In
mathematics, more specifically in
ring theory, a Euclidean domain (also called a Euclidean ring) is an
integral domain that can be endowed with a
Euclidean function which allows a suitable generalization of the
Euclidean division of
integers. This generalized Euclidean algorithm can be put to many of the same uses as Euclid's original algorithm in the
ring of integers: in any Euclidean domain, one can apply the Euclidean algorithm to compute the
greatest common divisor of any two elements. In particular, the greatest common divisor of any two elements exists and can be written as a linear combination
of them (
Bézout's identity). Also every
ideal in a Euclidean domain is
principal, which implies a suitable generalization of the
fundamental theorem of arithmetic: every Euclidean domain is a
unique factorization domain.
It is important to compare the
class of Euclidean domains with the larger class of
principal ideal domains (PIDs). An arbitrary PID has much the same "structural properties" of a Euclidean domain (or, indeed, even of the ring of integers), but when an explicit
algorithm for Euclidean division is known, one may use the
Euclidean algorithm and
extended Euclidean algorithm to compute greatest common divisors and
Bézout's identity. In particular, the existence of efficient algorithms for Euclidean division of integers and of
polynomials in one variable over a
field is of basic importance in
computer algebra.
So, given an integral domain R, it is often very useful to know that R has a Euclidean function: in particular, this implies that R is a PID. However, if there is no "obvious" Euclidean function, then determining whether R is a PID is generally a much easier problem than determining whether it is a Euclidean domain.
Euclidean domains appear in the following chain of
class inclusions:
Let R be an integral domain. A Euclidean function on R is a
functionf from R \ {0} to the non-negative integers satisfying the following fundamental division-with-remainder property:
(EF1) If a and b are in R and b is nonzero, then there exist q and r in R such that a = bq + r and either r = 0 or f (r) < f (b).
A Euclidean domain is an integral domain which can be endowed with at least one Euclidean function. A particular Euclidean function f is not part of the definition of a Euclidean domain, as, in general, a Euclidean domain may admit many different Euclidean functions.
In this context, q and r are called respectively a quotient and a remainder of the division (or Euclidean division) of a by b. In contrast with the case of
integers and
polynomials, the quotient is generally not uniquely defined, but when a quotient has been chosen, the remainder is uniquely defined.
Most algebra texts require a Euclidean function to have the following additional property:
(EF2) For all nonzero a and b in R, f (a) ≤ f (ab).
However, one can show that (EF1) alone suffices to define a Euclidean domain; if an integral domain R is endowed with a function g satisfying (EF1), then R can also be endowed with a function satisfying both (EF1) and (EF2) simultaneously. Indeed, for a in R \ {0} , one can define f (a) as follows:[1]
In words, one may define f (a) to be the minimum value attained by g on the set of all non-zero elements of the principal ideal generated by a.
A Euclidean function f is multiplicative if f (ab) = f (a) f (b) and f (a) is never zero. It follows that f (1) = 1. More generally, f (a) = 1 if and only if a is a
unit.
Notes on the definition
Many authors use other terms in place of "Euclidean function", such as "degree function", "valuation function", "gauge function" or "norm function".[2] Some authors also require the
domain of the Euclidean function to be the entire ring R;[2] however, this does not essentially affect the definition, since (EF1) does not involve the value of f (0). The definition is sometimes generalized by allowing the Euclidean function to take its values in any
well-ordered set; this weakening does not affect the most important implications of the Euclidean property.
The property (EF1) can be restated as follows: for any principal ideal I of R with nonzero generator b, all nonzero classes of the
quotient ringR/I have a representative r with f (r) < f (b). Since the possible values of f are well-ordered, this property can be established by showing that f (r) < f (b) for any r ∉ I with minimal value of f (r) in its class. Note that, for a Euclidean function that is so established, there need not exist an effective method to determine q and r in (EF1).
Examples
Examples of Euclidean domains include:
Any field. Define f (x) = 1 for all nonzero x.
Z, the ring of integers. Define f (n) = |n|, the
absolute value of n.[3]
Z[ i ], the ring of
Gaussian integers. Define f (a + bi) = a2 + b2, the
norm of the Gaussian integer a + bi.
K[[X]], the ring of
formal power series over the field K. For each nonzero
power seriesP, define f (P) as the
order of P, that is the degree of the smallest power of X occurring in P. In particular, for two nonzero power series P and Q, f (P) ≤ f (Q) if and only if PdividesQ.
Any
discrete valuation ring. Define f (x) to be the highest power of the
maximal idealM containing x. Equivalently, let g be a generator of M, and v be the unique integer such that g v is an
associate of x, then define f (x) = v. The previous example K[[X]] is a special case of this.
Examples of domains that are not Euclidean domains include:
Every domain that is not a
principal ideal domain, such as the ring of polynomials in at least two indeterminates over a field, or the ring of univariate polynomials with integer
coefficients, or the number ring Z[ √−5 ].
The
ring of integers of Q( √−19 ), consisting of the numbers a + b√−19/2 where a and b are integers and both even or both odd. It is a principal ideal domain that is not Euclidean.This was proved by
Theodore Motzkin and was the first case known.[6]
The ring A = RX, Y]/(X 2 + Y 2 + 1) is also a principal ideal domain[7] that is not Euclidean. To see that it is not a Euclidean domain, it suffices to show that for every non-zero prime , the map induced by the quotient map is not
surjective.[8]
Properties
Let R be a domain and f a Euclidean function on R. Then:
R is a
principal ideal domain (PID). In fact, if I is a nonzero
ideal of R then any element a of I \ {0} with minimal value (on that set) of f(a) is a generator of I.[9] As a consequence R is also a
unique factorization domain and a
Noetherian ring. With respect to general principal ideal domains, the existence of factorizations (i.e., that R is an
atomic domain) is particularly easy to
prove in Euclidean domains: choosing a Euclidean function f satisfying (EF2), x cannot have any decomposition into more than f(x) nonunit factors, so starting with x and repeatedly decomposing reducible factors is bound to produce a factorization into
irreducible elements.
Any element of R at which f takes its globally minimal value is invertible in R. If an f satisfying (EF2) is chosen, then the
converse also holds, and f takes its minimal value exactly at the invertible elements of R.
If Euclidean division is algorithmic, that is, if there is an
algorithm for computing the quotient and the remainder, then an
extended Euclidean algorithm can be defined exactly as in the case of integers.[10]
If a Euclidean domain is not a field then it has an element a with the following property: any element x not divisible by a can be written as x = ay + u for some unit u and some element y. This follows by taking a to be a non-unit with f(a) as small as possible. This strange property can be used to show that some principal ideal domains are not Euclidean domains, as not all PIDs have this property. For example, for d = −19, −43, −67, −163, the
ring of integers of is a PID which is not Euclidean, but the cases d = −1, −2, −3, −7, −11 are Euclidean.[11]
However, in many
finite extensions of Q with
trivialclass group, the ring of integers is Euclidean (not necessarily with respect to the absolute value of the field norm; see below).
Assuming the
extended Riemann hypothesis, if K is a finite
extension of Q and the ring of integers of K is a PID with an infinite number of units, then the ring of integers is Euclidean.[12]
In particular this applies to the case of
totally realquadratic number fields with trivial class group.
In addition (and without assuming ERH), if the field K is a
Galois extension of Q, has trivial class group and
unit rank strictly greater than three, then the ring of integers is Euclidean.[13]
An immediate
corollary of this is that if the
number field is Galois over Q, its class group is trivial and the extension has
degree greater than 8 then the ring of integers is necessarily Euclidean.
Norm-Euclidean fields
Algebraic number fieldsK come with a canonical norm function on them: the absolute value of the
field normN that takes an
algebraic elementα to the product of all the
conjugates of α. This norm maps the
ring of integers of a number field K, say OK, to the nonnegative
rational integers, so it is a candidate to be a Euclidean norm on this ring. If this norm satisfies the axioms of a Euclidean function then the number field K is called norm-Euclidean or simply Euclidean.[14][15] Strictly speaking it is the ring of integers that is Euclidean since fields are trivially Euclidean domains, but the terminology is standard.
If a field is not norm-Euclidean then that does not mean the ring of integers is not Euclidean, just that the field norm does not satisfy the axioms of a Euclidean function. In fact, the rings of integers of number fields may be divided in several classes:
Those that are not
principal and therefore not Euclidean, such as the integers of
Those that are principal and not Euclidean, such as the integers of
Those that are Euclidean and not norm-Euclidean, such as the integers of [16]
Those that are norm-Euclidean, such as
Gaussian integers (integers of )
The norm-Euclidean
quadratic fields have been fully classified; they are where takes the values