Discrete valuation

In mathematics, a discrete valuation is an integer valuation on a field K, that is a function

$\nu :K\to \mathbb {Z} \cup \{\infty \}$

satisfying the conditions

$\nu (x\cdot y)=\nu (x)+\nu (y)$

$\nu (x+y)\geq \min {\big \{}\nu (x),\nu (y){\big \}}$

$\nu (x)=\infty \iff x=0$

for all $x,y\in K$.

Note that often the trivial valuation which takes on only the values $0,\infty$ is explicitly excluded.

A field with a non-trivial discrete valuation is called a discrete valuation field.

Discrete valuation rings and valuations on fields

To every field with discrete valuation $\nu$ we can associate the subring

${\mathcal {O}}_{K}:=\left\{x\in K\mid \nu (x)\geq 0\right\}$

of $K$, which is a discrete valuation ring. Conversely, the valuation $\nu :A\rightarrow \mathbb {Z} \cup \{\infty \}$ on a discrete valuation ring $A$ can be extended in a unique way to a discrete valuation on the quotient field $K={\text{Quot}}(A)$; the associated discrete valuation ring ${\mathcal {O}}_{K}$ is just $A$.

Examples

• For a fixed prime $p$ and for any element $x\in \mathbb {Q}$ different from zero write $x=p^{j}{\frac {a}{b}}$ with $j,a,b\in \mathbb {Z}$ such that $p$ does not divide $a,b$, then $\nu (x)=j$ is a discrete valuation on $\mathbb {Q}$, called the p-adic valuation.

• Given a Riemann surface $X$, we can consider the field $K=M(X)$ of meromorphic functions $X\to \mathbb {C} \cup \{\infty \}$. For a fixed point $p\in X$, we define a discrete valuation on $K$ as follows: $\nu (f)=j$ if and only if $j$ is the largest integer such that the function $f(z)/(z-p)^{j}$ can be extended to a holomorphic function at $p$. This means: if $\nu (f)=j>0$ then $f$ has a root of order $j$ at the point $p$; if $\nu (f)=j<0$ then $f$ has a pole of order $-j$ at $p$. In a similar manner, one also defines a discrete valuation on the function field of an algebraic curve for every regular point $p$ on the curve.

More examples can be found in the article on discrete valuation rings.

References

• Fesenko, Ivan B.; Vostokov, Sergei V. (2002), Local fields and their extensions, Translations of Mathematical Monographs, vol. 121 (Second ed.), Providence, RI: American Mathematical Society, ISBN 978-0-8218-3259-2, MR 1915966