Coherent sheaves on elliptic curves.

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1 Coherent sheaves on elliptic curves. Aleksei Pakharev April 5, 2017 Abstract We describe the abelian category of coherent sheaves on an elliptic curve, and construct an action of a central extension of SL 2 (Z) on the derived category. Contents 1 Coherent sheaves on elliptic curve 1 2 (Semi)stable sheaves 2 3 Euler form 4 4 Derived category of coherent sheaves 5 5 SL 2 (Z) action 6 6 Classification of indecomposable sheaves 7 7 Braid group relations 8 1 Coherent sheaves on elliptic curve Definition 1.1. An elliptic curve over a field k is a nonsingular projective algebraic curve of genus 1 over k with a fixed k-rational point. Remark 1.2. If the characteristic of k is neither 2 nor 3, an elliptic curve can be alternatevily defined as the subvariety of P 2 k defined by an equation y2 z = x 3 pxz 2 qz 3, where p, q k, and the polynomial x 3 pxz 2 qz 3 is square-free. In this case, the fixed point is (0 : 1 : 0). Remark 1.3. Over the field of complex numbers, there is even a simpler description. An elliptic curve is precisely a quotient C/Λ of C by a nondegenerate lattice Λ C of rank 2. Remark 1.4. Any elliptic curve carries a structure of a group, with the fixed point being the identity. 1

2 Fix an elliptic curve X over a field k. We do not assume that k is algebraically closed, since the main example is the finite field F q. Recall that a coherent sheaf F on X is a sheaf of modules over O such that for every open affine U X the restriction F U is isomorphic to N for some finitely generated O(U)-module N. Example 1.5. The structure sheaf O is indeed a coherent sheaf. Also, one can consider the ideal sheaf m x = O( x) corresponding to a closed point x X. Then the cokernel of the inclusion O( x) O is the so called skyscrapper sheaf O x, which is coherent as well. Theorem 1.6. Coherent sheaves on X form an abelian category Coh(X). Theorem 1.7 (Global version of Serre theorem). Any coherent sheaf F on a smooth projective variety of dimension n over a field k admits a resolution F n F n 1... F 1 F 0 where each F i is finitely generated and locally free ( vector bundle). Theorem 1.8 (Grothendieck s finiteness theorem). Any coherent sheaf F on a smooth projective variety of dimension n over a field k has finite dimensional cohomologies over k. Corollary 1.9. For any coherent sheaves F and G the space Hom(F, G) has finite dimension over k, since Hom(F, G) = Γ(Hom(F, G), X) = H 0 (Hom(F, G), X). Theorem 1.10 (Grothendieck s vanishing theorem). Any coherent sheaf F on a smooth projective variety of dimension n over a field k has no i-th cohomologies for i > n. Definition An abelian category C is called hereditary if Ext 2 (, ) = 0. Corollary The category Coh(X) is hereditary. 2 (Semi)stable sheaves To a coherent sheaf we can associate two numbers, the Euler characteristic χ(f) and the rank rk(f). Definition 2.1. The Euler characteristic χ(f) is the alternating sum i ( 1)i dim k H i (F, X). In our case, it is equal to dim k H 0 (F, X) dim k H 1 (F, X). Definition 2.2. The rank rk(f) is the dimension of the stalk F ξ of F at a generic point ξ of X over the residue field. It is independent of ξ. Example 2.3. We have χ(o) = 0, rk(o) = 1, χ(o x ) = 1, rk(o x ) = 0. Proposition 2.4. Given a short exact sequence 0 F F F 0, we have χ(f) = χ(f ) + χ(f ) and rk(f) = rk(f ) + rk(f ). Definition 2.5. The slope µ(f) of a nontrivial coherent sheaf F is the quotient χ(f)/ rk(f). In the case rk(f) = 0 we set µ(f) =. Lemma 2.6. Given a short exact sequence 0 F F F 0, we have three options: 2

3 µ(f ) < µ(f) < µ(f ); µ(f ) = µ(f) = µ(f ); µ(f ) > µ(f) > µ(f ). Proof. We have µ(f ) = χ(f ) rk(f ), µ(f ) = χ(f ) rk(f ), µ(f) = χ(f) rk(f) = χ(f ) + χ(f ) rk(f ) + rk(f ) Since both rk(f ) and rk(f ) are nonnegative, we indeed get the lemma. Definition 2.7. A coherent sheaf F is called stable (resp. semistable) if for any nontrivial short exact sequence 0 F F F 0 we have µ(f ) < µ(f) (resp. µ(f ) µ(f)). General theory gives us the following Theorem 2.8 ([1] Harder-Narasimhan filtration). For a coherent sheaf F, there is a unique filtration 0 = F 0 F 1... F n F n+1 = F such that all A i = F i+1 /F i are semistable and µ(a i ) > µ(a i+1 ) for each i. In our case, we can derive much stronger proposition. Before stating it, note two useful statements. Proposition 2.9. If F and G are semistable sheaves, and µ(f) > µ(g), then Hom(F, G) = 0. Proof. Suppose we have a nontrivial map f : F G. Then µ(f) µ(f/ker f) = µ(im f) µ(g). Contradiction. Another property of Coh(X) we will need is Proposition 2.10 (Calabi-Yau property). For any two coherent sheaves F and G, there is an isomorphism Hom(F, G) Ext 1 (G, F). Proof. From Remark 1.4 we know that the canonical bundle K is trivial, K O. Also by Serre duality we get Hom(F, G) = Ext 0 (F, G) Ext 1 (G, F K) = Ext 1 (G, F). We are ready to prove 3

4 Theorem Any nontrivial coherent sheaf is a direct sum of indecomposable semistable sheaves. Proof. We only need to prove that any indecomposable sheaf is semistable. Suppose some indecomposable sheaf F is not semistable. Then the Harder-Narasimhan filtration of F is nontrivial. Consider only the case of length 1 filtration, it captures the main idea. So, we have a short exact sequence 0 F F F 0, where both F and F are semistable, and µ(f ) > µ(f ). By Proposition 2.9 we get Hom(F, F ) = 0. By Proposition 2.10 we obtain Ext 1 (F, F ) = Hom(F, F ) = 0. Therefore the exact sequence splits, contradiction with the assumption that F is indecomposable. Definition Denote the full subcategory of semistable coherent sheaves on X of slope µ by C µ. Proposition The category C µ is abelian, artinian, and closed under extensions. The simple objects in C µ are stable sheaves of slope µ. Corollary Coh(X) is the direct sum of all C µ (on the level of objects). 3 Euler form Since rk and χ are well defined on K 0 (Coh(X)), we can consider Definition 3.1. The Euler form F, G of two elements F, G K 0 (Coh(X)) is equal to dim Hom(F, G) dim Ext 1 (F, G). Proposition 3.2. We have F, G = rk(f)χ(g) χ(f) rk(g). Proof. First notice that the RHS only depends on the classes of F and G in the Grothendieck group K 0 (Coh(X)). Therefore it is sufficient to check the equality for some generators of the Grothendieck group, for example, for locally free sheaves. If F is locally free, the LHS reduces to χ(f G). Note that in the case of elliptic curve, the Hirzebruch-Riemann-Roch theorem gives us that χ(e) = deg(e) for any coherent sheaf E. Applying it here, we get LHS = χ(f G) = deg(f G) = rk(f) deg(g) deg(f) rk(g) = rk(f)χ(g) χ(f) rk(g) = RHS. Definition 3.3. The charge map is Z = (rk, χ): K 0 (Coh(X)) Z 2. It is surjective, since we have both (1, 0) and (0, 1) in the image. We have a canonical nondegenerate volume form on Z 2, (a, b), (c, d) = ad bc, and it is equal to the push-forward of the Euler form. Proposition 3.4. The kernel of the Euler form coincides with the kernel of Z, equivalently, K 0 (Coh(X))/ker, Z 2. 4

5 Also we can now write some relations between different C µ and C µ. Proposition 3.5. Suppose F and F are indecomposable, and Z(F) = (r, χ), Z(F ) = (r, χ ). If χ/r > χ /r, then Hom(F, F ) = 0, dim Ext 1 (F, F ) = χr χ r; If χ/r < χ /r, then dim Hom(F, F ) = χ r χr, Ext 1 (F, F ) = 0. Proof. By Proposition 2.10 and Proposition 2.9 we know that if χ/r > χ /r, then Hom(F, F ) = 0; if χ/r < χ /r, then Ext 1 (F, F ) = 0. Proposition 3.2 concludes the proof. 4 Derived category of coherent sheaves Let us show that Corollary 1.12 implies a neat description of the derived category D b (Coh(X)) of bounded complexes of coherent sheaves on X. Theorem 4.1. Suppose C is a hereditary abelian category. Then any object L D b (C) is isomorphic to the sum of its cohomologies, i. e. L = i Hi L[ i]. d i Proof. Let L be a complex... di 1 L i i+1 di+1 L.... Fix any i. We have a short exact sequence 0 ker d i 1 L i 1 im d i 1 0. Apply RHom(H i L, ). This gives rise to an exact sequence Ext 1 (H i L, L i 1 ) Ext 1 (H i L, im d i 1 ) Ext 2 (H i L, ker d i 1 ). Since Coh(X) is hereditary, we obtain a surjection from Ext 1 (H i L, L i 1 ) to Ext 1 (H i L, im d i 1 ). In particular, there exists M i such that the following diagram commutes Then the following morhphism 0 L i 1 M i H i L 0 0 im d i 1 ker d i H i L H i L 0 0 L i 1 M i 0 of complexes is a quasi-isomorphism. If we compose its inverse with the morphism 0 L i 1 M i 0 L i 2 L i 1 L i L i+1 we get a morphism H i L[ i] L in D b (Coh(X)) which is isomorphism in the i-th cohomology, and zero elsewhere. Therefore, if we sum up all this morphisms, we obtain an isomorphism i Hi L[ i] L. 5

6 Corollary 4.2. The derived category D b (Coh(X)) is the direct sum of Z copies of Coh(X), a sheaf F in the i-th copy goes to F. Since K 0 (D b (Coh(X))) = K 0 (Coh(X)), Z is defined on K 0 (D b (Coh(X))) as well. Note that Z(F[i]) = ( 1) i Z(F). Remark 4.3. The corollary works for any smooth projective curve X. Another example of a hereditary category is the category of representations of a quiver. 5 SL 2 (Z) action Proposition 3.2 suggests to define L, M = i ( 1)i dim Hom(L, M[i]) for any two objects L, M D b (Coh(X)). Therefore the Euler form is preserved by any autoequivalence of D b (Coh(X)). In other words, any autoequivalence f Aut(D b (Coh(X))) gives a corresponding automorphism of Z 2 preserving the volume form, i. e. gives an element π(f) SL 2 (Z). Definition 5.1. Say that an object E D b (Coh(X)) is spherical if Hom(E, E) = k (and consequently Hom(E, E[1]) = k). Example 5.2. The structure sheaf O and the skyscrapper sheaf at a rational k-point are spherical. Definition 5.3. A Forier-Mukai transform with a kernel L D b (Coh(X Y )) is a functor Φ L : D b (Coh(X)) D b (Coh(Y )) which sends an object F D b (Coh(X)) to Rπ 2 (π 1F L L), where π 1 : X Y X and π 2 : X Y X are the natural projections. Definition 5.4. For a spherical object E D b (Coh(X)), which is a complex of locally free sheaves, we can define a twist functor T E : D b (Coh(X)) D b (Coh(X)) to be equal to a Forier-Mukai transform with the kernel cone(e E O ) D b (Coh(X X)). Theorem 5.5 ([2]). For a spherical object E D b (Coh(X)) the twist functor T E is an exact equivalence which sends an object F to cone(rhom(e, F) L E ev F F). Remark 5.6. The evaluation works by applying ev : Ext i (E, F) E[ i] F on each grading. Let us see how T E acts on Grothendieck group. Proposition 5.7. The action of T E on K 0 (D b (Coh(X))) is given by [F] [F] E, F [E]. Proof. Indeed, [T E (F)] = [F] [RHom(E, F) L E] = [F] E, F [F]. Corollary 5.8. π(t O ) = ( ), π(t O x ) = ( ). Proof. Since Z 2 are generated by the charges of O and O x, we can check this on O and O x only. T O ([O]) = [O] O, O [O] = [O], T O ([O x ]) = [O x ] O, O x [O] = [O x ] [O], T Ox ([O]) = [O] O x, O [O x ] = [O] + [O x ], T Ox ([O x ]) = [O x ] O x, O x [O x ] = [O x ]. 6

7 Proposition 5.9. T Ox is in fact just the tensor product with O(x). Proof. The formula for the adjoint of a Forier-Mukai transform gives that the inverse of T Ox is the Forier-Mukai transform with the kernel cocone(o O (x,x) ). The map inside a cocone is nonzero. But any nonzero map O O (x,x) is a nonzero multiple of the natural surjection O O (x,x). Therefore the cocone is equal to the kernel of this map, or just O π1(o( x)). Now note that the sheaf O in the kernel trivializes all pullbacks and pushforwards we do to the identity maps between sheaves on X and on X. The proposition follows. The matrices π(t O ) and π(t Ox ) generate SL 2 (Z), therefore, π : Aut(D b (Coh(X))) SL 2 (Z) is surjective. 6 Classification of indecomposable sheaves Note that indecomposable torsion sheaves lie in C, and generate C. Moreover, we have Theorem 6.1. Indecomposable torsion sheaves are parametrized by a positive integer s > 0 and a closed point x X. The corresponding torsion sheaf is O/O( sx). Proof. Indeed, we reduce to the case of one point, then the local ring is PID, and the claim follows. In addition to that, SL 2 (Z) action allows us to prove Theorem 6.2. For each µ Q we have a canonical isomorphism C µ C. Proof. Indeed, let µ be equal to a/b for coprime a and b. Choose some γ SL 2 (Z) which sends (a, b) to (0, 1), and lift it to an autoequivalence f Aut(D b (Coh(X))) of the derived category. Take any indecomposable sheaf F C µ. Then f(f) is an indecomposable object in D b (Coh(X)) with the slope. Therefore, it is of form G[k], where G is a torsion sheaf, and k is some integer. Denote by f : C µ C a map which sends an indecomposable sheaf F to a sheaf G defined in this way. It is easy to see that if we begin with the inverse matrix f 1, then we get a map f 1 : C C µ which is inverse to f. Also f does not depend on a lift f. So C µ and C are canonically isomorphic. Summarizing, we have Theorem 6.3. Indecomposable sheaves are parametrized by a pair (rk, χ) in the right half of Z 2 and a closed point x X. Let us show how this describes indecomposable sheaves with charges (1, 1) and (1, 0). Proposition 6.4. T O (O) = O, T O (O(x)) = O x, T Ox (O) = O(x), T Ox (O x ) = O x. 7

8 Proof. The second line is a consequence of Proposition 5.9. The first line is an easy computation based on Theorem 5.5. Proposition 6.5. The indecomposable sheaves of charge (1, 1) are the sheaves O(x). The indecomposable sheaves of charge (1, 0) are the sheaves O(x y). Proof. The autoequivalence T 1 O maps the charge (0, 1) to (1, 1), so we can use it to obtain the indecomposables of charge (1, 1). Given an indecomposable O x of charge (0, 1), its image is O(x) by Proposition 6.4. The first part follows. Then we can apply T 1 O y to the latter indecomposables. We get that the indecomposables of charge (1, 0) are O(x y). 7 Braid group relations For matrices A = ( ) and B = ( ) we have the following relations ABA = BAB (AB) 3 = ( ) We expect similar relations to hold for T O and T Ox. Theorem 7.1 ([2]). where i: X X is the inverse map of X. T O T Ox T O T Ox T O T Ox (T O T Ox ) 3 i [1], We can prove the braid relation using the following Proposition 7.2 ([2]). Given two spherical objects E 1 and E 2, we have T E1 T E2 = T TE1 (E 2 )T E1 Proof. Using the computations in Proposition 6.4, we can write T O T Ox T O = T O T TOx (O)T Ox = T O T O(x) T Ox = T TO (O(x))T O T Ox = T Ox T O T Ox. This shows that T O and T Ox generate the group SL 2 (Z) in Aut(D b (Coh(X))), the central extension of SL 2 (Z) by Z. References [1] Harder, Narasimhan, On the cohomology groups of moduli spaces of vector bundles on curves, Math. Ann. 212, [2] Seidel, Thomas, Braid group actions on derived categories of coherent sheaves, arxiv:math/

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