The Stacks project

Proposition 27.26.13. Let $X$ be a quasi-compact scheme. Let $\mathcal{L}$ be an invertible sheaf on $X$. Set $S = \Gamma _*(X, \mathcal{L})$. The following are equivalent:

  1. $\mathcal{L}$ is ample,

  2. the open sets $X_ s$, with $s \in S_{+}$ homogeneous, cover $X$ and the associated morphism $X \to \text{Proj}(S)$ is an open immersion,

  3. the open sets $X_ s$, with $s \in S_{+}$ homogeneous, form a basis for the topology of $X$,

  4. the open sets $X_ s$, with $s \in S_{+}$ homogeneous, which are affine form a basis for the topology of $X$,

  5. for every quasi-coherent sheaf $\mathcal{F}$ on $X$ the sum of the images of the canonical maps

    \[ \Gamma (X, \mathcal{F} \otimes _{\mathcal{O}_ X} \mathcal{L}^{\otimes n}) \otimes _{\mathbf{Z}} \mathcal{L}^{\otimes -n} \longrightarrow \mathcal{F} \]

    with $n \geq 1$ equals $\mathcal{F}$,

  6. same property as (5) with $\mathcal{F}$ ranging over all quasi-coherent sheaves of ideals,

  7. $X$ is quasi-separated and for every quasi-coherent sheaf $\mathcal{F}$ of finite type on $X$ there exists an integer $n_0$ such that $\mathcal{F} \otimes _{\mathcal{O}_ X} \mathcal{L}^{\otimes n}$ is globally generated for all $n \geq n_0$,

  8. $X$ is quasi-separated and for every quasi-coherent sheaf $\mathcal{F}$ of finite type on $X$ there exist integers $n > 0$, $k \geq 0$ such that $\mathcal{F}$ is a quotient of a direct sum of $k$ copies of $\mathcal{L}^{\otimes - n}$, and

  9. same as in (8) with $\mathcal{F}$ ranging over all sheaves of ideals of finite type on $X$.

Proof. Lemma 27.26.11 is (1) $\Rightarrow $ (2). Lemmas 27.26.2 and 27.26.12 provide the implication (1) $\Leftarrow $ (2). The implications (2) $\Rightarrow $ (4) $\Rightarrow $ (3) are clear from Constructions, Section 26.8. Lemma 27.26.6 is (3) $\Rightarrow $ (1). Thus we see that the first 4 conditions are all equivalent.

Assume the equivalent conditions (1) – (4). Note that in particular $X$ is separated (as an open subscheme of the separated scheme $\text{Proj}(S)$). Let $\mathcal{F}$ be a quasi-coherent sheaf on $X$. Choose $s \in S_{+}$ homogeneous such that $X_ s$ is affine. We claim that any section $m \in \Gamma (X_ s, \mathcal{F})$ is in the image of one of the maps displayed in (5) above. This will imply (5) since these affines $X_ s$ cover $X$. Namely, by Lemma 27.17.2 we may write $m$ as the image of $m' \otimes s^{-n}$ for some $n \geq 1$, some $m' \in \Gamma (X, \mathcal{F} \otimes \mathcal{L}^{\otimes n})$. This proves the claim.

Clearly (5) $\Rightarrow $ (6). Let us assume (6) and prove $\mathcal{L}$ is ample. Pick $x \in X$. Let $U \subset X$ be an affine open which contains $x$. Set $Z = X \setminus U$. We may think of $Z$ as a reduced closed subscheme, see Schemes, Section 25.12. Let $\mathcal{I} \subset \mathcal{O}_ X$ be the quasi-coherent sheaf of ideals corresponding to the closed subscheme $Z$. By assumption (6), there exists an $n \geq 1$ and a section $s \in \Gamma (X, \mathcal{I} \otimes \mathcal{L}^{\otimes n})$ such that $s$ does not vanish at $x$ (more precisely such that $s \not\in \mathfrak m_ x \mathcal{I}_ x \otimes \mathcal{L}_ x^{\otimes n}$). We may think of $s$ as a section of $\mathcal{L}^{\otimes n}$. Since it clearly vanishes along $Z$ we see that $X_ s \subset U$. Hence $X_ s$ is affine, see Lemma 27.26.4. This proves that $\mathcal{L}$ is ample. At this point we have proved that (1) – (6) are equivalent.

Assume the equivalent conditions (1) – (6). In the following we will use the fact that the tensor product of two sheaves of modules which are globally generated is globally generated without further mention (see Modules, Lemma 17.4.3). By (1) we can find elements $s_ i \in S_{d_ i}$ with $d_ i \geq 1$ such that $X = \bigcup _{i = 1, \ldots , n} X_{s_ i}$. Set $d = d_1\ldots d_ n$. It follows that $\mathcal{L}^{\otimes d}$ is globally generated by

\[ s_1^{d/d_1}, \ldots , s_ n^{d/d_ n}. \]

This means that if $\mathcal{L}^{\otimes j}$ is globally generated then so is $\mathcal{L}^{\otimes j + dn}$ for all $n \geq 0$. Fix a $j \in \{ 0, \ldots , d - 1\} $. For any point $x \in X$ there exists an $n \geq 1$ and a global section $s$ of $\mathcal{L}^{j + dn}$ which does not vanish at $x$, as follows from (5) applied to $\mathcal{F} = \mathcal{L}^{\otimes j}$ and ample invertible sheaf $\mathcal{L}^{\otimes d}$. Since $X$ is quasi-compact there we may find a finite list of integers $n_ i$ and global sections $s_ i$ of $\mathcal{L}^{\otimes j + dn_ i}$ which do not vanish at any point of $X$. Since $\mathcal{L}^{\otimes d}$ is globally generated this means that $\mathcal{L}^{\otimes j + dn}$ is globally generated where $n = \max \{ n_ i\} $. Since we proved this for every congruence class mod $d$ we conclude that there exists an $n_0 = n_0(\mathcal{L})$ such that $\mathcal{L}^{\otimes n}$ is globally generated for all $n \geq n_0$. At this point we see that if $\mathcal{F}$ is globally generated then so is $\mathcal{F} \otimes \mathcal{L}^{\otimes n}$ for all $n \geq n_0$.

We continue to assume the equivalent conditions (1) – (6). Let $\mathcal{F}$ be a quasi-coherent sheaf of $\mathcal{O}_ X$-modules of finite type. Denote $\mathcal{F}_ n \subset \mathcal{F}$ the image of the canonical map of (5). By construction $\mathcal{F}_ n \otimes \mathcal{L}^{\otimes n}$ is globally generated. By (5) we see $\mathcal{F}$ is the sum of the subsheaves $\mathcal{F}_ n$, $n \geq 1$. By Modules, Lemma 17.9.7 we see that $\mathcal{F} = \sum _{n = 1, \ldots , N} \mathcal{F}_ n$ for some $N \geq 1$. It follows that $\mathcal{F} \otimes \mathcal{L}^{\otimes n}$ is globally generated whenever $n \geq N + n_0(\mathcal{L})$ with $n_0(\mathcal{L})$ as above. We conclude that (1) – (6) implies (7).

Assume (7). Let $\mathcal{F}$ be a quasi-coherent sheaf of $\mathcal{O}_ X$-modules of finite type. By (7) there exists an integer $n \geq 1$ such that the canonical map

\[ \Gamma (X, \mathcal{F} \otimes _{\mathcal{O}_ X} \mathcal{L}^{\otimes n}) \otimes _{\mathbf{Z}} \mathcal{L}^{\otimes -n} \longrightarrow \mathcal{F} \]

is surjective. Let $I$ be the set of finite subsets of $\Gamma (X, \mathcal{F} \otimes _{\mathcal{O}_ X} \mathcal{L}^{\otimes n})$ partially ordered by inclusion. Then $I$ is a directed partially ordered set. For $i = \{ s_1, \ldots , s_{r(i)}\} $ let $\mathcal{F}_ i \subset \mathcal{F}$ be the image of the map

\[ \bigoplus \nolimits _{j = 1, \ldots , r(i)} \mathcal{L}^{\otimes -n} \longrightarrow \mathcal{F} \]

which is multiplication by $s_ j$ on the $j$th factor. The surjectivity above implies that $\mathcal{F} = \mathop{\mathrm{colim}}\nolimits _{i \in I} \mathcal{F}_ i$. Hence Modules, Lemma 17.9.7 applies and we conclude that $\mathcal{F} = \mathcal{F}_ i$ for some $i$. Hence we have proved (8). In other words, (7) $\Rightarrow $ (8).

The implication (8) $\Rightarrow $ (9) is trivial.

Finally, assume (9). Let $\mathcal{I} \subset \mathcal{O}_ X$ be a quasi-coherent sheaf of ideals. By Lemma 27.22.3 (this is where we use the condition that $X$ be quasi-separated) we see that $\mathcal{I} = \mathop{\mathrm{colim}}\nolimits _\alpha I_\alpha $ with each $I_\alpha $ quasi-coherent of finite type. Since by assumption each of the $I_\alpha $ is a quotient of negative tensor powers of $\mathcal{L}$ we conclude the same for $\mathcal{I}$ (but of course without the finiteness or boundedness of the powers). Hence we conclude that (9) implies (6). This ends the proof of the proposition. $\square$


Comments (5)

Comment #224 by Ravi Vakil on

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Comment #225 by Ravi Vakil on

It might be nice to have a link the cohomological criterionness for ampleness in the case of finite type separated schemes over Noetherian rings. I have an argument in 18.7.1 in the March version of my notes in the proper case; but my argument uses properness. (I'm of course happy to contribute the source if it is helpful.) I've looked around a little bit and haven't found an argument in the more general case. (I've admittedly never used the more general case!)

Comment #226 by on

First of all, I'd be very happy to take a chunk of your book stating 18.7.1 and proving it. Second, I am not sure how to formulate the result in the nonproper case because for example in the case of there is no vanishing, yet the structure sheaf is relatively ample (as defined in 28.35.1). Right?

Comment #227 by on

You are absolutely right of course! I'd been blinded by assuming it was true. I remember thinking that at least my argument in the other direction works (cohomological vanishing implies relative ampleness), but now I should go and look and be careful. And about transporting 18.7.1 into the stacks project: I'm happy to do that too, and will figure out before long how to import it. (ps I like the bot defense --- very elegant solution)

Comment #228 by on

OK! If the technological barriers of adding stuff to the Stacks project LaTeX files becomes insurmountable, then feel free to just email a chunk of latex to stacks.project@gmail.com

We're very happy with any kind of feedback on this kind of issue too.


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