Math 764  Algebraic Geometry II  Homeworks
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Homeworks (Spring 2017)
Here are homework problems for Math 764 from Spring 2017 (by Dima Arinkin). I tried to convert the homeworks into the wiki format with pandoc. This does not always work as expected; in case of doubt, check the pdf files.
 Homework 1 (PDF), due February 3rd.
 Homework 2 (PDF), due February 10th.
 Homework 3 (PDF), due February 17th.
 Homework 4 (PDF), due February 24th.
 Homework 5 (PDF), due March 3rd.
 Homework 6 (PDF), due March 10th.
Homework 1
Due Friday, February 3rd
In all these problems, we fix a topological space ; all sheaves and presheaves are sheaves on .
 Example: Let be the unit circle, and let be the sheaf of functions on . Find the (sheaf) image and the kernel of the morphism Here is the polar coordinate on the circle.
 Sheaf operations: Let and be sheaves of sets. Recall that a morphism is a (categorical) monomorphism if and only if for any sheaf and any two morphisms , the equality implies . Show that is a monomorphism if and only if it induces injective maps on all stalks.
 Let and be sheaves of sets. Recall that a morphism is a (categorical) epimorphism if and only if for any sheaf and any two morphisms , the equality implies . Show that is a epimorphism if and only if it induces surjective maps on all stalks.
 Show that any morphism of sheaves can be written as a composition of an epimorphism and a monomorphism. (You should know what order of composition I mean here.)
 Let be a sheaf, and let be a subpresheaf of (thus, for every open set , is a subset of and the restriction maps for and agree). Show that the sheafification of is naturally identified with a subsheaf of .
 Let be a family of sheaves of abelian groups on indexed by a set (not necessarily finite). Show that the direct sum and direct product of this family exists in the category of sheaves of abelian groups. (E.g., a direct sum would be a sheaf of abelian groups together with a universal family of homomorphisms .) Do these operations agree with (a) taking stalks at a point (b) taking sections over an open subset ?

Locally constant sheaves:
Definition. A sheaf is constant over an open set if there is a subset such that the map (the germ of at ) gives a bijection between and for all .
is locally constant (on ) if every point of has a neighborhood on which is constant.
Recall that a covering space is a continuous map of topological spaces such that every has a neighborhood whose preimage is homeomorphic to for some discrete topological space . ( may depend on ; also, the homeomorphism is required to respect the projection to .)
Show that if is a covering space, its sheaf of sections is locally constant. Moreover, prove that this correspondence is an equivalence between the category of covering spaces and the category of locally constant sheaves. (If is pathwise connected, both categories are equivalent to the category of sets with an action of the fundamental group of .)
 Sheafification: (This problem may be hard, but it is still a good idea to try it) Prove or disprove the following statement (contained in the lecture notes). Let be a presheaf on , and let be its sheafification. Then every section can be represented as (the equivalence class of) the following gluing data: an open cover and a family of sections such that .
Homework 2
Due Friday, February 10th
Extension of a sheaf by zero. Let be a topological space, let be an open subset, and let be a sheaf of abelian groups on .
The extension by zero of (here is the embedding ) is the sheaf on that can be defined as the sheafification of the presheaf such that
 Is the sheafication necessary in this definition? (Or maybe is a sheaf automatically?)
 Describe the stalks of over all points of and the espace étalé of .
 Verify that is the left adjoint of the restriction functor from to : that is, for any sheaf on , there exists a natural isomorphism
(The restriction of a sheaf from to an open set is defined by for .)
Side question (not part of the homework): What changes if we consider the version of extension by zero for sheaves of sets (‘the extension by empty set’)?
Examples of affine schemes.
 Let be a finite collection of rings. Put . Describe the topological space in terms of ’s. What changes if the collection is infinite?
 Recall that the image of a regular map of varieties is constructible (Chevalley’s Theorem); that is, it is a union of locally closed sets. Give an example of a map of rings such that the image of a map is
(a) An infinite intersection of open sets, but not constructible.
(b) An infinite union of closed sets, but not constructible. (This part may be very hard.)
Contraction of a subvariety.
Let be a variety (over an algebraically closed field ) and let be a closed subvariety. Our goal is to construct a ringed space that is in some sense the result of ‘gluing’ together the points of . While can be described by a universal property, we prefer an explicit construction:
 The topological space is the ‘quotientspace’ : as a set, ; a subset is open if and only if is open. Here the natural projection is identity on and sends all of to the ‘center’ .
 The structure sheaf is defined as follows: for any open subset , is the algebra of functions such that the composition is a regular function that is constant along . (The last condition is imposed only if , in which case .)
In each of the following examples, determine whether the quotient is an algebraic variety; if it is, describe it explicitly.
 , (embedded as a line in ).
 , .
 , is a twopoint set (if you want a more challenging version, let be any finite set).
Homework 3
Due Friday, February 17th
 (Gluing morphisms of sheaves) Let and be two sheaves on the same space . For any open set , consider the restriction sheaves and , and let be the set of sheaf morphisms between them.
Prove that the presheaf on given by the correspondence is in fact a sheaf.
 (Gluing morphisms of ringed spaces) Let and be ringed spaces. Denote by the following presheaf on : its sections over an open subset are morphisms of ringed spaces where is considered as a ringed space. (And the notion of restriction is the natural one.) Show that is in fact a sheaf.
 (Affinization of a scheme) Let be an arbitrary scheme. Prove that there exists an affine scheme and a morphism that is universal in the following sense: any map form to an affine scheme factors through it.
 Let us consider direct and inverse limits of affine schemes. For simplicity, we will work with limits indexed by positive integers.
(a) Let be a collection of rings () together with homomorphisms . Consider the direct limit . Show that in the category of schemes,
(b) Let be a collection of rings () together with homomorphisms . Consider the inverse limit . Show that generally speaking, in the category of schemes,
 Here is an example of the situation from 4(b). Let be a field, and let , so that . Describe the direct limit in the category of ringed spaces. Is the direct limit a scheme?
 Let be a finite partially ordered set. Consider the following topology on : a subset is open if and only if whenever and , it must be that .
Construct a ring such that is homeomorphic to .
 Show that any quasicompact scheme has closed points. (It is not true that any scheme has closed points!)
 Give an example of a scheme that has no open connected subsets. In particular, such a scheme is not locally connected. Of course, my convention here is that the empty set is not connected...
Homework 4
Due Friday, February 24th
 Show that the following two definitions of quasiseparatedness of a scheme are equivalent:
 The intersection of any two quasicompact open subsets of is quasicompact;
 There is a cover of by affine open subsets whose (pairwise) intersections are quasicompact.
 In class, we gave the following definition: a scheme is integral if it is irreducible and reduced. Show that this is equivalent to the definition from Vakil’s notes: a scheme is integral if for any nonempty open , is a domain.
 Let us call a scheme locally irreducible if every point has an irreducible neighborhood. (Since a nonempty open subset of an irreducible space is irreducible, this implies that all smaller neighborhoods of this point are irreducible as well.) Prove or disprove the following claim: a scheme is irreducible if and only if it is connected and locally irreducible.
 Show that a locally Noetherian scheme is quasiseparated.
 Show that the following two definitions of a Noetherian scheme are equivalent:
 is a finite union of open affine sets, each of which is the spectrum of a Noetherian ring;
 is quasicompact and locally Noetherian.
 Show that any Noetherian scheme is a disjoint union of finitely many connected open subsets (the connected components of .) (A problem from the last homework shows that things might go wrong if we do not assume that is Noetherian.)
 A locally closed subscheme is defined as a closed subscheme of an open subscheme of . Accordingly, a locally closed embedding is a composition of a closed embedding followed by an open embedding (in this order). In principle, one can try to reverse the order, and consider open subschemes of closed subschemes of . Does this yield an equivalent definition?
Remark. The difficulty of such questions (and, sometimes, the answer to them) depends on the class of schemes one works with: often, very mild assumptions (such as, say, quasicompactness) would make the question easy. A complete answer to this problem would include both the mild assumptions that would make the two versions equivalent, and a description of what happens for general schemes.
Homework 5
Due Friday, March 3rd
 Fix a field , and put and . Consider the morphism given by . Describe the fiber product as explicitly as possible. (The answer may depend on .)
 (The Frobenius morphism.) Let be a scheme of characteristic : by definition, this means that in the structure sheaf of . Define the (absolute) Frobenius morphism as follows: it is the identity map on the underlying set, and the pullback equals for any (local) function .
Verify that this defines an affine morphism of schemes. Assuming is a scheme locally of finite type over a perfect field, verify that is a morphism of finite type (it is in fact finite, if you know what it means).
 (The relative Frobenius morphism.) Let be a morphism of schemes of characteristic . Put where the notation means that is considered as a scheme via the Frobenius map.
 Show that the Frobenius morphism naturally factors as the composition , where the first map is naturally a morphism of schemes over (while the second map, generally speaking, is not). The map is called the relative Frobenius morphism.
 Suppose , and is an affine variety (that is, an affine reduced scheme of finite type) over . Describe and the relative Frobenius explicitly in coordinates.
 Let be a scheme over . In this case, the absolute Frobenius is a morphism of schemes over (and it coincides with the relative Frobenius of over .
Consider the extension of scalars Then naturally extends to a morphism of schemes . Compare the map with the relative Frobenius of over .
 A morphism of schemes is surjective if it is surjective as a morphism of sets. Show that surjectivity is preserved by base changes. That is, if is surjective and is arbitrary, then is surjective.
 (Normalization) A scheme is normal if all of its local rings are integrally closed domains. Let be an integral scheme. Show that there exists a normal integral scheme together with a morphism that is universal in the following sense: any dominant morphism from a normal integral scheme to factors through . (Just like in the case of varieties, a morphism is dominant if its image is dense.)
 Let be a scheme of finite type over a field . For every field extension , put </p>
Show that is geometrically irreducible (that is, the morphism has geometrically irreducible fibers) if and only if is irreducible for all finite extensions .
Homework 6
Due Friday, March 10th
Sheaves of modules on ringed spaces.
Let be a ringed space, and let and be sheaves of modules. The tensor product of is the sheafification of the presheaf
 Prove that the stalks of are given by the tensor product: where . Conclude that the tensor product is a right exact functor (in each of the two arguments).
 Suppose that is locally free of finite rank. (That is to say, every point has a neighborhood such that . Prove that there exists a natural isomorphism Here is the dual of the locally free sheaf , and is the sheaf of homomorphisms. (Note that is not assumed to be quasicoherent.)
 (Projection formula) Let be a morphism of ringed spaces. Suppose is an module and is a locally free module of finite rank. Construct a natural isomorphism
Coherent sheaves on a noetherian scheme
 Let be a coherent sheaf on a loclly noetherian scheme .
Show that is locally free if and only if its stalks are free modules for all .
(b) Show that is locally free of rank one if and only if it is invertible: there exists a coherent sheaf such that .
 As in the previous problem, supposed be a coherent sheaf on a locally noetherian scheme . The fiber of at a point is the vector space for the natural map (where is the residue field of ). Denote by the dimension .
(a) Show that the function is upper semicontinuous: for every , the set is closed.
(b) Suppose is reduced. Show that is locally free if and only if is constant on each connected component of . (Do you see why we impose the assumption that is reduced here?)
 Let be a locally noetherian scheme and let be an open subset. Show that any coherent sheaf on can be extended to a coherent sheaf on on . (We say that is an extension of if .)
(If you need a hint for this problem, look at Problem II.5.15 in Hartshorne.)