# Krull ring

In commutative algebra, a Krull ring, or Krull domain, is a commutative ring with a well behaved theory of prime factorization. They were introduced by Wolfgang Krull in 1931. They are a higher-dimensional generalization of Dedekind domains, which are exactly the Krull domains of dimension at most 1.
In this article, a ring is commutative and has unity.

## Formal definition

Let $A$ be an integral domain and let $P$ be the set of all prime ideals of $A$ of height one, that is, the set of all prime ideals properly containing no nonzero prime ideal. Then $A$ is a Krull ring if
* $A_{\mathfrak {p}}$ is a discrete valuation ring for all ${\mathfrak {p}}\in P$,
* $A$ is the intersection of these discrete valuation rings (considered as subrings of the quotient field of $A$),
* any nonzero element of $A$ is contained in only a finite number of height 1 prime ideals.
It is also possible to characterize Krull rings by mean of valuations only:
An integral domain $A$ is a Krull ring if there exists a family $\{v_{i}\}_{i\in I}$ of discrete valuations on the field of fractions $K$ of $A$ such that:
* for any $x\in K\setminus \{0\}$ and all $i$, except possibly a finite number of them, $v_{i}(x)=0$,
* for any $x\in K\setminus \{0\}$, $x$ belongs to $A$ if and only if $v_{i}(x)\geq 0$ for all $i\in I$.
The valuations $v_{i}$ are called essential valuations of $A$.
The link between the two definitions is as follows: for every ${\mathfrak {p}}\in P$, one can associate a unique normalized valuation $v_{\mathfrak {p}}$ of $K$ whose valuation ring is $A_{\mathfrak {p}}$. Then the set ${\mathcal {V}}=\{v_{\mathfrak {p}}\}$ satisfies the conditions of the equivalent definition. Conversely, if the set ${\mathcal {V}}'=\{v_{i}\}$ is as above, and the $v_{i}$ have been normalized, then ${\mathcal {V}}'$ may be bigger than ${\mathcal {V}}$, but it must contain ${\mathcal {V}}$. In other words, ${\mathcal {V}}$ is the minimal set of normalized valuations satisfying the equivalent definition.

## Properties

With the notations above, let $v_{\mathfrak {p}}$ denote the normalized valuation corresponding to the valuation ring $A_{\mathfrak {p}}$, $U$ denote the set of units of $A$, and $K$ its quotient field.
* An element $x\in K$ belongs to $U$ if, and only if, $v_{\mathfrak {p}}(x)=0$ for every ${\mathfrak {p}}\in P$. Indeed, in this case, $x\not \in A_{\mathfrak {p}}{\mathfrak {p}}$ for every ${\mathfrak {p}}\in P$, hence $x^{-1}\in A_{\mathfrak {p}}$; by the intersection property, $x^{-1}\in A$. Conversely, if $x$ and $x^{-1}$ are in $A$, then $v_{\mathfrak {p}}(xx^{-1})=v_{\mathfrak {p}}(1)=0=v_{\mathfrak {p}}(x)+v_{\mathfrak {p}}(x^{-1})$, hence $v_{\mathfrak {p}}(x)=v_{\mathfrak {p}}(x^{-1})=0$, since both numbers must be $\geq 0$.
* An element $x\in A$ is uniquely determined, up to a unit of $A$, by the values $v_{\mathfrak {p}}(x)$, ${\mathfrak {p}}\in P$. Indeed, if $v_{\mathfrak {p}}(x)=v_{\mathfrak {p}}(y)$ for every ${\mathfrak {p}}\in P$, then $v_{\mathfrak {p}}(xy^{-1})=0$, hence $xy^{-1}\in U$ by the above property (q.e.d). This shows that the application $x\ {\rm {mod}}\ U\mapsto \left(v_{\mathfrak {p}}(x)\right)_{{\mathfrak {p}}\in P}$ is well defined, and since $v_{\mathfrak {p}}(x)\not =0$ for only finitely many ${\mathfrak {p}}$, it is an embedding of $A^{\times }/U$ into the free Abelian group generated by the elements of $P$. Thus, using the multiplicative notation "$\cdot$" for the later group, there holds, for every $x\in A^{\times }$, $x=1\cdot {\mathfrak {p}}_{1}^{\alpha _{1}}\cdot {\mathfrak {p}}_{2}^{\alpha _{2}}\cdots {\mathfrak {p}}_{n}^{\alpha _{n}}\ {\rm {mod}}\ U$, where the ${\mathfrak {p}}_{i}$ are the elements of $P$ containing $x$, and $\alpha _{i}=v_{{\mathfrak {p}}_{i}}(x)$.
* The valuations $v_{\mathfrak {p}}$ are pairwise independent. As a consequence, there holds the so-called weak approximation theorem, an homologue of the Chinese remainder theorem: if ${\mathfrak {p}}_{1},\ldots {\mathfrak {p}}_{n}$ are distinct elements of $P$, $x_{1},\ldots x_{n}$ belong to $K$ (resp. $A_{\mathfrak {p}}$), and $a_{1},\ldots a_{n}$ are $n$ natural numbers, then there exist $x\in K$ (resp. $x\in A_{\mathfrak {p}}$) such that $v_{{\mathfrak {p}}_{i}}(x-x_{i})=n_{i}$ for every $i$.
* A consequence of the weak approximation theorem is a characterization of when Krull rings are noetherian; namely, a Krull ring $A$ is noetherian if and only if all of its quotients $A/{\mathfrak {p}}$ by height-1 primes are noetherian.
* Two elements $x$ and $y$ of $A$ are coprime if $v_{\mathfrak {p}}(x)$ and $v_{\mathfrak {p}}(y)$ are not both $>0$ for every ${\mathfrak {p}}\in P$. The basic properties of valuations imply that a good theory of coprimality holds in $A$.
* Every prime ideal of $A$ contains an element of $P$.
* Any finite intersection of Krull domains whose quotient fields are the same is again a Krull domain.
* If $L$ is a subfield of $K$, then $A\cap L$ is a Krull domain.
* If $S\subset A$ is a multiplicatively closed set not containing 0, the ring of quotients $S^{-1}A$ is again a Krull domain. In fact, the essential valuations of $S^{-1}A$ are those valuation $v_{\mathfrak {p}}$ (of $K$) for which ${\mathfrak {p}}\cap S=\emptyset$.
* If $L$ is a finite algebraic extension of $K$, and $B$ is the integral closure of $A$ in $L$, then $B$ is a Krull domain.

## Examples

* Any unique factorization domain is a Krull domain. Conversely, a Krull domain is a unique factorization domain if (and only if) every prime ideal of height one is principal.
* Every integrally closed noetherian domain is a Krull domain. In particular, Dedekind domains are Krull domains. Conversely, Krull domains are integrally closed, so a Noetherian domain is Krull if and only if it is integrally closed.
* If $A$ is a Krull domain then so is the polynomial ring $A[x]$ and the formal power series ring $A[[x]]$.
* The polynomial ring $R[x_{1},x_{2},x_{3},\ldots ]$ in infinitely many variables over a unique factorization domain $R$ is a Krull domain which is not noetherian.
* Let $A$ be a Noetherian domain with quotient field $K$, and $L$ be a finite algebraic extension of $K$. Then the integral closure of $A$ in $L$ is a Krull domain (Mori–Nagata theorem).
* Let $A$ be a Zariski ring (e.g., a local noetherian ring). If the completion ${\widehat {A}}$ is a Krull domain, then $A$ is a Krull domain (Mori).
* Let $A$ be a Krull domain, and $V$ be the multiplicatively closed set consisting in the powers of a prime element $p\in A$. Then $S^{-1}A$ is a Krull domain (Nagata).

## The divisor class group of a Krull ring

Assume that $A$ is a Krull domain and $K$ is its quotient field. A prime divisor of $A$ is a height 1 prime ideal of $A$. The set of prime divisors of $A$ will be denoted $P(A)$ in the sequel. A (Weil) divisor of $A$ is a formal integral linear combination of prime divisors. They form an Abelian group, noted $D(A)$. A divisor of the form $div(x)=\sum _{p\in P}v_{p}(x)\cdot p$, for some non-zero $x$ in $K$, is called a principal divisor. The principal divisors of $A$ form a subgroup of the group of divisors (it has been shown above that this group is isomorphic to $A^{\times }/U$, where $U$ is the group of unities of $A$). The quotient of the group of divisors by the subgroup of principal divisors is called the divisor class group of $A$; it is usually denoted $C(A)$.
Assume that $B$ is a Krull domain containing $A$. As usual, we say that a prime ideal ${\mathfrak {P}}$ of $B$ lies above a prime ideal ${\mathfrak {p}}$ of $A$ if ${\mathfrak {P}}\cap A={\mathfrak {p}}$; this is abbreviated in ${\mathfrak {P}}|{\mathfrak {p}}$.
Denote the ramification index of $v_{\mathfrak {P}}$ over $v_{\mathfrak {p}}$ by $e({\mathfrak {P}},{\mathfrak {p}})$, and by $P(B)$ the set of prime divisors of $B$. Define the application $P(A)\to D(B)$ by
    $j({\mathfrak {p}})=\sum _{{\mathfrak {P}}|{\mathfrak {p}},\ {\mathfrak {P}}\in P(B)}e({\mathfrak {P}},{\mathfrak {p}}){\mathfrak {P}}$
(the above sum is finite since every $x\in {\mathfrak {p}}$ is contained in at most finitely many elements of $P(B)$). Let extend the application $j$ by linearity to a linear application $D(A)\to D(B)$. One can now ask in what cases $j$ induces a morphism ${\bar {j}}:C(A)\to C(B)$. This leads to several results. For example, the following generalizes a theorem of Gauss:
The application ${\bar {j}}:C(A)\to C(A[X])$ is bijective. In particular, if $A$ is a unique factorization domain, then so is $A[X]$.
The divisor class group of a Krull rings are also used to set up powerful descent methods, and in particular the Galoisian descent.

## Cartier divisor

A Cartier divisor of a Krull ring is a locally principal (Weil) divisor. The Cartier divisors form a subgroup of the group of divisors containing the principal divisors. The quotient of the Cartier divisors by the principal divisors is a subgroup of the divisor class group, isomorphic to the Picard group of invertible sheaves on Spec(A).
Example: in the ring k[x,y,z]/(xy–z$^2$) the divisor class group has order 2, generated by the divisor y=z, but the Picard subgroup is the trivial group.