Unique factorization domain

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In mathematics, a unique factorization domain (UFD) is, roughly speaking, a commutative ring in which every element can be uniquely written as a product of prime elements, analogous to the fundamental theorem of arithmetic for the integers. UFDs are sometimes called factorial rings, following the terminology of Bourbaki.

Formally, a unique factorization domain is defined to be an integral domain R in which every non-zero non-unit x of R can be written as a product of irreducible elements of R:

x = p1 p2 ... pn

and this representation is unique in the following sense: if q1,...,qm are irreducible elements of R such that

x = q1 q2 ... qm,

then m = n and there exists a bijective map φ : {1,...,n} -> {1,...,n} such that pi is associated to qφ(i) for i = 1, ..., n.

The uniqueness part is sometimes hard to verify, which is why the following equivalent definition is useful: a unique factorization domain is an integral domain R in which every non-zero non-unit can be written as a product of prime elements of R.

Examples

Most rings familiar from elementary mathematics are UFD's:

Here are some more examples of UFDs:

Despite these examples, very few integral domains are UFDs. Here is a counterexample:

  • The ring Z[√ −5] of all complex numbers of the form a + b √ −5, where a and b are integers. Then 6 factors as both (2)(3) and as (1 + √ −5) (1 − √ −5). These truly are different factorizations, because the only units in this ring are 1 and −1; thus, none of 2, 3, 1 + √ −5, and 1 − √ −5 are associate. It is not hard to show that all four factors are irreducible as well, though this may not be obvious. See also algebraic integer.

Most factor rings of a polynomial ring are not UFDs. Here is an example:

  • Let R be any commutative ring. Then R[X,Y,Z,W]/(XY-ZW) is not a UFD. It is clear that X, Y, Z, and W are all irreducibles, so the element XY=ZW has two factorizations into irreducible elements.

Properties

Additional examples of UFDs can be constructed as follows:

  • If R is a UFD, then so is the polynomial ring R[X]. By induction, we can show that the polynomial rings Z[X1, ..., Xn] as well as K[X1, ..., Xn] (K a field) are UFD's. (Any polynomial ring with more than one variable is an example of a UFD that is not a principal ideal domain.)

Some concepts defined for integers can be generalized to UFDs:

  • In UFD's, every irreducible element is prime. (In any integral domain, every prime element is irreducible, but the converse does not always hold.)
  • Any two (or finitely many) elements of a UFD have a greatest common divisor and a least common multiple. Here, a greatest common divisor of a and b is an element d which divides both a and b, and such that every other common divisor of a and b divides d. All greatest common divisors of a and b are associated.

Equivalent conditions for a ring to be a UFD

Under some circumstances, it is possible to give equivalent conditions for a ring to be a UFD.

  • An integral domain is a UFD if and only if the ascending chain condition holds for principal ideals, and any two elements of A have a least common multiple.

fr:Anneau factoriel

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