nLab
Euclidean domain

Idea

A Euclidean domain is an integral domain which admits a form of the Euclidean quotient-and-remainder algorithm familiar from school mathematics.

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 $q, r$.

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.

Examples

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

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

• Any field (trivially).

• 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 principal ideal domain.

Proof

Let $I \subseteq A$ be a nonzero 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 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 $(a_n) + (b_n) = (b_n)$.