Sheaf Cohomology Notes by Sven Balnojan to script by Andreas Gathmann 11th January, 2017, Mannheim Contents 0.1 Introduction.................................... 1 1 Generalizing Vector Bundles 1 1.1 (Quasi-)CoherentSheaves ............................ 2 1.2 Examples ..................................... 2 2SheafCohomology 3 2.1 SerreDuality ................................... 3 2.2 TwistingSheaves ................................. 3 2.3 DifferentialForms................................. 4 2.4 SheafCohomology ................................ 5 3Resources 7 0.1 Introduction Iquicklywalkthroughtheverybasicsofsheafcohomologyforsheavesthatgeneralizevector bundles. I mention how this is related to derived functors and mostly work with facts & examples. I mention Serre duality mostly because I need it in a seminar talk, but it’s an extremely important feature. 1GeneralizingVectorBundles I’I’m assuming you’ve heard all the terms that I now mention. So how do quasicoherent sheaves generalize vector bundles again? X -modules form a nice (i.e. abelian) category, so O do e.g. vector spaces. But locally free sheaves (= vector bundles) do not (see [Vakil2013, p. 362] for all this + counter example); We can enlarge that category to an abelian one however. So we have 1 X modules Qcoh(X) coh(X) vector bundles O − . (abelian) ⊃ (abelian) ⊃ (abelian) ⊃ (not abelian) So we enlarge the category and get an abelian one. Is this the smallest abelian enlarge- ment which is “local”? Turns out no, the category of coherent sheaves is even smaller. Remark 1. (i) The category is abelian so we can form the derived category, meaning we can consider the objects as “complexes of objects”. (ii) The category of vector spaces is abelian, but it’s also triangulated = semisimple. ) Semisimple categories are kind of boring (that every exact sequence of vector spaces splits is the incarnation of the semisimplicity), from the homological point of view, any complex is quasi-isomorphic to a complex with Hi =0for all i>0. (iii) The next simple categories have complexes (in their derived categories) with Hi =0 for all i>1. Those are called hereditary categories. 1.1 (Quasi-) Coherent Sheaves The class of X -modules is usually too large to study. Any ring R determines an affine O scheme specR = X with structure sheaf X .AnyR-module M determines a sheaf M˜ of O X -modules on X = specR.Iseverysheafof X -modules of this form? O O No, the ones that are are called quasicoherent. The ones that are associated to a finitely generated R-module are called coherent. 1.2 Examples We have a good idea on how coherent sheaves on the projective line look like. Theorem 2. Any coherent sheaf on P1(k) is the finite direct sum of line bundles and degree one skyscraper sheaves (as not every coherent sheaf is locally free we need the skyscraper sheaves; that’s the point we want to generalize vector bundles). Let’s look at an example of a non-coherent sheaf. 1 Example 3. We want to construct a non-coherent sheaf. So we fix a affine scheme X = Ak; We are look for a sheaf M˜ of X -modules (= an assignment: set X -module) that is not O 7! O induced by some X -module M. O If this were the case, then we’d know that M˜ (X)=M;Ifweconstructanon-trivialsheaf with no non-trivial global sections this then equals M =0;Whichissufficientbecausethe sheaf induced by the module M =0is the trivial sheaf. The sheaf in question is simply (the sheaf associated to the presheaf defined by): 00U U 2 . 7! (U)0/ U (OX 2 2 2SheafCohomology Why do we care about sheaf cohomology in this talk? For two reasons. Serre duality is an important feature of the categories we described above (cohX on • wpls, but not wps) and it’s formulated in terms of sheaf cohomology. (Related) We have an equivalence to a category given by Exti which is just a derived • functor. Indeed for the categories we are looking at, if we derive from the functor of global sections, we get the sheaf cohomology functor, so to determine Exti we can work with sheaf cohomology (as already mentioned in a talk before). 2.1 Serre Duality Why is Serre duality important? Serre duality is a “pretty” property relating two cohomology classes (a special case being the Poincaré duality which lets us compute cohomology classes in terms of homology classes). It states roughly: there is a unique !X (a line bundle = dualizing sheaf = canonical bundle; unique up to isomorphism) such that for all coh(X), i,thereisanatural F2 8 isomorphism i i n i (H (X, _ !X ) =) Ext ( ,!X ) = H − (X, )⇤. F ⌦ ⇠ F ⇠ F We start with a couple of reminders of basic algebraic geometry. 2.2 Twisting Sheaves Take a projective variety X Pn,it’shomogeneouscoordinateringS(X) (with grading by ⇢ degree, pieces denoted by S(d))andconsiderthelocalizationonthenon-zerohomogenous elements. It has a grading, so we can simply define the nth graded piece f K(n)= ; f S(d+n),g S(d), some d 0 and g =0 { g 2 2 ≥ 6 } and we define the twisting sheaf X (n) by O f (n)(U)= K(n); g(P ) =0 . OX { g 2 6 } P U \2 Example 4. (a) n (0) = n . OP OP (b) n ( 1) is for n =1e.g. with coordinates x0,x1 OP − 1 P1 ( 1)( (x0 : x1); x0 =0 ) x0 2O − { 6 } 3 (c) locally X (n) is isomorphic to X .In(b)e.g.wehavethemaps(onU,upperindices O O mean degrees): f 1 1 f 1 f 1 f 1 1 0 0 and 1 1 g 7! x0 · g g 7! g · x0 (d) however the sheaves are not globally isomorphic. If they where, then the global n n sections would be the same. But Γ(P , X ) = k while Γ(P , X (n)) is either empty for k O ⇠ k O n<0 or defined by homogeneous polynomials of degree n (that’s pretty evident, the twisting is really a shifting of the grading; “Shifting k by 1 produces non-holomorphic functions, − while shifting k by +1 produces polynomials of degree 1”). Some more interesting properties of twisting sheaves are the following. n Example 5. (i) Let X = P .Wehave X (d) X (l)= X (d + l) using the isomorphism O ⌦O O f f˜ ff˜ (U) . g ⌦OX g˜ 7! gg˜ (this defines a presheaf; associate to it the sheaf). This is enough a the tensor product of the modules is generated by the images of the tensor product . ⌦OX (U) (ii) We have X (d)_ = X ( d). The dual sheaf is defined (again by a presheaf) as O O − U Hom (U)( X (d), X ).Butthe X -linear homomorphisms are given exctly by mul- 7! OX O O O tiplication with sections of X ( d) (multiplication obviously gives an element in X ). O − O 2.3 Differential Forms If Y = Speck is a point, then we write ⌦X for the sheaf of differential forms.Inthatcase, for a smooth manifold this coincides with usual differential forms (we could also define the sheaf of relative differential forms and make this a special case). If X is a smooth scheme, then (if and only if) is ⌦X locally free of dimension n.Soit’savectorbundle,thecotangent bundle.Wedefine Definition 6. The canonical bundle of a smooth n-dimensional scheme X is the bundle n !X := ⌦X (top exterior power of 1-forms). ^ (It’s canonical because it’s canonically defined for any scheme and thus gives a method to compare schemes up to isomorphism). n n Example 7. !X on P . The canonical bundle = cotangent bundle of P is determined by an exact sequence '1 (n+1) '2 0 ⌦ n ( 1)⊕ 0 ! P ! O − ! O! n+1 (n+1) then a “basic AG theorem” yields !X = ( ( 1)⊕ ) = ( n 1). ⇠ ^ O − ⇠ O − − 4 Example 8. (i) (Of Serre duality, even though we haven’t defined the sheaf cohomology n 1 yet) Determine H − (X, _ !X ). _ is locally free, and !X = ( n 1).Sowehave OX ⌦ OX ⇠ O − − n 1 n 1 H − (X, _ ! )=Ext − ( , ( n 1)). OX ⌦ X OX O − − 1 By Serre duality the last expression is isomorphic to H (X, X )⇤.IfX is affine, then O 1 this is (dual to) zero; If X is projective and 1 dimensional, then H (X, X )=... In any O case, this is a lower cohomology group and simpler to calculate. n 1 n 1 The other way around we could try to determine H − (X, X );FirstdetermineH − (X, X ) 1 O O by the stuffabove, we only need to do H (X, _ ( n 1)) which again, is doable. OX ⌦O − − (ii) ??? (A doable calculation could be done for P1 or other curves) 2.4 Sheaf Cohomology We explain the statement above in slightly more detail now. Let 0 1 2 3 0 !F !F !F ! be an exact sequence of quasicoherent sheaves on some scheme X.IfX = SpecR is an affine scheme, then Γ:qcoh(X) Mod(R) is an exact functor (and an equivalence of categories). ! However if X = ProjS is projective, then Γ may not be exact. Example 9. Let X Pn be some smooth hypersurface of degree d.Denotetheinclusionby ⇢ i.Weusetheexactsequenceforthecotangentsheaftodeterminethepullbackbundlesglobal sections (= trivial). We then use the relative differential form sequence (which includes the pullback global sections!) to show that this sequence then is not exact. The cotangent sheaf is even defined by the exact sequence (n+1) 0 ⌦ n ( 1)⊕ 0 pulling back + global sections yields ! P !O − !O! (n+1) 0 Γ(i⇤⌦ n ) Γ( ( 1)⊕ ) Γ( ) 0 ! P ! O − ! O ! since ( 1) has no global sections (remember, global sections of (d) are “defined by O − O degree d polynomials”), the pullback bundle doesn’t either (as it’s an exact sequence!).