Proj construction

(Redirected from Serre twist sheaf)

In algebraic geometry, Proj is a construction analogous to the spectrum-of-a-ring construction of affine schemes, which produces objects with the typical properties of projective spaces and projective varieties. The construction, while not functorial, is a fundamental tool in scheme theory.

In this article, all rings will be assumed to be commutative and with identity.

Proj of a graded ring

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Proj as a set

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Let   be a commutative graded ring, where is the direct sum decomposition associated with the gradation. The irrelevant ideal of   is the ideal of elements of positive degree We say an ideal is homogeneous if it is generated by homogeneous elements. Then, as a set,  For brevity we will sometimes write   for  .

Proj as a topological space

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We may define a topology, called the Zariski topology, on   by defining the closed sets to be those of the form

 

where   is a homogeneous ideal of  . As in the case of affine schemes it is quickly verified that the   form the closed sets of a topology on  .

Indeed, if   are a family of ideals, then we have   and if the indexing set I is finite, then  

Equivalently, we may take the open sets as a starting point and define

 

A common shorthand is to denote   by  , where   is the ideal generated by  . For any ideal  , the sets   and   are complementary, and hence the same proof as before shows that the sets   form a topology on  . The advantage of this approach is that the sets  , where   ranges over all homogeneous elements of the ring  , form a base for this topology, which is an indispensable tool for the analysis of  , just as the analogous fact for the spectrum of a ring is likewise indispensable.

Proj as a scheme

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We also construct a sheaf on  , called the “structure sheaf” as in the affine case, which makes it into a scheme. As in the case of the Spec construction there are many ways to proceed: the most direct one, which is also highly suggestive of the construction of regular functions on a projective variety in classical algebraic geometry, is the following. For any open set   of   (which is by definition a set of homogeneous prime ideals of   not containing  ) we define the ring   to be the set of all functions

 

(where   denotes the subring of the ring of fractions   consisting of fractions of homogeneous elements of the same degree) such that for each prime ideal   of  :

  1.   is an element of  ;
  2. There exists an open subset   containing   and homogeneous elements   of   of the same degree such that for each prime ideal   of  :
    •   is not in  ;
    •  

It follows immediately from the definition that the   form a sheaf of rings   on  , and it may be shown that the pair ( ,  ) is in fact a scheme (this is accomplished by showing that each of the open subsets   is in fact an affine scheme).

The sheaf associated to a graded module

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The essential property of   for the above construction was the ability to form localizations   for each prime ideal   of  . This property is also possessed by any graded module   over  , and therefore with the appropriate minor modifications the preceding section constructs for any such   a sheaf, denoted  , of  -modules on  . This sheaf is quasicoherent by construction. If   is generated by finitely many elements of degree   (e.g. a polynomial ring or a homogenous quotient of it), all quasicoherent sheaves on   arise from graded modules by this construction.[1] The corresponding graded module is not unique.

The twisting sheaf of Serre

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A special case of the sheaf associated to a graded module is when we take   to be   itself with a different grading: namely, we let the degree   elements of   be the degree   elements of  , so and denote  . We then obtain   as a quasicoherent sheaf on  , denoted   or simply  , called the twisting sheaf of Serre. It can be checked that   is in fact an invertible sheaf.

One reason for the utility of   is that it recovers the algebraic information of   that was lost when, in the construction of  , we passed to fractions of degree zero. In the case Spec A for a ring A, the global sections of the structure sheaf form A itself, whereas the global sections of   here form only the degree-zero elements of  . If we define

 

then each   contains the degree-  information about  , denoted  , and taken together they contain all the grading information that was lost. Likewise, for any sheaf of graded  -modules   we define

 

and expect this “twisted” sheaf to contain grading information about  . In particular, if   is the sheaf associated to a graded  -module   we likewise expect it to contain lost grading information about  . This suggests, though erroneously, that   can in fact be reconstructed from these sheaves; as however, this is true in the case that   is a polynomial ring, below. This situation is to be contrasted with the fact that the spec functor is adjoint to the global sections functor in the category of locally ringed spaces.

Projective n-space

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If   is a ring, we define projective n-space over   to be the scheme

 

The grading on the polynomial ring   is defined by letting each   have degree one and every element of  , degree zero. Comparing this to the definition of  , above, we see that the sections of   are in fact linear homogeneous polynomials, generated by the   themselves. This suggests another interpretation of  , namely as the sheaf of “coordinates” for  , since the   are literally the coordinates for projective  -space.

Examples of Proj

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Proj over the affine line

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If we let the base ring be  , then has a canonical projective morphism to the affine line   whose fibers are elliptic curves except at the points   where the curves degenerate into nodal curves. So there is a fibration which is also a smooth morphism of schemes (which can be checked using the Jacobian criterion).

Projective hypersurfaces and varieties

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The projective hypersurface   is an example of a Fermat quintic threefold which is also a Calabi–Yau manifold. In addition to projective hypersurfaces, any projective variety cut out by a system of homogeneous polynomials in  -variables can be converted into a projective scheme using the proj construction for the graded algebra giving an embedding of projective varieties into projective schemes.

Weighted projective space

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Weighted projective spaces can be constructed using a polynomial ring whose variables have non-standard degrees. For example, the weighted projective space   corresponds to taking   of the ring   where   have weight   while   has weight 2.

Bigraded rings

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The proj construction extends to bigraded and multigraded rings. Geometrically, this corresponds to taking products of projective schemes. For example, given the graded rings with the degree of each generator  . Then, the tensor product of these algebras over   gives the bigraded algebra where the   have weight   and the   have weight  . Then the proj construction gives which is a product of projective schemes. There is an embedding of such schemes into projective space by taking the total graded algebra where a degree   element is considered as a degree   element. This means the  -th graded piece of   is the module In addition, the scheme   now comes with bigraded sheaves   which are the tensor product of the sheaves   where and  are the canonical projections coming from the injections of these algebras from the tensor product diagram of commutative algebras.

Global Proj

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A generalization of the Proj construction replaces the ring S with a sheaf of algebras and produces, as the result, a scheme which might be thought of as a fibration of Proj's of rings. This construction is often used, for example, to construct projective space bundles over a base scheme.

Assumptions

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Formally, let X be any scheme and S be a sheaf of graded  -algebras (the definition of which is similar to the definition of  -modules on a locally ringed space): that is, a sheaf with a direct sum decomposition

 

where each   is an  -module such that for every open subset U of X, S(U) is an  -algebra and the resulting direct sum decomposition

 

is a grading of this algebra as a ring. Here we assume that  . We make the additional assumption that S is a quasi-coherent sheaf; this is a “consistency” assumption on the sections over different open sets that is necessary for the construction to proceed.

Construction

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In this setup we may construct a scheme   and a “projection” map p onto X such that for every open affine U of X,

 

This definition suggests that we construct   by first defining schemes   for each open affine U, by setting

 

and maps  , and then showing that these data can be glued together “over” each intersection of two open affines U and V to form a scheme Y which we define to be  . It is not hard to show that defining each   to be the map corresponding to the inclusion of   into S(U) as the elements of degree zero yields the necessary consistency of the  , while the consistency of the   themselves follows from the quasi-coherence assumption on S.

The twisting sheaf

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If S has the additional property that   is a coherent sheaf and locally generates S over   (that is, when we pass to the stalk of the sheaf S at a point x of X, which is a graded algebra whose degree-zero elements form the ring   then the degree-one elements form a finitely-generated module over   and also generate the stalk as an algebra over it) then we may make a further construction. Over each open affine U, Proj S(U) bears an invertible sheaf O(1), and the assumption we have just made ensures that these sheaves may be glued just like the   above; the resulting sheaf on   is also denoted O(1) and serves much the same purpose for   as the twisting sheaf on the Proj of a ring does.

Proj of a quasi-coherent sheaf

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Let   be a quasi-coherent sheaf on a scheme  . The sheaf of symmetric algebras   is naturally a quasi-coherent sheaf of graded  -modules, generated by elements of degree 1. The resulting scheme is denoted by  . If   is of finite type, then its canonical morphism   is a projective morphism.[2]

For any  , the fiber of the above morphism over   is the projective space   associated to the dual of the vector space   over  .

If   is a quasi-coherent sheaf of graded  -modules, generated by   and such that   is of finite type, then   is a closed subscheme of   and is then projective over  . In fact, every closed subscheme of a projective   is of this form.[3]

Projective space bundles

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As a special case, when   is locally free of rank  , we get a projective bundle   over   of relative dimension  . Indeed, if we take an open cover of X by open affines   such that when restricted to each of these,   is free over A, then

 

and hence   is a projective space bundle. Many families of varieties can be constructed as subschemes of these projective bundles, such as the Weierstrass family of elliptic curves. For more details, see the main article.

Example of Global Proj

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Global proj can be used to construct Lefschetz pencils. For example, let   and take homogeneous polynomials   of degree k. We can consider the ideal sheaf   of   and construct global proj of this quotient sheaf of algebras  . This can be described explicitly as the projective morphism  .

See also

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References

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  1. ^ Ravi Vakil (2015). Foundations of Algebraic Geometry (PDF)., Corollary 15.4.3.
  2. ^ EGA, II.5.5.
  3. ^ EGA, II.5.5.1.