# 12. Linear systems Theorem Let X be a scheme over a ring A. (1) If φ: X P n A is an A-morphism then L = φ O P n

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1 12. Linear systems Theorem Let X be a scheme over a ring A. (1) If φ: X P n A is an A-morphism then L = φ O P n A (1) is an invertible sheaf on X, which is generated by the global sections s 0, s 1,..., s n, where s i = φ x i. (2) If L is an invertible sheaf on X, which is generated by the global sections s 0, s 1,..., s n, then there is a unique A-morphism φ: X P n A such that L = φ O P n A (1) and s i = φ x i. Proof. It is clear that L is an invertible sheaf. Since x 0, x 1,..., x n generate the ring A[x 0, x 1,..., x n ], it follows that x 0, x 1,..., x n generate the sheaf O P n A (1). Thus s 0, s 1,..., s n generate L. Hence (1). Now suppose that L is an invertible sheaf generated by s 0, s 1,..., s n. Let X i = { p X s i / m p L p }. Then X i is an open subset of X and the sets X 0, X 1,..., X n cover X. Define a morphism φ i : X i U i, where U i is the standard open subset of P n A. Since U i = Spec A[y 0, y 1,..., y n ], where y j = x j /x i, is affine, it suffices to give a ring homomorphism A[y 0, y 1,..., y n ] Γ(X i, O Xi ). We send y j to s j /s i, and extend by linearity. The ey observation is that the ratio is a well-defined element of O Xi, which does not depend on the choice of isomorphism L Xi O Xi. It is then straightforward to chec that φ i glue and that φ has the given properties. Example Let X = P 1, A =, L = O P 1 (2). In this case, global sections of L are generated by S 2, ST and T 2. This morphism is represented globally by [S : T ] [S 2 : ST : T 2 ]. The image is the conic XZ = Y 2 (provided the characteristic is not 2) inside P 2. More generally one can map P 1 into Pn by the invertible sheaf O P 1(n). More generally still, one can map P m into Pn using the invertible sheaf O P m (1). Corollary Aut(P n ) PGL(n + 1, ). 1

2 Proof. First note that PGL(n+1, ) acts naturally on P n and that this action is faithful. Now suppose that φ Aut(P h ). Let L = φ O P n (1). Since Cl(P n ) Z and O P n (1) generates Cl(P n ), it follows that L O P n (±1). As L is globally generated, we must have L O P n (1). Let s i = φ x i. Then s 0, s 1,..., s n is a basis for the -vector space H 0 (P h, O P n (1)). But then there is a matrix A = (a ij ) GL(n + 1, ) such that s i = ij a ij x j. Since the morphism φ is determined by s 0, s 1,..., s n, it follows that φ is determined by the class of A in GL(n + 1, ). Lemma Let φ: X P n A be an A-morphism. Then φ is a closed immersion if and only if (1) X i = X si is affine, and (2) the natural map of rings A[y 0, y 1,..., y n ] Γ(X i, O Xi ) which sends y i σ i σ j, is surjective. Proof. Suppose that φ is a closed immersion. Then X i is isomorphic to φ(x) U i, a closed subscheme of affine space. Thus X i is affine. Hence (1) and (2) follows as we have surjectivity on all of the localisations. Now suppose that (1) and (2) hold. Then X i is a closed subscheme of U i and so X is a closed subscheme of P n A. Theorem Let X be a projective scheme over an algebraically closed field and let φ: X P n be a morphism over, which is given by an invertible sheaf L and global sections s 0, s 1,..., s n which generate L. Let V Γ(X, L) be the space spanned by the sections Then φ is a closed immersion if and only if (1) V separates points: that is given p and q X there is σ V such that σ m P L p but σ / m q L q. (2) V separates tangent vectors: that is given p X the set spans m p /m 2 p. { σ V σ m p L p }, Proof. Suppose that φ is a closed immersion. Then we might as well consider X P n as a closed subscheme. In this case (1) is clear. Just pic a linear function on the whole of P n which vanishes at p but not at q (equivalently pic a hyperplane which contains p but not q). 2

3 Similarly linear functions on P n separate tangent vectors on the whole of projective space, so they certainly separate on X. Now suppose that (1) and (2) hold. Then φ is clearly injective. Since X is proper over Spec and P n is separated over Spec it follows that φ is proper. In particular φ(x) and φ is a homeomorphism onto φ(x). For the rest, see Hartshorne. Definition Let X be a noetherian scheme. We say that an invertible sheaf L is ample if for every coherent sheaf F there is an integer n 0 > 0 such that F L n is globally generated, for all n n 0. O X Lemma Let L be an invertible sheaf on a noetherian scheme. TFAE (1) L is ample. (2) L m is ample for all m > 0. (3) L m is ample for some m > 0. Proof. (1) implies (2) implies (3) is clear. So assume that M = L m is ample and let F be a coherent sheaf. For each 0 i m 1, let F i = F L i. By assumption there is an integer n i such that F i M n is globally generated for all n n i. Let n 0 be the maximum of the n i. If n (n 0 + 1)m, then we may write n = qm + i, where 0 i m 1 and q n 0 n i. But then F L m = F i M q, which is globally generated. Theorem Let X be a scheme of finite type over a noetherian ring A and let L be an invertible sheaf on X. Then L is ample if and only if L m is very ample for some m > 0. Proof. Suppose that L m is very ample. Then there is an immersion X P r A, for some positive integer r, and Lm = O X (1). Let X be the closure. If F is any coherent sheaf on X then there is a coherent sheaf F on X, such that F = F X. By Serre s result, F() is globally generated for some positive integer. It follows that F() is globally generated, so that L m is ample, and the result follows by (12.7). Conversely, suppose that L is ample. Given p X, pic an open affine neighbourhood U of p so that L U is free. Let Y = X U, give it the reduced induced strucure, with ideal sheaf I. Then I is coherent. Pic n > 0 so that I L n is globally generated. Then we may find s I L n not vanishing at p. We may identify s with s O U and then p U s U, an affine subset of X. 3

4 By compactness, we may cover X by such open affines and we may assume that n is fixed. Replacing L by L n we may assume that n = 1. Then there are global sections s 1, s 2,..., s H 0 (X, L) such that U i = U si is an open affine cover. Since X is of finite type, each B i = H 0 (U i, O Ui ) is a finitely generated A-module. Pic generators b ij. Then s n b ij lifts to s ij H 0 (X, L n ). Again we might as well assume n = 1. Now let P N A be the projective space with coordinates x 1, x 2,..., x and x ij. Locally we can define a map on each U i to the standard open affine, by the obvious rule, and it is standard to chec that this glues to an immersion. Definition Let L be an invertible sheaf on a smooth projective variety over an algebraically closed field. Let s H 0 (X, L). The divisor (s) of zeroes of s is defined as follows. By assumption we may cover X by open subsets U i over which we may identify s with f i O Ui. Note that (12.9) defines a Cartier divisor, which may safely be identified with the corresponding Weil divisor. Lemma Let X be a smooth projective variety over an algebraically closed field. Let D 0 be a divisor and let L = O X (D 0 ). (1) If s H 0 (X, L), s 0 then (s) D 0. (2) If D 0 and D D 0 then there is a global section s H 0 (X, L) such that D = (s). (3) If s i H 0 (X, L), i = 1 and 2, are two global sections then (s 1 ) = (s 2 ) if and only if s 2 = λs 1 where λ. Proof. The section s corresponds to a rational function f, O X (D) K. If D 0 is the Cartier divisor {(U i, f i )} then O X (D) is locally generated by f 1 i so that multiplication by f i induces an isomorphism with O Ui. D is then locally defined by ff i. But then D = D 0 + (f). Hence (1). Now suppose that D > 0 and D = D 0 + (f). Then (f) D 0 and so f O X (D) K and the divisor of zeroes of f is D. This is (2). Now suppose that (s 1 ) = (s 2 ). Then (f 1 ) = (f 2 ) and the rational function f 1 /f 2 has no zeroes nor poles. Since X is a projective variety, f 1 /f 2 = λ, a constant. Definition Let D 0 be a divisor. The complete linear system associated to D 0 is the set D 0 = { D Div(X) D 0, D D 0 }. 4

5 We have seen that D = P(H 0 (X, O X (D 0 ))). Thus D is naturally a projective space. Definition A linear system is any linear subspace of a complete linear system D 0. In other words, a linear system corresponds to a linear subspace, V H 0 (X, O X (D 0 )). We will then write V = { D D 0 D = (s), s V } P(V ) P(H 0 (X, O X (D 0 ))). Definition Let V be a linear system. The base locus of V is the intersection of the elements of V. Lemma Let X be a smooth projective variety over an algebraically closed field, and let V D 0 be a linear system. V generates O X (D 0 ) if and only if V is base point free. Proof. If V generates O X (D 0 ) then for every point x X we may find an element σ V such that σ(x) 0. But then D = (σ) does not contain x, and so the base locus is empty. Conversely suppose that the base locus is empty. The locus where V does not generated O X (D 0 ) is a closed subset Z of X. Pic x Z a closed point. By assumption we may find D V such that x / D. But then if D = (σ), σ(x) 0 and σ generates the stal L x, a contradiction. Thus Z is empty and O X (D 0 ) is globally generated. Example Consider O P 1(4). The complete linear system 4p defines a morphism into P 4, where p = [0 : 1] and q = [1 : 0], given by P 1 P 4, [S : T ] [S 4 : ST 3 : S 2 T 2 : ST 3 : T 4 ]. If we project from [0 : 0 : 1 : 0 : 0] we will get a morphism into P 3, [S : T ] [S 4 : ST 3 : ST 3 : T 4 ]. This corresponds to the sublinear system spanned by 4p, 3p + q, p + 3q, 4q. Consider O P 2(2) and the corresponding complete linear system. The map associated to this linear system is the Veronese embedding P 2 P 5, [X : Y : Z] [X 2 : Y 2 : Z 2 : Y Z : XZ : XY ]. Note also the notion of separating points and tangent directions becomes a little clearer in this more geometric setting. Separating points means that given x and y X, we can find D V such that x D and y / D. Separating tangent vectors means that given any irreducible length two zero dimensional scheme z, with support x, we can find D V such that x D but z is not contained in D. In fact the condition about separating tangent vectors is really the limiting case of separating points. 5

6 Thining in terms of linear systems also presents an inductive approach to proving global generation. Suppose that we consider the complete linear system D. Suppose that we can find Y D. Then the base locus of D is supported on Y. On the other hand suppose that I is the ideal sheaf of Y in X. Then there is an exact sequence 0 I O X O Y 0 As X is smooth D is Cartier and O X (D) is an invertible sheaf. Tensoring by locally free preserves exactness, so there are short exact sequences, 0 I(mD) O X (md) O Y (md) 0. Taing global sections, we get 0 H 0 (X, I(mD) H 0 (X, O X (md) H 0 (Y, O Y (md). At the level of linear systems there is therefore a linear map D D Y. The ey point is that we don t now that either map is surjective (in fact they are either both surjective or neither is surjective). In fact there are examples where the map is not surjective. There are certainly plenty of examples in general. Consider another example. Consider the problem of parametrising subvarieties or subschemes X of projective space P r. Any subscheme is determined by the homogeneous ideal I(X) of polynomials vanishing on X. As in the case of zero dimensional schemes, we would lie to reduce to the data of a vector subspace of fixed dimension in a fixed vector space. The obvious thing to consider is polynomials of degree d and the vector subspace of polynomials of polynomials of degree d vanishing on X. But how large should we tae d to be? The first observation is that if I is the ideal sheaf of X in P r then I d = H 0 (P r, I(d)), where I(d) is the Serre twist. To say that I d determines X, is essentially equivalent to saying that I(d) is globally generated. Fixing some data about X (in the case of zero dimensional schemes this would be the length) we would then lie a positive integer d 0 such that if d d 0 then two things are true: I(d) is globally generated. h 0 (P r, I(d)), the dimension of the space of global sections, is independent of X. 6

7 Now there is a short exact sequence Twisting by d, we get 0 I O P r O X 0. 0 I(d) O P r (d) O X (d) 0. Taing global sections gives another exact sequence. 0 H 0 (P r, I(d)) H 0 (P r, O P r (d)) H 0 (X, O X (d)). Again, it would be really nice if this exact sequence were exact on the right. Then global generation of I(d) would be reduced to global generation of O X (d) and one could read of h 0 (P r, I(d)) from h0 (X, O X (d)). 7

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