired chain.