# The Relative Proj Construction

Size: px
Start display at page:

Transcription

1 The Relative Proj Construction Daniel Murfet October 5, 2006 Earlier we defined the Proj of a graded ring. In these notes we introduce a relative version of this construction, which is the Proj of a sheaf of graded algebras S over a scheme X. This construction is useful in particular because it allows us to construct the projective space bundle associated to a locally free sheaf E, and it allows us to give a definition of blowing up with respect to an arbitrary sheaf of ideals. Contents 1 Relative Proj 1 2 The Sheaf Associated to a Graded Module Quasi-Structures The Graded Module Associated to a Sheaf Ring Structure Functorial Properties 21 5 Ideal Sheaves and Closed Subchemes 25 6 The Duple Embedding 27 7 Twisting With Invertible Sheaves 31 8 Projective Space Bundles 36 1 Relative Proj See our Sheaves of Algebras notes (SOA) for the definition of sheaves of algebras, sheaves of graded algebras and their basic properties. In particular note that a sheaf of algebras (resp. graded algebras) is not necessarily commutative. Although in SOA we deal with noncommutative algebras over a ring, here A-algebra will refer to a commutative algebra over a commutative ring A. Example 1. Let X be a scheme and F a sheaf of modules on X. In our Special Sheaves of Algebras (SSA) notes we defined the following structures: The relative tensor algebra T(F ), which is a sheaf of graded O X -algebras with the property that T 0 (F ) = O X. If F is quasi-coherent then so is T(F ) (SSA,Corollary 7). The relative symmetric algebra S(F ) which is a sheaf of commutative graded O X -algebras with the property that S 0 (F ) = O X. If F is quasi-coherent then S(F ) is quasi-coherent and locally generated by S 1 (F ) as an S 0 (F )-algebra (SSA,Corollary 16), (SSA,Corollary 18). 1

2 The relative polynomial sheaf F [x 1,..., x n ] for n 1. This is a sheaf of graded modules, which is quasi-coherent if F is (SSA,Corollary 44). If S is a sheaf of algebras on X then S [x 1,..., x n ] becomes a sheaf of graded algebras, which is commutative if S is. If S is a quasi-coherent sheaf of commutative algebras, then S [x 1,..., x n ] is locally finitely generated by S [x 1,..., x n ] 1 as a S [x 1,..., x n ] 0 -algebra (SSA,Corollary 46). Let X be a scheme and F a quasi-coherent sheaf of commutative graded O X -algebras. Then for any affine open subset X we have a graded O X ()-algebra F (), and therefore a scheme P rojf () together with a morphism of schemes π : P rojf () SpecO X () = (SOA,Proposition 40). Suppose V are affine open subsets of X. It follows immediately from (RAS,Lemma 9) that the following diagram is a pushout of rings (or equivalently that F () is a coproduct of O X (V )-algebras) O X (V ) O X () F (V ) F () Note that the isomorphism F (V ) OX (V ) O X () = F () is an isomorphism of graded O X ()- algebras, where the tensor product has the canonical grading, and F (V ) F () is a morphism of graded rings. If we write T = F (V ) OX (V ) O X () then T + is generated as a O X ()- module by ϕ(f (V ) + ) so we get morphisms of schemes P rojt P rojf (V ) and P rojf () P rojf (V ). It follows from our Proj Construction notes (Section 2 on P roj under pullback) that we have a commutative diagram, where the inner square is a pullback P rojf () P rojt P rojf (V ) SpecO X () SpecO X (V ) Therefore the outside diagram is also a pullback, and hence so is the following diagram P rojf () P rojf (V ) π V Hence ρ,v : P rojf () P rojf (V ) is an open immersion with open image π 1 V. Since ρ,v is the morphism of schemes induced by the morphism of graded rings F (V ) F () it is clear that if V W are affine, ρ V,W ρ,v = ρ,w. Also ρ, = 1 for any affine open subset X. For open affines, V X let X,V denote the open subset π 1 V of P rojf () with the induced scheme structure. We want to define an isomorphism of X-schemes O,V : X,V = XV,. We do this in several steps: π V Step 1 Let W be an affine open subset of V and let τ W be the following morphism of schemes over X τ W : π 1 W P rojf (W ) P rojf (V ) So τ W is an open immersion with open image π 1 V W. Let {W α} α Λ be the set of all affine open subsets of V, so that the open sets π 1 W α form an open cover of X,V. We want to show that we can glue the morphisms τ W over this cover. First we check a special case. 2

3 Step 2 Let W W be open affine subsets of V. We claim that τ W = π 1 W π 1 W P rojf (V ). It suffices to show that the following diagram commutes π 1 W π 1 W P rojf (W ) P rojf (W ) This is straightforward to check, using the pullback π 1 W = P rojf () W and properties of the morphisms ρ given above. Step 3 Now let W, W be arbitrary open affine subsets of V. We have to show the following diagram commutes π 1 (W W ) π 1 W π 1 W P rojf (V ) But we can cover W W with affine open subsets, and it suffices to check that both legs of this diagram agree when composed with the inclusions of these open subsets. Therefore we can reduce to the case already established in Step 2. This shows that we can glue the morphisms τ W to get a morphism of X-schemes τ : X,V P rojf (V ). Let θ,v : X,V = XV, be the factorisation through the open subset X V,. It is not hard to check that θ 1,V (π 1 V W ) = π 1 W for any open affine W V. Hence θ,v is an isomorphism, since it is locally the isomorphism π 1 W π 1 V W. So we have produced an isomorphism θ,v of schemes over X with the property that for any open affine subset W V the following diagram commutes X,V θ,v X V, (1) π 1 W π 1 V W where the bottom morphism is π 1 W P rojf (W ) π 1 V W. In particular if V then θ,v is the canonical isomorphism P rojf () = X V, induced by ρ,v. Take the family of schemes {P rojf ()} X indexed by the affine open subsets of X. For every pair of affine open subsets, V we have the isomorphism θ,v between the open subsets X,V and X V, of P rojf () and P rojf (V ) respectively. To glue all the family of schemes {P rojf ()} X along these open subsets, we first have to check that θ,v = θ 1 V, (see Ex 2.12 for the details). But one can check this locally, in which case it follows directly from the definition using (1). Note also that θ, = 1. Next let, V, W be open affine subsets of X. Since θ,v is a morphism of schemes over X, it is clear that θ,v (X,V X,W ) = X V, V V,W. Next we have to check that θ,w = θ V,W θ,v on X,V X,W, but it suffices to check this on π 1 Q for an affine open subset Q V W. But then the morphism π 1 Q π 1 W Q obtained from θ V,W θ,v is just the composite π 1 Q P rojf (Q) π 1Q P rojf (Q) π 1 V The middle two morphisms are inverse to each other, which shows that this composite is the same as the morphism determined by θ,w. Thus our family of schemes and patches satisfies the conditions of the Glueing Lemma, and we have a scheme ProjF together with open immersions ψ : P rojf () ProjF for each affine open subset X. These morphisms have the following properties: (a) The open sets Imψ cover ProjF. W Q 3

4 (b) For affine open subsets, V X we have ψ (X,V ) = Imψ Imψ V and ψ V XV, θ,v = ψ X,V. In particular for affine open subsets V we have X,V diagram commutes P rojf (V ) ψ V ProjF ρ,v ψ P rojf () = P rojf () so the following (2) The open sets Imψ are a nonempty open cover of ProjF, and it is a consequence of (b) above and the fact that θ,v is a morphism of schemes over X that the morphisms Imψ = P rojf () X can be glued (that is, for open affines, V the corresponding morphisms agree on Imψ Imψ V ). Therefore there is a unique morphism of schemes π : ProjF X with the property that for every open affine subset X the following diagram commutes P rojf () ψ ProjF (3) π π X In fact is easy to see that π 1 = Imψ, so the above diagram is also a pullback. In summary: Definition 1. Let X be a scheme and F a commutative quasi-coherent sheaf of graded O X - algebras. Then we can canonically associate to F a scheme π : ProjF X over X. For every open affine subset X there is an open immersion ψ : P rojf () ProjF with the property that the diagram (3) is a pullback and the diagram (2) commutes for any open affines V. ProjF P rojf () ψ = Lemma 1. The morphism π : ProjF X is separated. If F is locally finitely generated as an O X -algebra then π is of finite type. Proof. Separatedness is a local property and for open affine X the morphism SpecF () is separated (TPC,Proposition 1), so it follows that π is also separated. The same argument holds for the finite type property, using (TPC,Proposition 3). Corollary 2. If X is noetherian and F is locally finitely generated as an O X -algebra then ProjF is noetherian. Proof. Follows immediately from Lemma 1 and (Ex. 3.13g). 4

5 Corollary 3. If X is noetherian and F is locally finitely generated by F 1 as an O X -algebra, then the morphism π : ProjF X is proper. Proof. For each open affine X, the morphism π : P rojf () is a projective morphism (TPC,Lemma 19) and therefore proper (H, 4.9),(TPC,Proposition 3). But since ProjF is noetherian by Corollary 2 and properness is local (H,4.8) it follows that π is also proper. Let X be a nonempty topological space. It is easy to check that X is irreducible if and only if every nonempty open set V is dense. We have another useful characterisation of irreducible spaces Lemma 4. Let X be a nonempty topological space and { α } α Λ a nonempty open cover of X by nonempty open sets α. Then for X to be irreducible it is necessary and sufficient that α be irreducible for every α, and α β for every pair α, β. Proof. The condition is clearly necessary. To see that it is sufficient, let V be a nonempty open subset of X. To show V is dense in X, it suffices to show that V α for every α. But there must exist an index γ with V γ, in which case V γ is dense in γ, and therefore must meet the nonempty open subset α γ γ for every other index α. Therefore α γ V is nonempty, which shows that V α is nonempty for every index α. Definition 2. Let X be a scheme and F a commutative quasi-coherent sheaf of graded O X - algebras. Let Λ be the set of nonempty affine open subsets X with F () + 0. We say F is relevant if it is locally an integral domain, Λ is nonempty and if for every pair, V Λ there is W Λ with W V. Proposition 5. If F is relevant then ProjF is an integral scheme. Proof. Since F is relevant there is a nonempty affine open subset W X with P rojf (W ) an integral scheme (TPC,Proposition 4). In particular ProjF is nonempty and since we can cover it with integral schemes, it is reduced. sing Lemma 4 it suffices to show that Imψ Imψ V is nonempty for every pair of nonempty open affine subsets, V X with F () + 0, F (V ) + 0. By assumption there is a nonempty affine open subset W V with F (W ) + 0. Then Imψ W = P rojf (W ) is nonempty and contained in Imψ Imψ V, as required. 2 The Sheaf Associated to a Graded Module Throughout this section X is a scheme and S a commutative quasi-coherent sheaf of graded O X -algebras locally generated by S 1 as an S 0 -algebra. Let GrMod(S ) denote the category of all sheaves of graded S -modules and QcoGrMod(S ) the full subcategory of quasi-coherent sheaves of graded S -modules (SOA,Definition 2). Note that these are precisely the sheaves of graded S -modules that are quasi-coherent as sheaves of O X -modules, so in this section there is no harm in simply calling these sheaves quasi-coherent (SOA,Proposition 19). In this section we define a functor : QcoGrMod(S ) Mod(ProjS ) which is the relative version of the functor SGrMod Mod(P rojs) for a graded ring S. Note that QcoGrMod(S ) is an abelian category (SOA,Proposition 47). Lemma 6. Let X be a scheme and V affine open subsets. If S is a commutative quasicoherent sheaf of graded O X -algebras, and M a quasi-coherent sheaf of graded S -modules, then the following morphism of graded S ()-modules is an isomorphism M (V ) S (V ) S () M () (4) a b b a (5) 5

6 Proof. Both M (V ) and S () are graded S (V )-modules (SOA,Proposition 40) so there is certainly a morphism of graded S ()-modules with a b b a. From (RAS,Lemma 9) we know that there are isomorphisms of O X ()-modules S (V ) OX (V ) O X () = S () M (V ) OX (V ) O X () = M () So at least we have an isomorphism of abelian groups M (V ) S (V ) S () = M (V ) S (V ) (S (V ) OX (V ) O X ()) = (M (V ) S (V ) S (V )) OX (V ) O X () = M (V ) OX (V ) O X () = M () It is easily checked that this map agrees with (4), which is therefore an isomorphism. Proposition 7. If M is a quasi-coherent sheaf of graded S -modules then there is a canonical quasi-coherent sheaf of modules M on ProjS with the property that for every affine open subset X there are isomorphisms (ψ ) M () = M Imψ M = M () ψ where ψ : P rojs () ProjS is the canonical open immersion and ψ : P rojs () Imψ the induced isomorphism. Proof. Let X be an affine open subset. Then S () is a commutative graded O X ()-algebra with degree d component S d () and M () is a graded S ()-module with degree n component M n () (SOA,Proposition 40). Therefore we have a quasi-coherent sheaf of modules M () on P rojs (). Let M denote the direct image (ψ ) M () where ψ : P rojs () = Imψ is induced by ψ. We want to glue the M over the open sets Imψ as ranges over all affine open subsets of X. Let W be open affine subsets and ρ W, : P rojs (W ) P rojs () the canonical open immersion. sing Lemma 6 and the fact that S is generated by S 1 as a S 0 -algebra we have an isomorphism of sheaves of modules on P rojs (W ) α W, : ρ W, (M () ) = (M () S () S (W )) = M (W ) [V, m/s] b/t (b m W ) /ts W for open T P rojs (W ), V ρ W, (T ) sections m M (), s S () and b, t S (W ) pairwise homogenous of the same degree. Let ρ W, : P rojs (W ) = X,W be the isomorphism induced by ρ W, and note that ψ,w = ψ W (ρ W, ) 1 where ψ,w : X,W = Imψ W is induced by ψ. Therefore we have an isomorphism of sheaves of modules ϕ,w : M ImψW M W on Imψ W M W = (ψ W ) (M (W ) ) = (ψ W ) ρ W, (M () ) = (ψ W ) (ρ W, ) 1 (M () X,W ) = (ψ W (ρ W, ) 1 ) (M () X,W ) = (ψ,w ) (M () X,W ) = (ψ ) (M () ) ImψW = M ImψW using α W, and (MRS,Proposition 110). Given open V X,W and sections m M (), s S () homogenous of the same degree, we have ϕ,w : M ImψW M W m/s m W /s W 6

7 Clearly ϕ, = 1, and if Q W are open affine subsets then ϕ,q = ϕ W,Q ϕ,w ImψQ. This means that for open affine, V X the isomorphisms ϕ 1 V,W ϕ,w for open affine W V glue together to give an isomorphism of sheaves of modules ϕ,v : M Imψ Imψ V M V Imψ Imψ V (6) ϕ V,W ϕ,v ImψW = ϕ,w for affine open W V (7) The notation is unambiguous, since this definition agrees with the earlier one in the case V. By construction these isomorphisms can be glued (GS,Proposition 1) to give a canonical sheaf of modules M on ProjS and a canonical isomorphism of sheaves of modules µ : M Imψ M for every open affine X. These isomorphisms are compatible in the following sense: we have µ V = ϕ,v µ on Imψ Imψ V for any open affine, V X. It is clear that M is quasi-coherent since the modules M () are. Example 2. For every n Z we have the quasi-coherent sheaf of graded S -modules S (n), and we denote by O(n) the sheaf of modules S (n) on ProjS. It follows from (H,5.12) that O(n) is an invertible sheaf and moreover for every open affine X we have a canonical isomorphism ψ O(n) = O(n). Proposition 8. If β : M N is a morphism of quasi-coherent sheaves of graded S -modules then there is a canonical morphism β : M N of sheaves of modules on ProjS and this defines an additive functor : QcoGrMod(S ) Mod(ProjS ). Proof. For every affine open X, β : M () N () is a morphism of graded S ()- modules, and therefore gives a morphism β : M () N () of sheaves of modules on P rojs (). Let b : M N be the morphism (ψ ) β (notation of Proposition 7). We have to show that for open affine, V X and T = Imψ Imψ V the following diagram commutes b T M T N T M V T bv T N V T In suffices to check this on sections of the form m/s, which is trivial, so the diagram commutes. Therefore there is a unique morphism of sheaves of modules β with the property that for every affine open X the following diagram commutes (GS,Proposition 6) M Imψ eβ Imψ N Imψ µ M N b µ sing this unique property it is easy to check that defines an additive functor. Proposition 9. The additive functor : QcoGrMod(S ) Mod(ProjS ) is exact. Proof. Since QcoGrMod(S ) is an abelian subcategory of GrMod(S ) (SOA,Proposition 47), a sequence is exact in the former category if and only if it is exact in the latter, which by (LC,Corollary 10) is if and only if it is exact as a sequence of sheaves of S -modules. So suppose we have an exact sequence of quasi-coherent graded S -modules M ϕ M ψ M 7

8 Since this is exact as a sequence of sheaves of S -modules, it is exact as a sequence of sheaves of modules on X, and therefore using (MOS,Lemma 5) we see that for affine open X the following sequence of graded S ()-modules is exact M () ϕ M () ψ M () Since the functor : S ()GrMod Mod(P rojs ()) is exact, we have an exact sequence of sheaves of modules on P rojs () M () fϕ M () fψ M () If ψ : P rojs () ProjS is the canonical open immersion and ψ : P rojs () Imψ the induced isomorphism then applying (ψ ) and using the natural isomorphism (ψ ) M () = M Imψ we see that the following sequence of sheaves of modules on Imψ is exact M Imψ eϕ Imψ M Imψ e ψ Imψ M Imψ It now follows from (MRS,Lemma 38) that the functor is exact. Proposition 10. Let X be a scheme and n 0 an integer. Then ProjO X [x 0,..., x n ] = P n X. That is, there is a pullback diagram ProjO X [x 0,..., x n ] ζ P n Z π X SpecZ (8) Moreover if O(1) is the invertible sheaf of Proposition 7 then there is a canonical isomorphism of sheaves of modules ζ O(1) = O(1). Proof. Let S be the commutative quasi-coherent sheaf of graded O X -algebras O X [x 0,..., x n ]. For an affine open subset X, there is a canonical isomorphism of graded O X ()-algebras S () = O X ()[x 0,..., x n ] (SSA,Proposition 45). The canonical morphism of graded rings z : Z[x 0,..., x n ] O X ()[x 0,..., x n ] therefore induces a morphism of schemes ζ : P rojs () P n Z fitting into the following pullback diagram (TPC,Proposition 16) P rojs () ζ P n Z π SpecZ sing the naturality of (SSA,Proposition 45) in it is easy to see that the ζ glue to give a morphism of schemes ζ : ProjS P n Z. sing our notes on the local nature of pullbacks in Section 2.3 we see that (8) is a pullback, as required. Finally we have to show ζ O(1) = O(1). With the notation of Proposition 7, for affine open X there is an isomorphism of sheaves of modules on Imψ ϑ : (ζ O(1)) Imψ = (ζ (ψ ) 1 ) O(1) = ((ψ ) 1 ) ζ O(1) = (ψ ) (ζ O(1)) = (ψ ) O(1) = O(1) Imψ 8

9 sing (MRS,Proposition 111), (MRS,Remark 8), (MRS,Proposition 107), (MPS,Proposition 13), and finally µ. We need to know the action of ϑ on special sections of the following form: suppose Q Imψ is open and that b, t S () are homogenous of the same degree such that b/t Γ(ψ 1 Q, P rojs ()). Denote also by b/t the corresponding section of ProjS. Suppose a, s Z[x 0,..., x n ] are homogenous, with a of degree d+1, s of degree d, and a/s Γ(ψ 1 T, O(1)) for some open T with ζ(q) T. Then we have ϑ Q [T, a/s] b/t Γ(Q, ζ O(1)) (9) ( [T, a/s] b/t ) = z (a)b /z (s)t (10) The section on the right is the section of ProjS s twisting sheaf corresponding to z (a)b /z (s)t Γ(ψ 1 Q, O(1)) via µ. To show that the isomorphisms ϑ glue we need only show that for affine W we have ϑ ImψW = ϑ W. We can reduce immediately to sections of the form (9) for Q Imψ W. The compatibility conditions on the morphisms µ, µ W (see Proposition 7) mean that the section [T, a/s] b/t is the same as [T, a W /s W ] b W /t W (the sections now being the images under µ W instead of µ ). But ( ) [T, a W /s W ] b W /t W = z W (a W )b W /zw (s W )t W ϑ W Q and by the same argument, the section on the right is the same section of ProjS s twisting sheaf as z (a)b /z (s)t. This shows that ϑ ImψW = ϑ W and therefore there is an isomorphism ϑ : ζ O(1) O(1) of sheaves of modules on ProjS unique with the property that for affine open X, ϑ Imψ = ϑ. Remark 1. In general O(1) is not very ample on ProjF relative to X. See (7.10) and (Ex 7.14). Remark 2. Taking n = 0 in Proposition 10 we see that the structural morphism ProjO X [x 0 ] X is an isomorphism of schemes. Definition 3. If F is a sheaf of modules on ProjS then F (n) denotes the tensor product F O(n) for n Z. If F is quasi-coherent, then so is F (n) for every n Z. Proposition 11. If M, N are quasi-coherent sheaves of graded S -modules then M S N is a quasi-coherent sheaf of graded S -modules and there is a canonical isomorphism of sheaves of modules on ProjS natural in M, N ϑ : M ProjS N (M S N ) m/s n/t (m n) /st Proof. We showed in (SOA,Proposition 46) that M S N is a quasi-coherent sheaf of graded S - modules. For affine open X let ψ : P rojs () ProjS be the canonical open immersion and ψ : P rojs () Imψ the induced isomorphism. Then using (SOA,Proposition 46) we see that there is an isomorphism of sheaves of modules on Imψ ϑ : ( M ProjS N ) Imψ = M Imψ N Imψ = (ψ ) M () (ψ ) N () = (ψ ) (M () N () ) = (ψ ) ((M () S () N ()) ) = (ψ ) ((M S N )() ) = (M S N ) Imψ Note that we use the results of (MPS,Proposition 3) which means we need the standing hypothesis that S is locally generated by S 1 as an S 0 -algebra. If Q Imψ is open and m M (), s S () such that m/s defines an element of Γ(ψ 1 Q, M () ) then identify this with a section of 9

10 M (Q) via the canonical isomorphism µ of Proposition 7. Similarly suppose n N (), s S () give a section n/t N (Q). Then m/s n/t Γ(Q, M ProjS N ) (11) ϑ Q( m/s n/t) = (m n) /st (12) To glue the morphisms ϑ using (GS,Corollary 5) it suffices to show that for affine open W we have ϑ ImψW = ϑ W. We can reduce immediately to sections of the form (11) for Q Imψ W, in which case we use the compatibility conditions on the morphisms µ, µ W in an argument similar to the one given in the proof of Proposition 10. There is an isomorphism ϑ : M ProjS N (M S N ) of sheaves of modules on ProjS unique with the property that for affine open X, ϑ Imψ = ϑ. sing this unique property it is not difficult to check that ϑ is natural in both variables. Lemma 12. There is an equality O(0) = S = O ProjS of sheaves of modules on ProjS. Proof. If one inspects the glueing constructions given in (GS,Proposition 1) and our solution to (H,Ex.2.12) then it is not difficult to see that proving the equality S = O ProjS amounts to showing that for affine open subsets, V X and open Q Imψ Imψ V the following two morphisms of abelian groups agree (ϕ V, ) Q, (θ #,V ) ψ 1 V Q : O P rojs (V )(ψ 1 V Q) O P rojs ()(ψ 1 Q) where ϕ V, is the isomorphism defined in Proposition 7 and θ,v is the isomorphism defined in the construction of ProjS. If ψ V, : X V, Imψ Imψ V denotes the isomorphism induced by ψ V : P rojf (V ) ProjS then we are trying to show that ϕ V, = (ψ V, ) θ,v as morphisms of sheaves of abelian groups on Imψ Imψ V, so we can reduce to the case where V and consider only special sections of the form m/s. Both morphisms map this section to m /s, so we are done. Corollary 13. For integers m, n Z there are canonical isomorphisms of sheaves of modules on ProjS τ m,n : O(m) O(n) O(m + n) a/b c/d ac/bd κ m,n : F (m)(n) F (m + n) (m a/b) c/d m ( ac/bd) ρ n : (M (n)) M (n) (a m) /sb m/s where F is a sheaf of modules on ProjS and M is a quasi-coherent sheaf of graded S -modules. The latter two isomorphisms are natural in F and M respectively. Proof. sing (MRS,Lemma 102) and Proposition 11 we have the first isomorphism of sheaves of modules τ m,n : O(m) O(n) = S (m) S (n) = (S (m) S (n)) = S (m + n) = O(m + n) Given an open affine subset X, Q Imψ and a, b, c, d S () homogenous with a/b Γ(ψ 1 Q, O(m)) respectively c/d Γ(ψ 1 Q, O(n)) we denote by a/b, c/d the corresponding sections of O(m), O(n) on Q. Then it is not hard to check that τ m,n Q ( a/b c/d) = ac/bd. sing τ m,n we have an isomorphism of sheaves of modules κ m,n : F (m)(n) = (F O(m)) O(n) = F (O(m) O(n)) = F O(m + n) = F (m + n) If m F (Q) then it is not hard to check that κ m,n Q ((m a/b) c/d) = m ac/bd. Finally using (MRS,Lemma 104) we have an isomorphism of sheaves of modules ρ n : (M (n)) = (M S S (n)) = M S (n) = a/b M O(n) = M (n) 10

11 If m M (), s S () are homogenous of the same degree and a, b S () are homogenous with a of degree d + n and b of degree d such that m/s Γ(ψ 1 Q, M () ) and a/b Γ(ψ 1 Q, O(n)) then (ρ n Q ) 1 maps m/s a/b to (a m) /sb. Naturality of the latter two isomorphisms is not difficult to check. Proposition 14. Let M, N, T be quasi-coherent sheaves of graded S -modules. We claim that the following diagram of sheaves of modules on ProjS commutes ( M N ) T ϑ 1 (M S N ) T λ M ( N T ) 1 ϑ M (N S T ) ϑ ((M S N ) S T ) eλ (M S (N S T )) ϑ Proof. For open affine X let ψ, ψ be as in Proposition 7 and identify the sheaves of modules (ψ ) M () and M Imψ by the isomorphism µ given there. Similarly for N and T. Then we can reduce to checking commutativity of the diagram on sections of the form ( m/s n/t) p/q ( Γ Q, ( M N ) T ) for affine open X, open Q Imψ and m M (), n N (), p N () and s, t, q S (). Since ϑ Imψ = ϑ this is not difficult to check. Corollary 15. For integers n, e, d Z the following diagram of sheaves of modules on ProjS commutes (O(n) O(e)) O(d) λ O(n) (O(e) O(d)) τ n,e 1 1 τ e,d O(n + e) O(d) O(n) O(e + d) τ n+e,d τ n,e+d O(n + e + d) Proof. The same used in our Section 2.5 notes works mutatis mutandis, using (MRS,Lemma 103). Corollary 16. Let F be a sheaf of modules on ProjS and n, e, d Z integers. following diagram of sheaves of modules on ProjS commutes Then the (F (n) O(e)) O(d) λ F (n) (O(e) O(d)) κ n,e 1 1 τ e,d F (n + e) O(d) F (n) O(e + d) κ n+e,d κ n,e+d F (n + e + d) Proof. Once again the proof given in our Section 2.5 notes works mutatis mutandis. 11

12 Lemma 17. Let e Z and n > 0 be given. Then there is a canonical isomorphism of sheaves of modules on ProjS ζ : O(e) n O(ne) f 1 /s1 f n /sn f 1 f n /s1 s n Proof. The proof is by induction on n. If n = 1 the result is trivial, so assume n > 1 and the result is true for n 1. Let ζ be the isomorphism O(e) n = O(e) O(e) (n 1) = O(e) O(ne e) = O(ne) sing the inductive hypothesis and the explicit form of the isomorphism τ : O(e) O(ne e) = O X (ne) one checks that we have the desired isomorphism. Lemma 18. Let e Z and n, m > 0 be given. We claim that the following diagram of sheaves of modules on ProjS commutes O(e) m O(e) n O(e) (m+n) O(em) O(en) O(em + en) Proof. After reducing to sections of the form (f 1 /s1 f m /sm ) (g 1 /t1 g n /tn ) this is straightforward. Lemma 19. Let M be a quasi-coherent sheaf of graded S -modules and π : ProjS X the structural morphism. There are canonical morphisms of sheaves of abelian groups on X natural in M α : M 0 π M α n : M n π M (n) β n : M n π M (n) n Z n Z In particular we have a morphism of sheaves of abelian groups α d : S d π O(d) for every d 0. For d = 0 we have a morphism of sheaves of rings α 0 : S 0 π O ProjS. Proof. For affine open X we have a morphism of abelian groups M 0 () Γ(P rojs (), M () ) = Γ(π 1, M ) m m/1 (µ ) 1 m/1) Imψ ( where µ : M Imψ M is the canonical isomorphism. It is not difficult to check that this morphism is natural in the affine open set, so we can glue to obtain the required morphism of sheaves of abelian groups α. To define α n just replace M by M (n). Naturality of these morphisms in M is not difficult to check. To define β n for n Z we glue together the following morphisms of abelian groups M n () Γ(P rojs (), M (n)() ) = Γ(π 1, M (n)) = Γ(π 1, M (n)) m (ρ n µ 1 ) Imψ ( m/1) using the isomorphism ρ n defined in Corollary 13. Naturality in M is not difficult to check, using the naturality of ρ n. 12

13 Remark 3. Set Y = ProjS. Then for n = 0 we have O(n) = O Y and τ m,0 : O(m) O Y O(m) is the canonical isomorphism a b b a. Similarly κ m,0 : F (m) O Y F (m) is the canonical isomorphism. Also if s Γ(, S d ), r Γ(, S e ) and α d : S d π O(d) and α e : S e π O(e) are as above then τ d,e π 1 (αd (s) α e (r)) = α d+e (sr) Lemma 20. Let M be a quasi-coherent sheaf of graded S -modules. Then the following diagram of sheaves of modules on ProjS commutes for m, n Z M (m)(n) κ m,n M (m + n) M (m) (n) M (m)(n) M (m + n) Proof. After reducing to sections of the form ( m/s a/b) c/d this is straightforward. Lemma 21. Let M be a quasi-coherent sheaf of graded S -modules, X an affine open subset and n, d Z. Then the following diagram of sheaves of modules on Imψ commutes ψ (M () (n) O(d)) ψ (M () (n)) ψ O(d) ψ (M () (n + d)) ψ (M ()(n) ) O(d) Imψ ψ (M ()(n + d) ) M (n) Imψ O(d) Imψ M (n + d) Imψ M (n + d) Imψ ( ) M (n) O(d) Imψ where ψ : P rojs () ProjS is the canonical open immersion and ψ : P rojs () Imψ the induced isomorphism. Proof. Once again by reduction to sections of the form ( m/s a/b) c/d. Lemma 22. Let M be a quasi-coherent sheaf of graded S -modules. Then the following diagram of sheaves of abelian groups on X commutes M n Z S d χ M n+d π β n α d M (n) OX π O(d) β n+d ( ) π M (n) O(d) π M (n + d) π κ n,d 13

14 Proof. See (SGR,Section 2.3) for the definition of the tensor product of sheaves of abelian groups Z. We induce the morphism of sheaves of abelian groups χ using the bilinear map M n () S d () M n+d () defined by (m, s) s m. The morphism β n α d is similarly induced by the bilinear map (m, s) β n(m) α d (s). It suffices to check commutativity on sections of the form m s, which follows from Lemma 20. Definition 4. Let F be a sheaf of modules on ProjS and X an affine open subset. Let ψ : P rojs () ProjS be the canonical open immersion, ψ : P rojs () Imψ the induced isomorphism with inverse λ. Then we denote by F the sheaf of modules λ (F Imψ ) on P rojs (). If ψ : F F is a morphism of sheaves of modules then denote by ψ the morphism λ (ψ Imψ ) : F F. This defines an additive functor which clearly preserves quasi-coherentness. ( ) : Mod(ProjS ) Mod(P rojs ()) Lemma 23. Let F be a sheaf of modules on ProjS and X an affine open subset. Then for n Z there is an isomorphism of sheaves of modules on P rojs () natural in F γ : F (n) F (n) Proof. The isomorphism is given by the composite F (n) = λ (F (n) Imψ ) = λ (F Imψ O(n) Imψ ) = F λ (O(n) Imψ ) = F (n) Since by construction (ψ ) O(n) = O(n) Imψ and therefore O(n) = λ (O(n) Imψ ). Naturality in F means that the following diagram commutes for a morphism φ : F F of sheaves of modules F (n) F (n) φ(n) φ (n) F (n) F (n) It suffices to check commutativity on sections of the form m a/b, which is straightforward. Lemma 24. Let F be a sheaf of modules on ProjS and X an affine open subset. Then for n, d Z the following diagram of sheaves of modules on P rojs () commutes F (n)(d) κ n,d F (n + d) F (n) (d) F (n)(d) κ n,d F (n + d) Proof. By reduction to sections of the form (m a/b) c/d. 2.1 Quasi-Structures See (MRS,Section 2.1) for the definition of the equivalence relation of quasi-isomorphism on sheaves of graded S -modules and for the definition of quasi-isomorphism,quasi-monomorphism and quasi-epimorphism. Proposition 25. Let M, N be quasi-coherent sheaves of graded S -modules. If M N then there is a canonical isomorphism of sheaves of modules M = N on ProjS. In particular if M 0 then M = 0. 14

15 Proof. Suppose that M N and let κ : M {d} N {d} be an isomorphism of sheaves of graded S -modules for some d 0. Then for affine open X, κ : M (){d} N (){d} is an isomorphism of graded S ()-modules (SOA,Lemma 48) and therefore M () N (). Let γ : M () N () be the canonical isomorphism of sheaves of modules on P rojs () (MPS,Proposition 18). Denote by γ the induced isomorphism (ψ ) γ : M N (notation of Proposition 7). We claim that the following diagram commutes for affine open W M ImψW γ Imψ W N ImψW (13) ϕ,w M W γ W N W ϕ,w Let m M (), s S () be homogenous of the same degree, assume f S () 1 and let Q Imψ be open with (ψ ) 1 Q D + (sf). It suffices to check that the diagram commutes on the section m/s of M (Q), which is straightforward. Therefore there is a unique isomorphism γ : M N of sheaves of modules on ProjS with the property that the following diagram commutes for every affine open X (GS,Proposition 6) γ Imψ M Imψ N Imψ M γ N which completes the proof. Corollary 26. Let φ : M N be a morphism of quasi-coherent sheaves of graded S -modules. Then (i) φ is a quasi-monomorphism = φ : (ii) φ is a quasi-epimorphism = φ : (iii) φ is a quasi-isomorphism = φ : M N is a monomorphism. M N is an epimorphism. M N is an isomorphism. Proof. These statements follow immediately from Proposition 25, Proposition 9, (MRS,Lemma 100), and (SOA,Proposition 47). 3 The Graded Module Associated to a Sheaf If S is a graded ring generated by S 1 as an S 0 -algebra then the functor : SGrMod Mod(P rojs) has a right adjoint Γ : Mod(P rojs) SGrMod (AAMPS,Proposition 2). In this section we study a relative version of the functor Γ. Throughout this section X is a scheme, S a commutative quasi-coherent sheaf of graded O X -algebras locally generated by S 1 as an S 0 -algebra, and π : ProjS X the structural morphism. If F is a sheaf of modules on ProjS then define the following sheaf of modules on X Γ (F ) = n Z π (F (n)) For d 0 let α d : S d π O(d) be the morphism of sheaves of abelian groups defined in Lemma 19 and for m, n Z let κ m,n : F (m) O(n) F (m + n) be the isomorphism defined in Corollary 13. For open X and s Γ(, S d ), m Γ(π 1, F (n)) we define the section s m Γ(π 1, F (d + n)) by s m = κ n,d π 1 (m α d (s)) (14) 15

19 isomorphism. The proof reduces to checking that for n Z the following diagram commutes Φ(n) M (n) F (n) M (n) Ũ M (n)() M () (n) ϕ (n) F (n) By reduction to the case of sections of the form m/s a/b this is not difficult to check. Proposition 33. Suppose S is locally finitely generated as an O X -algebra. Then we have an adjoint pair of functors QcoGrMod(S ) e Γ ( ) Qco(ProjS ) Γ ( ) (16) For a quasi-coherent sheaf of graded S -modules M the unit is the morphism η : M Γ (M ) defined above. Proof. We already know that the morphism M Γ (M ) is natural in M, so it suffices to show that the pair (M, η) is a reflection of M along Γ ( ). Suppose we are given a morphism φ : M Γ (G ) of sheaves of graded S -modules for a quasi-coherent sheaf G of modules on ProjS. For affine open X we a morphism of graded S ()-modules φ : M () Γ (G )() and a pair of adjoints S ()GrMod e Γ ( ) Mod(P rojs ()) Γ ( ) (17) sing the isomorphism of Lemma 31, the composite M () Γ (G )() = Γ (G ) induces a morphism ϕ : M () G of sheaves of modules on P rojs (). Taking the image of this morphism under the functor (ψ ) and composing with the isomorphism (ψ ) M () = M Imψ we have a morphism of sheaves of modules on Imψ Φ : M Imψ G Imψ m/s (νκ d, d ) Q (φ (m) Q 1/s) where m M (), s S () are homogenous of degree d such that m/s Γ(ψ 1 Q, M () ) and ν : G (0) G is the canonical isomorphism. sing what by now is a standard technique (see for example the proof of Proposition 11), we show that for affine open W, Φ ImψW = Φ W. Therefore there is a unique morphism Φ : M G of sheaves of modules on ProjS with the property that Φ Imψ = Φ for every open affine X. For affine open consider the following 19

20 diagram Γ (G )() Γ (G ) φ M () Γ (Φ) Γ (ϕ ) η Γ ( M )() Γ (M () ) using this diagram, Lemma 32, Proposition 29 and the adjunction in (17) one shows that Φ is the unique morphism of sheaves of modules on ProjS making the following diagram commute φ M Γ (G ) η Γ (Φ) Γ ( M ) which completes the proof that we have an adjunction. The natural bijection Hom(M, Γ (G )) Hom( M, G ) is given by the construction φ Φ of the proof. Corollary 34. Suppose S is locally finitely generated by S 1 as an S 0 -algebra with S 0 = O X. Then for a quasi-coherent sheaf of modules G on ProjS the counit ε : Γ (G ) G is an isomorphism. Proof. For an affine open subset X there is an isomorphism of sheaves of modules on P rojs () Γ (G ) Ũ = Γ (G )() = Γ (G ) If ε G : Γ (G ) G is the counit of the adjunction (16) and ε G : Γ (G ) G is the counit of the adjunction (17) then by construction the following diagram of sheaves of modules on P rojs () commutes ε G Γ (G ) G (ε G ) Γ (G ) Ũ Since G is quasi-coherent, ε G is an isomorphism (AAMPS,Proposition 13) and therefore ε G Imψ is an isomorphism for every affine open X. It follows that ε is an isomorphism, as required. Corollary 35. Let X be a scheme of finite type over a field k and S a commutative quasi-coherent sheaf of graded O X -algebras locally finitely generated by S 1 as an O X -algebra. If M is a locally quasi-finitely generated quasi-coherent sheaf of graded S -modules then the unit η : M Γ (M ) is a quasi-isomorphism. That is, there is an integer d 0 such that for every n d we have an isomorphism of O X -modules η n : M n π ( M (n)) Proof. See (SOA,Definition 14) for the definition of locally quasi-finitely generated. Note that by Lemma 28, Γ (M ) a quasi-coherent sheaf of graded S -modules. Since X is noetherian it is quasi-compact, so we can find an affine open cover 1,..., n of X. By an argument similar to the one given in (MOS,Lemma 2) it suffices to show that there exists a d 0 such that η{d} i = η i {d} (SOA,Lemma 48) is an isomorphism of graded S ( i )-modules for 1 i n. So it suffices to show that η i : M ( i ) Γ (M )( i ) is a quasi-isomorphism for each i, which follows from Proposition 29 and (AEMPS,Corollary 3). 20

21 3.1 Ring Structure In this section set Y = ProjS and for n Z write ÔY (n) for the sheaf of modules O Y O(n) on Y. There is a natural isomorphism ÔY (n) = O(n). With this distinction, we have Γ (O Y ) = n Z π (ÔY (n)) For m, n Z let Λ m,n denote the following isomorphism of modules Λ m,n : ÔY (m) ÔY (n) = O(m) O(n) = O(m + n) = Ô(m + n) As above let γ (O Y ) denote the coproduct of presheaves of abelian groups n Z π (ÔY (n)). For an open subset X and a Γ(π 1, ÔY (m)) and b Γ(π 1, ÔY (n)) we define a b = Λ m,n π 1 (a b) Γ(π 1, ÔY (n + m)) If 1 denotes the element 1 1 of Γ(π 1, Ô(0)) and c Γ(π 1, ÔY (e)) then we have a(b + c) = ab + ac (a + b)c = ac + bc a(bc) = (ab)c 1a = a1 = a We use the analogue of Corollary 15 for Ô, which is easily checked by reducing to special sections. It is then not hard to check that γ (O Y )() is a commutative ring with the product (x y) i = x m y n = Λ m,n π 1 (x m y n ) m+n=i m+n=i With this definition γ (O Y ) is a presheaf of rings. Therefore Γ (O Y ) becomes a sheaf of Z-graded rings. It is not hard to check that η : S Γ (O Y ) is a morphism of sheaves of Z-graded rings. If we let Γ (O Y ) denote the graded submodule Γ (O Y ){0} of Γ (O Y ) then it is not hard to check that Γ (O Y ) is also a subsheaf of rings. In fact Γ (O Y ) is a commutative sheaf of graded O X - algebras and there is an induced morphism η : S Γ (O Y ) of sheaves of graded O X -algebras. If S is locally finitely generated as an O X -algebra then by Lemma 28 and (SOA,Lemma 48), Γ (O Y ) is a commutative quasi-coherent sheaf of graded O X -algebras. Corollary 36. Let X be a scheme of finite type over a field k and S a commutative quasi-coherent sheaf of graded O X -algebras locally finitely generated by S 1 as an O X -algebra. The morphism of sheaves of graded O X -algebras η : S Γ (O Y ) is a quasi-isomorphism. Proof. This follows immediately from Corollary Functorial Properties Proposition 37. Let X be a scheme and ξ : F G a morphism of commutative quasi-coherent sheaves of graded O X -algebras. Then canonically associated with ξ is an open set G(ξ) ProjG and a morphism of schemes Projξ : G(ξ) ProjF over X. For every open affine subset X we have G(ξ) Imψ = ψ (G(ξ )) and the following diagram is a pullback G(ξ) Projξ ProjF (18) ψ G(ξ ) P rojξ P rojf () 21

22 Proof. For an open affine X it is clear that ξ : F () G () is a morphism of graded O X ()-algebras, inducing a morphism of schemes P rojξ : G(ξ ) P rojf () over. It is not difficult to check that given open affines V we have G(ξ V ) P rojg () = G(ξ ) (19) where we identify P rojg () with an open subset of P rojg (V ). sing our notes on the Proj construction it is immediate that the following diagram commutes G(ξ V ) P rojf (V ) (20) G(ξ ) P rojf () Let G(ξ) be the open subset ψ (G(ξ )) of ProjG, where the union is over all affine open subsets of X. sing (19) we see that G(ξ) Imψ = ψ (G(ξ )) and (20) implies that the morphisms ψ (G(ξ )) = G(ξ ) P rojf () ProjF glue to give the morphism Projξ with the desired properties (as usual it suffices to check this for open affines V, which is straightforward). Lemma 38. Let X be a scheme and ξ : F G and ρ : G H morphisms of commutative quasi-coherent sheaves of graded O X -algebras. Then G(ρξ) G(ρ) and the following diagram commutes Projρξ G(ρξ) ProjF (Projρ) G(ρξ) Projξ G(ξ) Moreover if 1 : F F is the identity then G(1) = ProjF and Proj1 = 1. If ξ : F G is an isomorphism then G(ξ) = ProjF and Projξ is an isomorphism with inverse Proj(ξ 1 ). Proof. By intersecting with the open sets Imψ it is not hard to check that G(ρξ) G(ρ). It is also clear that (Projρ)G(ρξ) G(ξ), so we get the induced morphism G(ρξ) G(ξ) in the diagram. To check that the above diagram commutes, cover G(ρξ) in open subsets ψ (G(ρ ξ )) and after a little work reduce to the same result for morphisms of P roj of graded rings induced by morphisms of graded rings, which we checked in our Proj Construction notes. The remaining claims of the Lemma are then easily checked. Proposition 39. Let X be a scheme and φ : F G a morphism of commutative quasi-coherent sheaves of graded O X -algebras that is an epimorphism of sheaves of modules. Then G(φ) = ProjG and the induced morphism ProjG ProjF is a closed immersion. Proof. This follows immediately from (H, Ex.3.12), (MOS,Lemma 2) and the local nature of the closed immersion property. Example 3. Let X be a scheme, S a commutative quasi-coherent sheaf of graded O X -algebras, and J a quasi-coherent sheaf of homogenous S -ideals (MRS,Definition 22). Consider the following exact sequence in the abelian category GrMod(S ) 0 J S S /J 0 Here S /J is the sheafification of the presheaf of S -modules S ()/J (). For every open subset X, J () is an ideal of S (), so S ()/J () is a O X ()-algebra and S /J a sheaf of O X -algebras, which becomes a commutative quasi-coherent sheaf of graded O X -algebras with the canonical grading given in (LC,Corollary 10) and S S /J is a morphism of sheaves of graded O X -algebras. It follows from Proposition 39 that the induced morphism of schemes Proj(S /J ) ProjS is a closed immersion. Note that for affine open X it follows from (MOS,Lemma 5) that (S /J )() = S ()/J () as graded O X ()-algebras. 22

23 Proposition 40. Let X be a scheme and φ : S T a morphism of sheaves of graded O X - algebras satisfying the conditions of Section 2. Let Φ : Z ProjS be the induced morphism of schemes and M a quasi-coherent sheaf of graded S -modules. Then there is a canonical isomorphism of sheaves of modules on Z natural in M ϑ : Φ M (M S T ) Z [T, m/s] b/t (m b) /φ (s)t Proof. The morphism φ : S T induces an additive functor S T : GrMod(S ) GrMod(T ) which is left adjoint to the restriction of scalars functor (MRS,Proposition 105). This functor maps quasi-coherent sheaves of graded S -modules to quasi-coherent sheaves of graded T -modules (SOA,Corollary 22),(SOA,Proposition 19), so at least the claim makes sense. Set Z = G(φ), and for affine open X let Φ : G(φ ) P rojs () be induced by the morphism of graded rings φ : S () T (). Let σ be the following isomorphism of sheaves of modules defined in (MPS,Proposition 6) σ : Φ (M () ) (M () S () T ()) G(φ ) It follows from (SOA,Proposition 46) that there is an isomorphism of graded T ()-modules M () S () T () (M S T )() defined by a b a b. Let χ : P rojt () ProjT and ψ : P rojs () ProjS be the canonical open immersions, write Z for χ (G(φ )) and let Φ : Z Imψ be induced by Φ. These morphisms fit into the following diagram ProjT ProjS Z Φ Z Φ χ ψ G(φ ) P rojs () Φ From Proposition 7 we have isomorphisms of sheaves of modules µ : M Imψ (ψ ) M () µ : (M S T ) Z (χ ) (M S T )() G(φ ) 23

### Relative Affine Schemes

Relative Affine Schemes Daniel Murfet October 5, 2006 The fundamental Spec( ) construction associates an affine scheme to any ring. In this note we study the relative version of this construction, which

### Section Higher Direct Images of Sheaves

Section 3.8 - Higher Direct Images of Sheaves Daniel Murfet October 5, 2006 In this note we study the higher direct image functors R i f ( ) and the higher coinverse image functors R i f! ( ) which will

### Section Blowing Up

Section 2.7.1 - Blowing Up Daniel Murfet October 5, 2006 Now we come to the generalised notion of blowing up. In (I, 4) we defined the blowing up of a variety with respect to a point. Now we will define

### Modules over a Scheme

Modules over a Scheme Daniel Murfet October 5, 2006 In these notes we collect various facts about quasi-coherent sheaves on a scheme. Nearly all of the material is trivial or can be found in [Gro60]. These

### Modules over a Ringed Space

Modules over a Ringed Space Daniel Murfet October 5, 2006 In these notes we collect some useful facts about sheaves of modules on a ringed space that are either left as exercises in [Har77] or omitted

### The Proj Construction

The Proj Construction Daniel Murfet May 16, 2006 Contents 1 Basic Properties 1 2 Functorial Properties 2 3 Products 6 4 Linear Morphisms 9 5 Projective Morphisms 9 6 Dimensions of Schemes 11 7 Points of

### Synopsis of material from EGA Chapter II, 4. Proposition (4.1.6). The canonical homomorphism ( ) is surjective [(3.2.4)].

Synopsis of material from EGA Chapter II, 4 4.1. Definition of projective bundles. 4. Projective bundles. Ample sheaves Definition (4.1.1). Let S(E) be the symmetric algebra of a quasi-coherent O Y -module.

### Synopsis of material from EGA Chapter II, 3

Synopsis of material from EGA Chapter II, 3 3. Homogeneous spectrum of a sheaf of graded algebras 3.1. Homogeneous spectrum of a graded quasi-coherent O Y algebra. (3.1.1). Let Y be a prescheme. A sheaf

### Concentrated Schemes

Concentrated Schemes Daniel Murfet July 9, 2006 The original reference for quasi-compact and quasi-separated morphisms is EGA IV.1. Definition 1. A morphism of schemes f : X Y is quasi-compact if there

### The Segre Embedding. Daniel Murfet May 16, 2006

The Segre Embedding Daniel Murfet May 16, 2006 Throughout this note all rings are commutative, and A is a fixed ring. If S, T are graded A-algebras then the tensor product S A T becomes a graded A-algebra

### Schemes via Noncommutative Localisation

Schemes via Noncommutative Localisation Daniel Murfet September 18, 2005 In this note we give an exposition of the well-known results of Gabriel, which show how to define affine schemes in terms of the

### Section Projective Morphisms

Section 2.7 - Projective Morphisms Daniel Murfet October 5, 2006 In this section we gather together several topics concerned with morphisms of a given scheme to projective space. We will show how a morphism

### Algebraic Geometry Spring 2009

MIT OpenCourseWare http://ocw.mit.edu 18.726 Algebraic Geometry Spring 2009 For information about citing these materials or our Terms of Use, visit: http://ocw.mit.edu/terms. 18.726: Algebraic Geometry

### THE GROTHENDIECK GROUP OF A QUANTUM PROJECTIVE SPACE BUNDLE

THE GROTHENDIECK GROUP OF A QUANTUM PROJECTIVE SPACE BUNDLE IZURU MORI AND S. PAUL SMITH Abstract. We compute the Grothendieck group of non-commutative analogues of projective space bundles. Our results

### Chern classes à la Grothendieck

Chern classes à la Grothendieck Theo Raedschelders October 16, 2014 Abstract In this note we introduce Chern classes based on Grothendieck s 1958 paper [4]. His approach is completely formal and he deduces

### Lecture 9: Sheaves. February 11, 2018

Lecture 9: Sheaves February 11, 2018 Recall that a category X is a topos if there exists an equivalence X Shv(C), where C is a small category (which can be assumed to admit finite limits) equipped with

### SERRE FINITENESS AND SERRE VANISHING FOR NON-COMMUTATIVE P 1 -BUNDLES ADAM NYMAN

SERRE FINITENESS AND SERRE VANISHING FOR NON-COMMUTATIVE P 1 -BUNDLES ADAM NYMAN Abstract. Suppose X is a smooth projective scheme of finite type over a field K, E is a locally free O X -bimodule of rank

### Synopsis of material from EGA Chapter II, 5

Synopsis of material from EGA Chapter II, 5 5. Quasi-affine, quasi-projective, proper and projective morphisms 5.1. Quasi-affine morphisms. Definition (5.1.1). A scheme is quasi-affine if it is isomorphic

### Algebraic Geometry Spring 2009

MIT OpenCourseWare http://ocw.mit.edu 18.726 Algebraic Geometry Spring 2009 For information about citing these materials or our Terms of Use, visit: http://ocw.mit.edu/terms. 18.726: Algebraic Geometry

### Direct Limits. Mathematics 683, Fall 2013

Direct Limits Mathematics 683, Fall 2013 In this note we define direct limits and prove their basic properties. This notion is important in various places in algebra. In particular, in algebraic geometry

### Derived Algebraic Geometry IX: Closed Immersions

Derived Algebraic Geometry I: Closed Immersions November 5, 2011 Contents 1 Unramified Pregeometries and Closed Immersions 4 2 Resolutions of T-Structures 7 3 The Proof of Proposition 1.0.10 14 4 Closed

### arxiv: v1 [math.ag] 24 Sep 2018

PURITY IN CATEGORIES OF SHEAVES MIKE PREST AND ALEXANDER SLÁVIK arxiv:1809.08981v1 [math.ag] 24 Sep 2018 Abstract. We consider categorical and geometric purity for sheaves of modules over a scheme satisfying

### Basic Modern Algebraic Geometry

Version of December 13, 1999. Final version for M 321. Audun Holme Basic Modern Algebraic Geometry Introduction to Grothendieck s Theory of Schemes Basic Modern Algebraic Geometry Introduction to Grothendieck

### Derived Categories Of Quasi-coherent Sheaves

Derived Categories Of Quasi-coherent Sheaves Daniel Murfet October 5, 2006 In this note we give a careful exposition of the basic properties of derived categories of quasicoherent sheaves on a scheme.

### ALGEBRAIC GEOMETRY: GLOSSARY AND EXAMPLES

ALGEBRAIC GEOMETRY: GLOSSARY AND EXAMPLES HONGHAO GAO FEBRUARY 7, 2014 Quasi-coherent and coherent sheaves Let X Spec k be a scheme. A presheaf over X is a contravariant functor from the category of open

### Derived Categories Of Sheaves

Derived Categories Of Sheaves Daniel Murfet October 5, 2006 We give a standard exposition of the elementary properties of derived categories of sheaves on a ringed space. This includes the derived direct

### FORMAL GLUEING OF MODULE CATEGORIES

FORMAL GLUEING OF MODULE CATEGORIES BHARGAV BHATT Fix a noetherian scheme X, and a closed subscheme Z with complement U. Our goal is to explain a result of Artin that describes how coherent sheaves on

### Scheme theoretic vector bundles

Scheme theoretic vector bundles The best reference for this material is the first chapter of [Gro61]. What can be found below is a less complete treatment of the same material. 1. Introduction Let s start

### PART II.1. IND-COHERENT SHEAVES ON SCHEMES

PART II.1. IND-COHERENT SHEAVES ON SCHEMES Contents Introduction 1 1. Ind-coherent sheaves on a scheme 2 1.1. Definition of the category 2 1.2. t-structure 3 2. The direct image functor 4 2.1. Direct image

### Introduction and preliminaries Wouter Zomervrucht, Februari 26, 2014

Introduction and preliminaries Wouter Zomervrucht, Februari 26, 204. Introduction Theorem. Serre duality). Let k be a field, X a smooth projective scheme over k of relative dimension n, and F a locally

### 1 Replete topoi. X = Shv proét (X) X is locally weakly contractible (next lecture) X is replete. D(X ) is left complete. K D(X ) we have R lim

Reference: [BS] Bhatt, Scholze, The pro-étale topology for schemes In this lecture we consider replete topoi This is a nice class of topoi that include the pro-étale topos, and whose derived categories

### SOME OPERATIONS ON SHEAVES

SOME OPERATIONS ON SHEAVES R. VIRK Contents 1. Pushforward 1 2. Pullback 3 3. The adjunction (f 1, f ) 4 4. Support of a sheaf 5 5. Extension by zero 5 6. The adjunction (j!, j ) 6 7. Sections with support

### COARSENINGS, INJECTIVES AND HOM FUNCTORS

COARSENINGS, INJECTIVES AND HOM FUNCTORS FRED ROHRER It is characterized when coarsening functors between categories of graded modules preserve injectivity of objects, and when they commute with graded

### Exercises of the Algebraic Geometry course held by Prof. Ugo Bruzzo. Alex Massarenti

Exercises of the Algebraic Geometry course held by Prof. Ugo Bruzzo Alex Massarenti SISSA, VIA BONOMEA 265, 34136 TRIESTE, ITALY E-mail address: alex.massarenti@sissa.it These notes collect a series of

### IndCoh Seminar: Ind-coherent sheaves I

IndCoh Seminar: Ind-coherent sheaves I Justin Campbell March 11, 2016 1 Finiteness conditions 1.1 Fix a cocomplete category C (as usual category means -category ). This section contains a discussion of

### DERIVED CATEGORIES OF COHERENT SHEAVES

DERIVED CATEGORIES OF COHERENT SHEAVES OLIVER E. ANDERSON Abstract. We give an overview of derived categories of coherent sheaves. [Huy06]. Our main reference is 1. For the participants without bacground

### Formal power series rings, inverse limits, and I-adic completions of rings

Formal power series rings, inverse limits, and I-adic completions of rings Formal semigroup rings and formal power series rings We next want to explore the notion of a (formal) power series ring in finitely

### Math 248B. Applications of base change for coherent cohomology

Math 248B. Applications of base change for coherent cohomology 1. Motivation Recall the following fundamental general theorem, the so-called cohomology and base change theorem: Theorem 1.1 (Grothendieck).

### SUMMER COURSE IN MOTIVIC HOMOTOPY THEORY

SUMMER COURSE IN MOTIVIC HOMOTOPY THEORY MARC LEVINE Contents 0. Introduction 1 1. The category of schemes 2 1.1. The spectrum of a commutative ring 2 1.2. Ringed spaces 5 1.3. Schemes 10 1.4. Schemes

### Preliminary Exam Topics Sarah Mayes

Preliminary Exam Topics Sarah Mayes 1. Sheaves Definition of a sheaf Definition of stalks of a sheaf Definition and universal property of sheaf associated to a presheaf [Hartshorne, II.1.2] Definition

### Algebraic Geometry Spring 2009

MIT OpenCourseWare http://ocw.mit.edu 18.726 Algebraic Geometry Spring 2009 For information about citing these materials or our Terms of Use, visit: http://ocw.mit.edu/terms. 18.726: Algebraic Geometry

### MATH 8253 ALGEBRAIC GEOMETRY WEEK 12

MATH 8253 ALGEBRAIC GEOMETRY WEEK 2 CİHAN BAHRAN 3.2.. Let Y be a Noetherian scheme. Show that any Y -scheme X of finite type is Noetherian. Moreover, if Y is of finite dimension, then so is X. Write f

### 1 Notations and Statement of the Main Results

An introduction to algebraic fundamental groups 1 Notations and Statement of the Main Results Throughout the talk, all schemes are locally Noetherian. All maps are of locally finite type. There two main

### THE SMOOTH BASE CHANGE THEOREM

THE SMOOTH BASE CHANGE THEOREM AARON LANDESMAN CONTENTS 1. Introduction 2 1.1. Statement of the smooth base change theorem 2 1.2. Topological smooth base change 4 1.3. A useful case of smooth base change

### INTRO TO TENSOR PRODUCTS MATH 250B

INTRO TO TENSOR PRODUCTS MATH 250B ADAM TOPAZ 1. Definition of the Tensor Product Throughout this note, A will denote a commutative ring. Let M, N be two A-modules. For a third A-module Z, consider the

### Sheaves of Modules. Chi-yu Cheng. This is a supplement for the study of Hartshorne s Algebraic Geometry

Sheaves of Modules Chi-yu Cheng This is a supplement for the study of Hartshorne s Algebraic Geometry Currently a graduate student at the University of Washington 1 1 Preliminary Observations Observation

### Motivic integration on Artin n-stacks

Motivic integration on Artin n-stacks Chetan Balwe Nov 13,2009 1 / 48 Prestacks (This treatment of stacks is due to B. Toën and G. Vezzosi.) Let S be a fixed base scheme. Let (Aff /S) be the category of

### which is a group homomorphism, such that if W V U, then

4. Sheaves Definition 4.1. Let X be a topological space. A presheaf of groups F on X is a a function which assigns to every open set U X a group F(U) and to every inclusion V U a restriction map, ρ UV

### RELATIVE AMPLENESS IN RIGID-ANALYTIC GEOMETRY

RELATIVE AMPLENESS IN RIGID-ANALYTIC GEOMETRY BRIAN CONRAD 1. Introduction 1.1. Motivation. The aim of this paper is to develop a rigid-analytic theory of relative ampleness for line bundles, and to record

### We can choose generators of this k-algebra: s i H 0 (X, L r i. H 0 (X, L mr )

MODULI PROBLEMS AND GEOMETRIC INVARIANT THEORY 43 5.3. Linearisations. An abstract projective scheme X does not come with a pre-specified embedding in a projective space. However, an ample line bundle

### FOUNDATIONS OF ALGEBRAIC GEOMETRY CLASS 24

FOUNDATIONS OF ALGEBRAIC GEOMETRY CLASS 24 RAVI VAKIL CONTENTS 1. Vector bundles and locally free sheaves 1 2. Toward quasicoherent sheaves: the distinguished affine base 5 Quasicoherent and coherent sheaves

### SMA. Grothendieck topologies and schemes

SMA Grothendieck topologies and schemes Rafael GUGLIELMETTI Semester project Supervised by Prof. Eva BAYER FLUCKIGER Assistant: Valéry MAHÉ April 27, 2012 2 CONTENTS 3 Contents 1 Prerequisites 5 1.1 Fibred

### Introduction to Chiral Algebras

Introduction to Chiral Algebras Nick Rozenblyum Our goal will be to prove the fact that the algebra End(V ac) is commutative. The proof itself will be very easy - a version of the Eckmann Hilton argument

### The Picard Scheme and the Dual Abelian Variety

The Picard Scheme and the Dual Abelian Variety Gabriel Dorfsman-Hopkins May 3, 2015 Contents 1 Introduction 2 1.1 Representable Functors and their Applications to Moduli Problems............... 2 1.2 Conditions

### FOUNDATIONS OF ALGEBRAIC GEOMETRY CLASS 24

FOUNDATIONS OF ALGEBRAIC GEOMETR CLASS 24 RAVI VAKIL CONTENTS 1. Normalization, continued 1 2. Sheaf Spec 3 3. Sheaf Proj 4 Last day: Fibers of morphisms. Properties preserved by base change: open immersions,

### CATEGORICAL GROTHENDIECK RINGS AND PICARD GROUPS. Contents. 1. The ring K(R) and the group Pic(R)

CATEGORICAL GROTHENDIECK RINGS AND PICARD GROUPS J. P. MAY Contents 1. The ring K(R) and the group Pic(R) 1 2. Symmetric monoidal categories, K(C), and Pic(C) 2 3. The unit endomorphism ring R(C ) 5 4.

### CHAPTER 1. Étale cohomology

CHAPTER 1 Étale cohomology This chapter summarizes the theory of the étale topology on schemes, culminating in the results on l-adic cohomology that are needed in the construction of Galois representations

### Cohomology and Base Change

Cohomology and Base Change Let A and B be abelian categories and T : A B and additive functor. We say T is half-exact if whenever 0 M M M 0 is an exact sequence of A-modules, the sequence T (M ) T (M)

### Section Divisors

Section 2.6 - Divisors Daniel Murfet October 5, 2006 Contents 1 Weil Divisors 1 2 Divisors on Curves 9 3 Cartier Divisors 13 4 Invertible Sheaves 17 5 Examples 23 1 Weil Divisors Definition 1. We say a

### Université Paris Sud XI Orsay THE LOCAL FLATTENING THEOREM. Master 2 Memoire by Christopher M. Evans. Advisor: Prof. Joël Merker

Université Paris Sud XI Orsay THE LOCAL FLATTENING THEOREM Master 2 Memoire by Christopher M. Evans Advisor: Prof. Joël Merker August 2013 Table of Contents Introduction iii Chapter 1 Complex Spaces 1

### Non characteristic finiteness theorems in crystalline cohomology

Non characteristic finiteness theorems in crystalline cohomology 1 Non characteristic finiteness theorems in crystalline cohomology Pierre Berthelot Université de Rennes 1 I.H.É.S., September 23, 2015

### Algebra Qualifying Exam Solutions January 18, 2008 Nick Gurski 0 A B C 0

1. Show that if B, C are flat and Algebra Qualifying Exam Solutions January 18, 2008 Nick Gurski 0 A B C 0 is exact, then A is flat as well. Show that the same holds for projectivity, but not for injectivity.

### Basic results on Grothendieck Duality

Basic results on Grothendieck Duality Joseph Lipman 1 Purdue University Department of Mathematics lipman@math.purdue.edu http://www.math.purdue.edu/ lipman November 2007 1 Supported in part by NSA Grant

### Lecture 9 - Faithfully Flat Descent

Lecture 9 - Faithfully Flat Descent October 15, 2014 1 Descent of morphisms In this lecture we study the concept of faithfully flat descent, which is the notion that to obtain an object on a scheme X,

### 43 Projective modules

43 Projective modules 43.1 Note. If F is a free R-module and P F is a submodule then P need not be free even if P is a direct summand of F. Take e.g. R = Z/6Z. Notice that Z/2Z and Z/3Z are Z/6Z-modules

### HARTSHORNE EXERCISES

HARTSHORNE EXERCISES J. WARNER Hartshorne, Exercise I.5.6. Blowing Up Curve Singularities (a) Let Y be the cusp x 3 = y 2 + x 4 + y 4 or the node xy = x 6 + y 6. Show that the curve Ỹ obtained by blowing

### 1. Algebraic vector bundles. Affine Varieties

0. Brief overview Cycles and bundles are intrinsic invariants of algebraic varieties Close connections going back to Grothendieck Work with quasi-projective varieties over a field k Affine Varieties 1.

### ALGEBRAIC K-THEORY HANDOUT 5: K 0 OF SCHEMES, THE LOCALIZATION SEQUENCE FOR G 0.

ALGEBRAIC K-THEORY HANDOUT 5: K 0 OF SCHEMES, THE LOCALIZATION SEQUENCE FOR G 0. ANDREW SALCH During the last lecture, we found that it is natural (even just for doing undergraduatelevel complex analysis!)

### FOUNDATIONS OF ALGEBRAIC GEOMETRY CLASS 2

FOUNDATIONS OF ALGEBRAIC GEOMETRY CLASS 2 RAVI VAKIL CONTENTS 1. Where we were 1 2. Yoneda s lemma 2 3. Limits and colimits 6 4. Adjoints 8 First, some bureaucratic details. We will move to 380-F for Monday

### CHAPTER 1. AFFINE ALGEBRAIC VARIETIES

CHAPTER 1. AFFINE ALGEBRAIC VARIETIES During this first part of the course, we will establish a correspondence between various geometric notions and algebraic ones. Some references for this part of the

### Lecture 2 Sheaves and Functors

Lecture 2 Sheaves and Functors In this lecture we will introduce the basic concept of sheaf and we also will recall some of category theory. 1 Sheaves and locally ringed spaces The definition of sheaf

### What is an ind-coherent sheaf?

What is an ind-coherent sheaf? Harrison Chen March 8, 2018 0.1 Introduction All algebras in this note will be considered over a field k of characteristic zero (an assumption made in [Ga:IC]), so that we

### Some remarks on Frobenius and Lefschetz in étale cohomology

Some remarks on obenius and Lefschetz in étale cohomology Gabriel Chênevert January 5, 2004 In this lecture I will discuss some more or less related issues revolving around the main idea relating (étale)

### QUOT SCHEMES SIDDHARTH VENKATESH

QUOT SCHEMES SIDDHARTH VENKATESH Abstract. These are notes for a talk on Quot schemes given in the Spring 2016 MIT-NEU graduate seminar on the moduli of sheaves on K3-surfaces. In this talk, I present

### MATH 101A: ALGEBRA I PART C: TENSOR PRODUCT AND MULTILINEAR ALGEBRA. This is the title page for the notes on tensor products and multilinear algebra.

MATH 101A: ALGEBRA I PART C: TENSOR PRODUCT AND MULTILINEAR ALGEBRA This is the title page for the notes on tensor products and multilinear algebra. Contents 1. Bilinear forms and quadratic forms 1 1.1.

### Construction of M B, M Dol, M DR

Construction of M B, M Dol, M DR Hendrik Orem Talbot Workshop, Spring 2011 Contents 1 Some Moduli Space Theory 1 1.1 Moduli of Sheaves: Semistability and Boundedness.............. 1 1.2 Geometric Invariant

### Algebraic varieties and schemes over any scheme. Non singular varieties

Algebraic varieties and schemes over any scheme. Non singular varieties Trang June 16, 2010 1 Lecture 1 Let k be a field and k[x 1,..., x n ] the polynomial ring with coefficients in k. Then we have two

### Elementary (ha-ha) Aspects of Topos Theory

Elementary (ha-ha) Aspects of Topos Theory Matt Booth June 3, 2016 Contents 1 Sheaves on topological spaces 1 1.1 Presheaves on spaces......................... 1 1.2 Digression on pointless topology..................

### REFLEXIVITY AND RIGIDITY FOR COMPLEXES, II: SCHEMES

REFLEXIVITY AND RIGIDITY FOR COMPLEXES, II: SCHEMES LUCHEZAR L. AVRAMOV, SRIKANTH B. IYENGAR, AND JOSEPH LIPMAN Abstract. We prove basic facts about reflexivity in derived categories over noetherian schemes;

### Representable presheaves

Representable presheaves March 15, 2017 A presheaf on a category C is a contravariant functor F on C. In particular, for any object X Ob(C) we have the presheaf (of sets) represented by X, that is Hom

### Duality, Residues, Fundamental class

Duality, Residues, Fundamental class Joseph Lipman Purdue University Department of Mathematics lipman@math.purdue.edu May 22, 2011 Joseph Lipman (Purdue University) Duality, Residues, Fundamental class

### D-MATH Algebraic Geometry FS 2018 Prof. Emmanuel Kowalski. Solutions Sheet 1. Classical Varieties

D-MATH Algebraic Geometry FS 2018 Prof. Emmanuel Kowalski Solutions Sheet 1 Classical Varieties Let K be an algebraically closed field. All algebraic sets below are defined over K, unless specified otherwise.

### COMMUTATIVE ALGEBRA LECTURE 1: SOME CATEGORY THEORY

COMMUTATIVE ALGEBRA LECTURE 1: SOME CATEGORY THEORY VIVEK SHENDE A ring is a set R with two binary operations, an addition + and a multiplication. Always there should be an identity 0 for addition, an

### where Σ is a finite discrete Gal(K sep /K)-set unramified along U and F s is a finite Gal(k(s) sep /k(s))-subset

Classification of quasi-finite étale separated schemes As we saw in lecture, Zariski s Main Theorem provides a very visual picture of quasi-finite étale separated schemes X over a henselian local ring

### Factorization of birational maps for qe schemes in characteristic 0

Factorization of birational maps for qe schemes in characteristic 0 AMS special session on Algebraic Geometry joint work with M. Temkin (Hebrew University) Dan Abramovich Brown University October 24, 2014

### Algebraic Geometry Spring 2009

MIT OpenCourseWare http://ocw.mit.edu 18.726 Algebraic Geometry Spring 2009 For information about citing these materials or our Terms of Use, visit: http://ocw.mit.edu/terms. 18.726: Algebraic Geometry

### Exercises from Vakil

Exercises from Vakil Ian Coley February 6, 2015 2 Sheaves 2.1 Motivating example: The sheaf of differentiable functions Exercise 2.1.A. Show that the only maximal ideal of the germ of differentiable functions

### MATH 101B: ALGEBRA II PART A: HOMOLOGICAL ALGEBRA

MATH 101B: ALGEBRA II PART A: HOMOLOGICAL ALGEBRA These are notes for our first unit on the algebraic side of homological algebra. While this is the last topic (Chap XX) in the book, it makes sense to

### Weil-étale Cohomology

Weil-étale Cohomology Igor Minevich March 13, 2012 Abstract We will be talking about a subject, almost no part of which is yet completely defined. I will introduce the Weil group, Grothendieck topologies

### SCHEMES. David Harari. Tsinghua, February-March 2005

SCHEMES David Harari Tsinghua, February-March 2005 Contents 1. Basic notions on schemes 2 1.1. First definitions and examples.................. 2 1.2. Morphisms of schemes : first properties.............

### PERVERSE SHEAVES. Contents

PERVERSE SHEAVES SIDDHARTH VENKATESH Abstract. These are notes for a talk given in the MIT Graduate Seminar on D-modules and Perverse Sheaves in Fall 2015. In this talk, I define perverse sheaves on a

### THE DERIVED CATEGORY OF A GRADED GORENSTEIN RING

THE DERIVED CATEGORY OF A GRADED GORENSTEIN RING JESSE BURKE AND GREG STEVENSON Abstract. We give an exposition and generalization of Orlov s theorem on graded Gorenstein rings. We show the theorem holds

### A course in. Algebraic Geometry. Taught by C. Birkar. Michaelmas 2012

A course in Algebraic Geometry Taught by C. Birkar Michaelmas 2012 Last updated: May 30, 2013 1 Disclaimer These are my notes from Caucher Birkar s Part III course on algebraic geometry, given at Cambridge

### NOTES ON ALGEBRAIC GEOMETRY MATH 202A. Contents Introduction Affine varieties 22

NOTES ON ALGEBRAIC GEOMETRY MATH 202A KIYOSHI IGUSA BRANDEIS UNIVERSITY Contents Introduction 1 1. Affine varieties 2 1.1. Weak Nullstellensatz 2 1.2. Noether s normalization theorem 2 1.3. Nullstellensatz

### Curves on P 1 P 1. Peter Bruin 16 November 2005

Curves on P 1 P 1 Peter Bruin 16 November 2005 1. Introduction One of the exercises in last semester s Algebraic Geometry course went as follows: Exercise. Let be a field and Z = P 1 P 1. Show that the

### VECTOR BUNDLES ON THE PROJECTIVE LINE AND FINITE DOMINATION OF CHAIN COMPLEXES

VECTOR BUNDLES ON THE PROJECTIVE LINE AND FINITE DOMINATION OF CHAIN COMPLEXES THOMAS HÜTTEMANN Abstract. We present an algebro-geometric approach to a theorem on finite domination of chain complexes over

### Matsumura: Commutative Algebra Part 2

Matsumura: Commutative Algebra Part 2 Daniel Murfet October 5, 2006 This note closely follows Matsumura s book [Mat80] on commutative algebra. Proofs are the ones given there, sometimes with slightly more

### 3. The Sheaf of Regular Functions

24 Andreas Gathmann 3. The Sheaf of Regular Functions After having defined affine varieties, our next goal must be to say what kind of maps between them we want to consider as morphisms, i. e. as nice