The Segre Embedding. Daniel Murfet May 16, 2006

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1 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 in a canonical way with the grading given by (TES,Lemma 13). That is, S A T is the coproduct of the morphisms of A-modules S d A T e S A T for d, e 0. The canonical morphisms p 1 : S S A T, p 2 : T S A T are then morphisms of graded A-algebras. Definition 1. Let S, T be graded A-algebras. We define their cartesian product, denoted S A T, to be the following graded A-algebra: as an A-module it is the sum of the images of the A-module morphisms S d A T d S A T for all d 0. This is an A-subalgebra of S A T which is a graded A-algebra with grading (S A T ) d = Sd A T d for d 0. The scheme P roj(s A T ) is covered by open subsets D + (f g) for f S, g T homogenous of the same degree d > 0. It is not hard to check that the following are well-defined morphisms of A-algebras ϕ f,g : S (f) (S A T ) (f g) ψ f,g : T (g) (S A T ) (f g) s/f n (s g n )/(f g) n t/g n (f n t)/(f g) n If h S, k T are homogenous of the same degree e > 0 then it is readily checked that the following diagram commutes (the vertical morphisms are the canonical ring morphisms) ϕ f,g S (f) (S A T ) (f g) ψ f,g T (g) S (fh) ϕ fh,gk (S A T ) (fh gk) T (gk) ψ fh,gk Therefore the morphisms Spec(ϕ f,g ) and Spec(ψ f,g ) glue to give morphisms of schemes over A Φ : P roj(s A T ) P rojs, Ψ : P roj(s A T ) P rojt Here Φ is the unique morphism of schemes making the left square in the following diagram commute for every pair of homogenous elements of the same positive degree f S, g T, and similarly for Ψ and the right square P rojs Φ Ψ P roj(s A T ) P rojt SpecS (f) Spec(ϕ f,g ) Spec((S A T ) (f g) ) Spec(ψ f,g ) SpecT (g) 1

2 Lemma 1. Let S be a graded ring and f S d for some d > 0. If h S e for e > 0 then the isomorphism D + (f) = SpecS (f) identifies the open subsets D + (fh) and D(h d /f e ). P rojs D + (f) SpecS (f) D +(fh) ( ) h d D f e Proof. If p P rojs is a homogenous prime ideal with f / p then the image of p in SpecS (f) is the prime ideal ps f S (f). It is clear that this prime belongs to D(h d /f e ) if and only if h d / p, so if and only if p D + (fh) = D + (f) D + (h). Lemma 2. Let S, T be graded A-algebras and Φ, Ψ as above. If f S, g T are homogenous of degree d > 0 then Φ 1 D + (f) Ψ 1 D + (g) = D + (f g). Proof. The inclusion is obvious, since by construction we have Φ(D + (f g)) D + (f) and Ψ(D + (f g)) D + (g). For the reverse inclusion let p be a homogenous prime of S A T with Φ(p) D + (f) and Ψ(p) D + (g). There exists homogenous h S, k T of the same degree e > 0 such that p D + (h k). Using Lemma 1 and the definition of the morphisms ϕ h,k, ψ h,k we see that h d g e / p and f e k d / p. Therefore (f e g e )(h d k d ) = f e h d g e k d / p and hence f g / p, as required. Proposition 3. Let S, T be graded A-algebras, and suppose that S is generated by S 1 as an S 0 - algebra and that T is generated by T 1 as a T 0 -algebra. Then P roj(s A T ) = P rojs A P rojt, so we have a pullback diagram P roj(s A T ) Ψ P rojt Φ P rojs SpecA Proof. By the hypotheses on S, T the open sets of the form D + (f), D + (g) for f S 1, g T 1 give open covers of P rojs and P rojt respectively. By the local nature of products and Lemma 2 it is enough to show that D + (f g) = D + (f) A D + (g) or equivalently Spec((S A T ) (f g) ) = Spec(T (g) ) A Spec(S (f) ), for every pair of homogenous elements f S 1, g T 1. This amounts to showing that the following diagram is a pushout of rings A S (f) (1) T (g) ϕ f,g ψ f,g (S A T ) (f g) We show (1) is a pushout by showing that the ring morphism S (f) A T (g) (S A T ) (f g) defined by s/f n t/g m (sf m g n t)/(f g) n+m is an isomorphism of rings. The proof is motivated by the technique used in (TPC,Proposition 15). Consider the following well-defined A-bilinear map S f T g (S A T ) f g (s/f n, t/g m ) (sf m tg n )/(f g) n+m 2

3 This induces a morphism of A-algebras S f A T g (S A T ) f g. The canonical maps S S f, T T g are morphisms of A-algebras, so we have a morphism of A-algebras S A T S f A T g defined by s t s/1 t/1. This sends f g to a unit, so there is an induced morphism of A-algebras (S A T ) f g S f A T g given by (s t)/(f g) n s/f n t/g n. Since we have already constructed the inverse, this is an isomorphism of A-algebras. The rings S f, T g are -graded, and are therefore graded A-modules. Hence S f A T g is a graded A-module and therefore also a -graded ring. It is not hard to check that (S A T ) f g = S f A T g is an isomorphism of -graded rings, so it induces an isomorphism of degree zero subrings (S A T ) (f g) = (Sf A T g ) 0. The injective morphism of A-algebras S A T S A T, which certainly does not preserve grade, nonetheless localises and restricts to give an injective ring morphism (S A T ) (f g) (S A T ) (f g) = (Sf A T g ) 0 defined by (s t)/(f g) n s/f n t/g n for s S n, t T n. Let α : S f T g S f A T g be the canonical morphism of groups (GRM,Section 6). The kernel of α is the abelian group P generated by elements (a x) y x (a y) where x, y are homogenous. The morphism S (f) T (g) S f T g is injective since S (f), T (g) are direct summands of S f, T g respectively, and tensor products preserve colimits. Therefore the group S (f) T (g) is isomorphic to its image in S f T g, which is mapped by α onto the image of (S A T ) (f g) in (S f A T g ) 0. So there is an isomorphism of abelian groups (S (f) T (g) )/P = (S A T ) (f g) where P = P (S (f) T (g) ). We can write S f T g as the following direct sum S f T g = (S f ) p (T g ) q p,q Therefore it is not hard to see that P is generated as an abelian group by elements (a x) y x (a y) where x, y are homogenous of degree zero, that is, x S (f), y T (g). Hence there is an isomorphism of abelian groups S (f) A T (g) = (S(f) T (g) )/P = (S A T ) (f g) s/f n t/g m (sf m g n t)/(f g) n+m This shows that (1) is a pushout, and completes the proof. Lemma 4. Let S, T be graded A-algebras. If S is generated by elements {s i } i I S 1 as an S 0 -algebra and T is generated by {t j } j J T 1 as a T 0 -algebra, then S A T is generated by {s i t j } (S A T ) 1 as an (S A T ) 0 -algebra. Corollary 5. Let A be a ring and fix integers m, n 1. There is a canonical closed immersion P m A A P n A of schemes over A, called the Segre embedding. A Proof. The pullback we have in mind is P roj(a[x 0,..., x m ] A A[y 0,..., y n ]) = P m A A P n A. Consider the following morphism of graded A-algebras γ : A[{z ij } 0 i m,0 j n ] A[x 0,..., x m ] A A[y 0,..., y n ] z ij x i y j which is surjective since the latter ring is generated as an A-algebra by the elements x i y j. Therefore the morphism of A-schemes induced by γ is the desired closed immersion. Proposition 6. Let be a scheme and fix integers m, n 1. There is a canonical closed immersion P m P n of schemes over, called the Segre embedding. Proof. When we say canonical we mean that once you select specific pullbacks P m, Pn, Pm the definition of the closed immersion involves no arbitrary choices. Consider P n and Pmn+m+n 3

4 the following commutative diagram P n P m α P n Pm P n P n P m P m Spec where α is induced into the bottom pullback to make the diagram commute. Using standard properties of pullbacks, we see that every face of this cube is a pullback. That is, P n P m = P m P m (P n P m ) P n P m = P n P n (P n P m ) Therefore there is a unique morphism of schemes over, P n P m following diagram commute P n P m P mn+m+n making the (2) α P n Pm P mn+m+n where the bottom morphism is the closed immersion of Corollary 5. Once again using standard properties of pullbacks we see that (2) is a pullback, and therefore the top morphism is a closed immersion, which completes the proof. Corollary 7. Let be a scheme and fix integers m, n 1. There is a canonical closed immersion P mn+m+n of schemes over. P n P m Proof. By definition we have P n P m = Pm P n = P m P n Let α : P m Pn be the Segre embedding. Then the morphism 1 α : P n P m = (Pm P n ) P mn+m+n is the desired closed immersion (LocP,Proposition 1). = P mn+m+n 4

5 Proposition 8. The composition of projective morphisms is projective. Proof. Let f : Y and g : Y be projective morphisms, so that we have integers m, n 1 and a commutative diagram f P m Y f Y g P n g where f, g are closed immersions. The morphism P m g : Pm Y Pm P of (TPC,Section 5) is a closed n immersion, and using Corollary 7 we have a commutative diagram with the top morphism also a closed immersion P m P n P mn+m+n P m g P m Y P n f g Y This shows that gf is projective and completes the proof. f Lemma 9. Projective morphisms are stable under pullback. That is, if f : Y is projective and there is a pullback diagram g f Y f Y then f is projective. Proof. This follows immediately from the construction of the morphisms P n f and the fact that closed immersions are stable under pullback. in (TPC,Section 5) Proposition 10. If f : Y and g : Y are quasi-projective with Y noetherian, then g f is quasi-projective. Proof. Use the proof of Proposition 8, except now f, g are immersions and we use the stability of immersions under pullback (SI,Lemma 15) and composition (SI,Lemma 16). We need Y noetherian so that P m Y is noetherian, which is the technical condition of (SI,Lemma 16). Proposition 11. Let P be a property of morphisms of schemes such that (a) a closed immersion has P. (b) the composition of two morphisms having P has P. (c) P is stable under base extension. Then the following holds (d) the product of two morphisms having P has P. (e) if f : Y and g : Y are two morphisms, and if g f has P and g is separated, then f has P. 5

6 Proof. (d) Let f : Y and f : Y be morphisms of schemes over a scheme S and form the following diagram S Y (Y S Y f S f ) Y (Y S Y ) Y S Y Y f f Y S Using (b) and (c) it is easy to check that f S f has P. For (e) we consider, Y as schemes over and f as a morphism of -schemes. Since g is separated over the graph morphism Γ f : Y is a closed immersion. Therefore Γ f has P and using the definition of the graph morphism and (b), (c) we see that f also has P. Corollary 12. We have the following properties of projective morphisms (a) a closed immersion is projective. (b) the composition of two projective morphisms is projective. (c) projective morphisms are stable under base extension. (d) the product of projective morphisms is projective. (e) if f : Y and g : Y are two morphisms, and if g f is projective and g is separated, then f is projective. 6

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