The Stacks project

Definition 111.49.1. Throughout, let $S$ be any scheme and let $X$ be a Noetherian, integral scheme.

1. A Weil divisor on $X$ is a formal linear combination $\Sigma n_ i[Z_ i]$ of prime divisors $Z_ i$ with integer coefficients.

2. A prime divisor is a closed subscheme $Z \subset X$, which is integral with generic point $\xi \in Z$ such that ${\mathcal O}_{X, \xi }$ has dimension $1$. We will use the notation ${\mathcal O}_{X, Z} = {\mathcal O}_{X, \xi }$ when $\xi \in Z \subset X$ is as above. Note that ${\mathcal O}_{X, Z} \subset K(X)$ is a subring of the function field of $X$.

3. The Weil divisor associated to a rational function $f \in K(X)^\ast$ is the sum $\Sigma v_ Z(f)[Z]$. Here $v_ Z(f)$ is defined as follows

   1. If $f \in {\mathcal O}_{X, Z}^\ast$ then $v_ Z(f) = 0$.

   2. If $f \in {\mathcal O}_{X, Z}$ then

      $$v_ Z(f) = \text{length}_{{\mathcal O}_{X, Z}}({\mathcal O}_{X, Z}/(f)).$$

   3. If $f = \frac{a}{b}$ with $a, b \in {\mathcal O}_{X, Z}$ then

      $$v_ Z(f) = \text{length}_{{\mathcal O}_{X, Z}}({\mathcal O}_{X, Z}/(a)) - \text{length}_{{\mathcal O}_{X, Z}}({\mathcal O}_{X, Z}/(b)).$$

4. An effective Cartier divisor on a scheme $S$ is a closed subscheme $D \subset S$ such that every point $d\in D$ has an affine open neighbourhood $\mathop{\mathrm{Spec}}(A) = U \subset S$ in $S$ so that $D \cap U = \mathop{\mathrm{Spec}}(A/(f))$ with $f \in A$ a nonzerodivisor.

5. The Weil divisor $[D]$ associated to an effective Cartier divisor $D \subset X$ of our Noetherian integral scheme $X$ is defined as the sum $\Sigma v_ Z(D)[Z]$ where $v_ Z(D)$ is defined as follows

   1. If the generic point $\xi$ of $Z$ is not in $D$ then $v_ Z(D) = 0$.

   2. If the generic point $\xi$ of $Z$ is in $D$ then

      $$v_ Z(D) = \text{length}_{{\mathcal O}_{X, Z}}({\mathcal O}_{X, Z}/(f))$$

      where $f \in {\mathcal O}_{X, Z} = {\mathcal O}_{X, \xi }$ is the nonzerodivisor which defines $D$ in an affine neighbourhood of $\xi$ (as in (4) above).

6. Let $S$ be a scheme. The sheaf of total quotient rings ${\mathcal K}_ S$ is the sheaf of ${\mathcal O}_ S$-algebras which is the sheafification of the pre-sheaf ${\mathcal K}'$ defined as follows. For $U \subset S$ open we set ${\mathcal K}'(U) = S_ U^{-1}{\mathcal O}_ S(U)$ where $S_ U \subset {\mathcal O}_ S(U)$ is the multiplicative subset consisting of sections $f \in {\mathcal O}_ S(U)$ such that the germ of $f$ in ${\mathcal O}_{S, u}$ is a nonzerodivisor for every $u\in U$. In particular the elements of $S_ U$ are all nonzerodivisors. Thus ${\mathcal O}_ S$ is a subsheaf of ${\mathcal K}_ S$, and we get a short exact sequence

   $$0 \to {\mathcal O}_ S^\ast \to {\mathcal K}_ S^\ast \to {\mathcal K}_ S^\ast /{\mathcal O}_ S^\ast \to 0.$$

7. A Cartier divisor on a scheme $S$ is a global section of the quotient sheaf ${\mathcal K}_ S^\ast /{\mathcal O}_ S^\ast$.

8. The Weil divisor associated to a Cartier divisor $\tau \in \Gamma (X, {\mathcal K}_ X^\ast /{\mathcal O}_ X^\ast )$ over our Noetherian integral scheme $X$ is the sum $\Sigma v_ Z(\tau )[Z]$ where $v_ Z(\tau )$ is defined as by the following recipe

   1. If the germ of $\tau$ at the generic point $\xi$ of $Z$ is zero – in other words the image of $\tau$ in the stalk $({\mathcal K}^\ast /{\mathcal O}^\ast )_\xi$ is “zero” – then $v_ Z(\tau ) = 0$.

   2. Find an affine open neighbourhood $\mathop{\mathrm{Spec}}(A) = U \subset X$ so that $\tau |_ U$ is the image of a section $f \in {\mathcal K}(U)$ and moreover $f = a/b$ with $a, b \in A$. Then we set

      $$v_ Z(f) = \text{length}_{{\mathcal O}_{X, Z}}({\mathcal O}_{X, Z}/(a)) - \text{length}_{{\mathcal O}_{X, Z}}({\mathcal O}_{X, Z}/(b)).$$