# Examples of Non-Noetherian Valuation Rings

For valuation rings I know examples which are Noetherian.

I know there are good standard non Noetherian Valuation Rings. Can anybody please give some examples of rings of this kind?

I am very eager to know. Thanks.

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Consider the tower of domains

$K[x]\subset K[x^{1/2}]\subset \ldots \subset K[x^{1/2^k}]\subset\ldots$

where $K$ is a field and $x$ is transcendental over $K$. Every ring in the chain is a polynomial ring in one variable over $K$. Thus the localizations $O_k:=K[x^{1/2^k}]_{P_k}$, where $P_k$ is the prime ideal genrated by $x^{1/2^k}$ are discrete valuation rings. Since $P_{k+1}\cap K[x^{1/2^k}]=P_k$ one has $O_k\subset O_{k+1}$ and $M_{k+1}\cap O_k =M_k$ for the maximal ideals $M_k$ of the rings $O_k$.

Now $O:=\bigcup\limits_k O_k$ is a non-noetherian valuation ring of the field $K(x^{1/2^k} : k\in\mathbb{N})$. The value group of an associated valuation is order-isomorphic to the subgroup $\{z/2^k : z\in\mathbb{Z}, k\in\mathbb{N}\}\subset\mathbb{Q}$. Hence this example yields a non-noetherian valuation ring of Krull dimension $1$.

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Any valuation rings that have dimension $\geq 2$ are not Noetherian. The dimension of a valuation ring is equal to the rank of its value group.

To get a simple example of a valuation ring that has dimension $2$, take $R = k[x,y]$, where $k$ is a field. Define the standard valuation $v: k(x,y) \rightarrow \mathbb{Z}^2$ with $v(x) = (1,0) \leq v(y) = (0,1)$, and take the value of a polynomial as the minimal values among those of its monomials. The value group is $\mathbb{Z}^2$, which has rank $2$. So the valuation ring is not Noetherian. This example is "standard" in the sense that it is encountered more often. However, Hagen's example is more interesting.

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mr.bigproblem's answer is incorrect. First of all $k[x,y]$ is Noetherian. Furthermore it is not a valuation ring as $\frac{x}{y} \in k(x,y)$ but $\frac{x}{y} \notin k[x,y]$ and also $(\frac{x}{y})^{-1} \notin k[x,y]$.
In order to obtain a non Noetherian valuation ring, take $\mathbb{Z}^2$ with the lexicographic order. Define the valuation $v:k(x,y)^* \to \mathbb{Z}^2$ as follows: for any $a \in k^*$ and $0 \le n,m \in \mathbb{Z}$ set $v(ax^ny^m)=(n,m)$. For a polynomial $\: f=\sum f_i \in k[x,y]$ set $v(f)= \inf \{f_0,...,f_d\}$ where the $f_i$ are distinct monomials. Finally for a rational function $f \in k(x,y)$ there are $g,h \in k[x,y]$ such that $f= \frac{g}{h}$ set $v(f)= v(g)-v(h)$. The corresponding valuation ring $R_v= \{f \:|\: v(f) \ge 0\}\cup \{0\}$ contains $k[x,y]$, but it also contains $xy^{-1}$ since $(0,0) < (1,-1)$. In fact $xy^n \in R_v$ for any $n \in \mathbb{Z}$. It follows that $R_v=k[x,y,x/y,x/y^2,x/y^3...]$.
It does not seem that mr.bigproblem is taking $k[x,y]$ as his example of a non-Noetherian valuation ring - the valuation is defined later. +1 for writing up the example in detail though –  user115654 Feb 27 at 20:14