# nLab Euclidean domain

Contents

### Context

#### Algebra

higher algebra

universal algebra

# Contents

## Idea

A Euclidean domain is an integral domain which admits a form of the Euclidean quotient-and-remainder algorithm familiar from school mathematics, as well as a form of the extended Euclidean algorithm for calculating the greatest common divisor and Bézout coefficients in a Bézout domain.

## Definition

A Euclidean domain is an integral domain $A$ for which there exists a function $d: A \setminus \{0\} \to \mathbb{N}$ to the natural numbers, often called a degree function, such that given $f, g \in A$ with $g \neq 0$, there exist $q, r \in A$ such that $f = q \cdot g + r$ and either $r = 0$ or $d(r) \lt d(g)$. (One may harmlessly stipulate that $d(0) = 0$; what to do with the zero element varies from author to author.) There is no uniqueness requirement for $q, r$.

One could posit, instead of the property that for all $f \in A$ and $g \in A \setminus \{0\}$, there exist $q, r \in A$ such that $f = q \cdot g + r$ and either $r = 0$ or $d(r) \lt d(g)$; that the integral domain has the structure of a function $(-)\div(-):A \times A \setminus \{0\} \to A$ called the division function, and a function $(-)\ \%\ (-):A \times A \setminus \{0\} \to A$ called the remainder function, such that for all $f \in A$ and $g \in A \setminus \{0\}$, $f = (f \div g) \cdot g + (f\ \%\ g)$ and either $f\ \%\ g = 0$ or $d(f\ \%\ g) \lt d(g)$. There might be multiple such division and remainder functions for the integral domain $A$.

Some authors also add the requirement that $d(a) \leq d(a b)$ for all nonzero $a, b$. There is no loss of generality in assuming it; every Euclidean domain admits such a degree function $d'$, defining $d'(a) = \min \{d(a b): b \in A, b \neq 0\}$. We’ll use it freely below, if and when we need to.

### In constructive mathematics

In constructive mathematics, there are different types of integral domains, yielding different types of Euclidean domains: the degree function, division, function, and remainder function are no longer valued in $A \setminus \{0\}$ in one variable, but in $\{x \in A \vert x \neq 0\}$, $\{x \in A \vert x \# 0\}$, or some other definition, depending on what the base integral domain ends up being (classical, Heyting, discrete, residue, et cetera).

The definition of the Euclidean domain further bifurcates in constructive mathematics due to the disjunctive condition above. These statements, which are equivalent in the presence of excluded middle, include:

• $(r = 0) \vee (d(r) \lt d(g))$
• $\not((r \neq 0) \wedge (d(r) \geq d(g)))$
• $(d(r) \geq d(g)) \to (r = 0)$

or equivalently,

• $(f\ \%\ g = 0) \vee (d(f\ \%\ g) \lt d(g))$
• $\not((f\ \%\ g \neq 0) \wedge (d(f\ \%\ g) \geq d(g))$
• $(d(f\ \%\ g) \geq d(g)) \to (f\ \%\ g = 0)$

## Examples

• The (rational) integers $\mathbb{Z}$.

• Any field (trivially).

• The ring of polynomials $k[x]$ over a discrete field $k$, using the ordinary polynomial degree.

• On the other hand, if $k$ is a Heyting field which is not a discrete field, not every polynomial in $k[x]$ apart from zero can be proven to have a well-defined degree, and so is not an Euclidean domain.

• The real polynomial ring $\mathbb{R}[x]$ is only an Euclidean domain in the presence of the analytic limited principle of omniscience, which states that it has decidable apartness.

• Any discrete valuation ring: letting $\pi$ be a generator of the maximal ideal, put $d(x) = n$ where $x = u \pi^n$, with $u$ a unit.

• The Gaussian integers $\mathbb{Z}[i]$, with $d(a + b i) = a^2 + b^2$ (the norm).

• The Eisenstein integers? $\mathbb{Z}[\omega]$, where $\omega$ is a primitive cube root of unity, with $d(a + b \omega) = a^2 - a b + b^2$ (the norm).

## Properties

###### Proposition

A Euclidean domain is a Bézout domain.

###### Proof

Let $I \subseteq A$ be a nonzero finitely generated ideal, and suppose $d(g)$ is the minimum degree taken over all nonzero $g \in I$. For such a $g$ and any $f \in I$, we may write $f = q g + r$ where either $r = 0$ or $d(r) \lt d(g)$ (which is impossible since $r \in I$ and $d(g)$ is minimal). So $r = 0$ it is, and thus $I = (g)$.

This proof uses the well-orderedness of $\mathbb{N}$. It also suggests the practical procedure known as the extended Euclidean algorithm: given two elements $a, b$ in a Euclidean domain $A$, this algorithm provides a generator $d$ of the ideal $I = (a) + (b)$, called a greatest common divisor of $a, b$, as follows. One constructs a sequence of pairs $(a_n, b_n)$ of elements of $I$, with $(a_0, b_0) = (a, b)$, and $(a_{n+1}, b_{n+1}) = (b_n, r_n)$ where $a_n = b_n q_n + r_n$ for some choice of $q_n, r_n$ such that $d(r_n) \lt d(b_n)$ and $r_n \neq 0$. If no such choices for $q_n, r_n$ exist, then the sequence terminates at $(a_n, b_n)$, and $b_n$ generates $I$, i.e, $(b_n) = I$. Notice that the sequence terminates after finitely many steps since we cannot have an infinite descent of natural numbers

$d(r_0) \gt d(r_1) \gt \ldots,$

again by well-orderedness of $\mathbb{N}$.

The proof that this algorithm produces a gcd consists in the observation that at each step we have

$(a_{n+1}) + (b_{n+1}) = (a_n) + (b_n),$

and that from the definition of Euclidean domain, the only reason why the sequence would terminate at $(a_n, b_n)$ is that $a_n = b_n q_n + 0$ for some $q_n$, and in that case $(a_n) + (b_n) = (b_n)$.

## Generalisations

Euclidean domains could be generalised to the case where the structure is only a rig instead of a ring; these objects could be called Euclidean cancellative rigs.