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Let \(X\) be a noetherian scheme, let \(\delta\) be a coherent locally free sheaf on \(X\), and let \(\pi: \mathbf{P}(\delta) \rightarrow X\) be the corresponding projective space bundle. Show that there is a natural 1-1 correspondence between sections of \(\pi\) (i.e., morphisms \(\sigma: X \rightarrow\) \(\mathbf{P}(\delta)\) such that \(\pi \quad \sigma=\) id \(_{\mathrm{x}}\) ) and quotient invertible sheaves \(\delta \rightarrow \mathscr{L} \rightarrow 0\) of \(\delta\)

Short Answer

Expert verified
To show that there is a natural 1-1 correspondence between sections of \( \pi \) and quotient invertible sheaves of \(\delta\), start by inspecting the case of a trivial sheaf, then make the observations from the simpler case to help you work on the main, more general case. You then need to show that given a section, you can construct a quotient line bundle and vice versa. Do this by constructing a well-defined map and its inverse, showing that they are indeed inverse to each other and form a bijection.

Step by step solution

01

Understanding Definitions

Firstly, make sure that all concepts mentioned in the exercise are clear: a locally free sheaf is a sheaf of modules that is isomorphic to a direct sum of sheaves of rank one modules over some cover of the scheme. A Noetherian scheme is a scheme such that all of its local rings are Noetherian. An invertible sheaf or a line bundle, is a locally free sheaf of rank 1. A morphism of schemes \(f: X \rightarrow Y\) is said to have a section if there exists a morphism \(g: Y \rightarrow X\) such that \(f \circ g: Y \rightarrow Y\) is the identity.
02

The Case of a Trivial Sheaf

Start by thinking about a much simpler case where \(\delta\) is a trivial sheaf, i.e., \(\delta = \mathscr{O}_X^n\) for some positive integer \(n\). In this case \(\mathbf{P}(\delta)\) is the \(n-1\)-dimensional projective space over \(X\). A section of \(\pi\) in this case is a homomorphism from \(X\) to \(\mathbf{P}(\delta)\) that picks out one line in each fibre, and a quotient line bundle of \(\delta\) is just a line subbundle in an obvious way.
03

The General Case

Now consider the more general case where \(\delta\) is coherent and locally free. Locally, \(\delta\) is free and hence any quotient line bundle can be defined locally, similarly as for a trivial sheaf. A section of \(\pi\), defined globally, will then determine a quotient line bundle of \(\delta\) that is also defined globally.
04

Proving the Correspondence

Proving the correspondence bijectively includes proving it in both ways: a section \(\sigma\) of \(\pi\) does produce a quotient line bundle \(\mathscr{L}\) of \(\delta\), and for a quotient line bundle there is a section \(\sigma\) with the given properties. Show that the constructed map is well-defined. Then construct the inverse map, and show that these two maps are indeed inverse to each other. Thus, they form a bijection.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Noetherian Scheme
Let's start with the concept of a Noetherian scheme, which forms the foundation for many theorems in algebraic geometry. A scheme is a space that may be covered by 'patches,' known as affine schemes, glued together in a certain fashion. Now, if each of these affine schemes is Noetherian, which intuitively means they are built upon rings with the property that every ascending chain of ideals eventually stabilizes, then the scheme itself is considered Noetherian.

In other words, a Noetherian scheme is an algebraic structure that ensures a level of finiteness or manageability. This property is especially useful when working with sheaves and morphisms because it provides a guarantee that various processes, like the resolution of sheaves, will eventually come to an end. An analogy could be made to a bookshelf that has limited space; it can only fit so many books, much like a Noetherian scheme 'fits' only so many ideal 'layers'.
Coherent Locally Free Sheaf
Next, let's unpack the notion of a coherent locally free sheaf. Think of a sheaf as a book that provides information depending on where you are in a certain space; the concept is similarly applied to schemes. A sheaf is locally free if around every point, you can find a 'neighborhood' where the sheaf looks like a bunch of copies of the structure sheaf, much like finding a chapter in a book that individually makes sense within the context.

A coherent sheaf is one step beyond being locally free. It adds an element of finiteness, ensuring that locally, not only is the sheaf free (split into nice, well-behaved pieces), but it can also be described by a finite number of generators. It's akin to saying a chapter is not only coherent on its own but it is also made up of a finite set of key points or theses. For algebraic geometers, working with coherent locally free sheaves is like having a manual that is neatly organized and constructed with clearly defined, finite sections.
Section of a Morphism
Moving on to the concept of a section of a morphism. In the realm of schemes, a morphism is akin to a function, but for spaces; it takes points in one space and 'maps' them to another. Imagine you have a city map, and for every neighborhood, there's an arrow pointing back to the city's name on the map -- that's your 'section'; it reverses the direction of the morphism. More formally, given a morphism of schemes \( f: X \rightarrow Y \), a section is another morphism \( g: Y \rightarrow X \) such that \( f \circ g \) (first apply \( g \), then \( f \) — much like following the arrow on the map to the city name and back) is the identity on \( Y \).

This notion is vital when one discusses projective bundles; having a section means being able to pick out a single choice consistently over the whole scheme. It's like being able to pick out one specific attraction in every neighborhood of the city, no matter which map you're looking at.
Quotient Invertible Sheaf
Lastly, let's delve into the concept of a quotient invertible sheaf. To understand this, picture a pie (our sheaf) and imagine slicing a piece out of it. This piece is akin to our 'quotient', and the fact that it is 'invertible' suggests that in some sense, this process is reversible. In a mathematical context, we start with our coherent locally free sheaf, \( \delta \), and we carve out of it a new sheaf, \( \mathscr{L} \), where the latter sheaf can be thought of as this single 'slice'.

A quotient invertible sheaf is important because it captures the idea of localization: focusing on one 'slice' and its properties, while still retaining the structure and context of the original 'pie.' In the context of the exercise, relating sections of a projective space bundle to quotient invertible sheaves means defining a correspondence between picking out consistent 'slices' over the scheme and morphisms that take the whole scheme back to these specific 'slices' within a projective space.

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Most popular questions from this chapter

Let \(X\) be a regular nocthcrian scheme, and \(\delta\) a locally free coherent sheaf of rank \(\geqslant 2\) on \(X\) (a) Show that Pic \(\mathbf{P}(\delta) \cong \operatorname{Pic} X \times \mathbf{Z}\) (b) If \(f^{\prime}\) is another locally free coherent sheafon \(X\). show that \(\mathrm{P}(\mathcal{E)} \cong \mathbf{P}(\mathcal{E} \text { ' ) lover } X\) ' if and only if there is an invertible sheaf \(\mathscr{Y}\) on \(X\) such that \(\mathscr{E}^{\prime} \cong \delta \otimes \mathscr{Y}\)

(a) Let \(X\) be a scheme over a scheme \(Y\), and let \(\mathscr{L}, \mathscr{M}\) be two very ample invertible sheaves on \(X .\) Show that \(\mathscr{L} \otimes \mathscr{M}\) is also very ample. [Hint: Use a Segre embedding. \(]\) (b) Let \(f: X \rightarrow Y\) and \(g: Y \rightarrow Z\) be two morphisms of schemes. Let \(\mathscr{L}\) be a very ample invertible sheaf on \(X\) relative to \(Y\), and let \(\mathscr{M}\) be a very ample invertible sheaf on \(Y\) relative to \(Z\). Show that \(\mathscr{L} \otimes f^{*} . \mathscr{M}\) is a very ample invertible sheaf on \(X\) relative to \(Z\).

Examples of Valuation Rings. Let \(k\) be an algebraically closed field. (a) If \(K\) is a function field of dimension 1 over \(k(I, \$ 6),\) then every valuation ring of \(K / k\) (except for \(K\) itself) is discrete. Thus the set of all of them is just the abstract nonsingular curve \(C_{K}\) of \((\mathrm{I}, \$ 6)\) (b) If \(K / k\) is a function field of dimension two, there are several different kinds of valuations. Suppose that \(X\) is a complete nonsingular surface with function field \(K\) (1) If \(Y\) is an irreducible curve on \(X\), with generic point \(x_{1},\) then the local ring \(R=C_{x_{1}, x}\) is a discrete valuation ring of \(K k\) with center at the (nonclosed) point \(x_{1}\) on \(X\) (2) If \(f: X^{\prime} \rightarrow X\) is a birational morphism, and if \(Y^{\prime}\) is an irreducible curve in \(X^{\prime}\) whose image in \(X\) is a single closed point \(x_{0},\) then the local ring \(R\) of the generic point of \(Y^{\prime}\) on \(X^{\prime}\) is a discrete valuation ring of \(K k\) with center at the closed point \(x_{0}\) on \(X\) (3) Let \(r_{0} \in X\) be a closed point. Let \(f: X_{1} \rightarrow X\) be the blowing-up of \(x_{0}\) (I. \(\$ 4)\) and let \(E_{1}=f^{-1}\left(r_{0}\right)\) be the exceptional curve. Choose a closed point \(x_{1} \in E_{1},\) let \(f_{2}: X_{2} \rightarrow X_{1}\) be the blowing-up of \(x_{1},\) and let \(E_{2}=\) \(f_{2}^{-1}\left(x_{1}\right)\) be the exceptional curve. Repeat. In this manner we obtain a sequence of varieties \(X\), with closed points \(x_{i}\) chosen on them, and for each \(i,\) the local ring \(C_{1,1,1}, x_{1},\) dominates \(C_{x_{1}, x_{1}},\) Let \(R_{0}=\bigcup_{1=0}^{x} C_{x_{1}, x_{1}}\) Then \(R_{0}\) is a local ring, so it is dominated by some valuation ring \(R\) of \(K / k\) by \((\mathrm{I}, 6.1 \mathrm{A}) .\) Show that \(R\) is a valuation ring of \(K / k\). and that it has center \(x_{0}\) on \(X .\) When is \(R\) a discrete valuation ring? Note. We will see later (V.Ex. 5.6) that in fact the \(R_{0}\) of (3) is already a valuation ring itself, so \(R_{0}=R\). Furthermore, every valuation ring of \(K, k\) (except for \(K\) itself) is one of the three kinds just described.

Prove the analogue of (5.6) for formal schemes, which says, if \(\mathcal{K}\) is an affine formal scheme. and if \\[ 0 \rightarrow \widetilde{\psi} \rightarrow \widetilde{\psi} \rightarrow \widetilde{\psi}^{\prime \prime} \rightarrow 0 \\] is an exact sequence of \(C_{\mathrm{r}}\) -modules. and if \(\tilde{\psi}\) is coherent, then the sequence of global sections \\[ 0 \rightarrow \Gamma\left(\boldsymbol{x}, \widetilde{\boldsymbol{x}}^{\prime}\right) \rightarrow \Gamma(\boldsymbol{x}, \widetilde{\boldsymbol{\varphi}}) \rightarrow \Gamma\left(\boldsymbol{x}, \widetilde{x}^{\prime \prime}\right) \rightarrow 0 \\] is exact. For the proof. proceed in the following steps. (a) Let 3 be an ideal of definition for \(\mathfrak{X},\) and for each \(n>0\) consider the exact sequence \\[ 0 \rightarrow \mathfrak{F}^{\prime} / \mathfrak{I}^{n} \mathfrak{F}^{\prime} \rightarrow \mathfrak{F} / \mathfrak{I}^{n} \mathfrak{F}^{\prime} \rightarrow \mathfrak{F}^{\prime \prime} \rightarrow 0 \\] Use \((5.6),\) slightly modified. to show that for every open affine subset \(21 \subseteq\) the sequence is exact. (b) Now pass to the limit, using \((9.1),(9.2),\) and \((9.6) .\) Conclude that \(\widetilde{x} \cong \lim \tilde{x} \sqrt{x}_{x}\) and that the sequence of global sections above is exact.

A morphism \(f: X \rightarrow Y\) of schemes is quasi-compact if there is a cover of \(Y\) by open affines \(V_{i}\) such that \(f^{-1}\left(V_{i}\right)\) is quasi-compact for each \(i .\) Show that \(f\) is quasicompact if and only if for every open affine subset \(V \subseteq Y, f^{-1}(V)\) is quasi-compact.

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