Hodge Theory via Deformations of Affine Cones and Fano Threefolds Enrico Fatighenti University of Warwick Supervisor: Miles Reid [email protected] Abstract Deformation Theory and Hodge Theory are known to be closely related. Examples of this friendship can be found, for example, in the theory of the Variation of Hodge Structures, or in the Griffiths’s Residue Theory. The latter identifies the Hodge Structures of a hypersurface with the Milnor Algebra of the hypersurface itself - an important deformation-theoretical invariant. The aim of the present work is to show how to compute effectively (part of) the Hodge Structure of a smooth projective variety X, under some mild conditions, by looking at some graded component of the infinitesimal deformation/obstruction module of the affine cone over X. As an application of this result, we computed the whole Hodge Structure of Fano Threefolds. Back in the day: the hypersurface case One of the most important contact points between Hodge Theory and Deformation Theory is the Griffiths’s Residue Theorem: if X is a smooth projective hypersurface, it establishes an isomorphism, given by a generalisation of the Poincar´e Residue map, p−1,n−p Hprim (X) ∼ = Mf where the subscript prim stands for the primitive cohomology, and the object on the right is called the Milnor Algebra, an important deformation-theoretical invariant for the hypersurface, defined as Deformations of the affine cone vs Deformations of the projective variety Hodge Theory kicks in: Primitive Cohomology of X inside TA1 Up to now we considered the deformations of A, so a pretty natural question arises: what do they shares with the deformations of X? Now, the TA1 module is naturally Z-graded, and Schlessinger first showed that the degree 0 component of the TA1 represents the projective deformations of X inside the fixed Pn (and similar for the TA2 ). This is quite evident if one considers the case of a hypersurface: it is quite easy to show that, in this case, Hypotheses first: let X be (quasi)smooth in a (weighted) projective space and assume • ωX ∼ = OX (α), some α ∈ Z, some D ∼ = OX (1) ample; TA1 := Mf (d) where d is the degree of the defining polynomial. This simply means that, in order to deform the projective hypersurface X = V (f ), we may deform the affine cone simply adding a polynomial of the same degree. Any other choice of degree will bring the affine cone to something that is not a cone (think to the example xy + ε = 0 in C2.) or, more conceptually, as TA1 := Ext1A(Ω1A, OA) In a similar fashion, we can define the T 2-module, that measures the infinitesimal obstructions to extending deformations of A, as TA2 := Ext2A(Ω1A, OA) and same for TA0 , that measures the infinitesimal automorphisms of A. • Quasi-linear sections of weighted Grassmannians. If the hypotheses above are verified, we can identify TA1 ∼ = H 1(U, ΘU ) where U is the complement of the vertex in the affine cone and Θ is the holomorphic tangent sheaf. The key step is now to relate the data on X to the one on U . This is done via the exact sequence . . . → H 1(X, OX (α)) → (TA1 )α → H 1(X, ΘX (α)) → H 2(X, OX (α)) → . . . and, thanks to our vanishing hypotheses, Serre duality and diagram chasing, we show 1 In order to generalise the previous setting, we looked back to some old papers by Schlessinger, Wahl et al. (see[2] and [3]), in which they considered the deformations of an affine isolated singularity. In particular, if X is a smooth projective variety, we can consider the affine cone A over X: under the identification D ∼ = OX (1), for some D ∈ Pic (X), we can think to A as Spec(R(X, D)), whereas X =Proj (A). We can define the module TA1 (the first cotangent sheaf), the set of isomorphism classes of first order infinitesimal deformations of A, by the sequence 0 → ΘA → ΘCn+1|A → NA → TA1 , • Smooth Fano varieties; Passing in cohomology and using some standard argument it is possible to express everything in terms of the cohomology of X. Hence we have a sequence of Z-graded modules and, restricting to degree α, we get with Jf = (f0, . . . , fn), that is the Jacobian ideal of f , generated by its partial derivatives. As said, the Milnor Algebra Mf contains the deformation data for X. The purpose of this work is to generalise this setting to any codimension case. Our Champion: The T • hn−1,0(X) where dim X = n ≥ 2; What type of varieties satisfies the previous hypotheses? We identify a bunch of significative examples (altough there are many others) • Complete intersections; 0 → ΘU |X → ΘU → π ∗(ΘX ) → 0. Mf := C[x0, . . . , xn]/Jf Theorem. Let X be smooth, projective satisfying the hypotheses above. Then n−1,1 (TA1 )α ∼ = Hprim (X) and totally similar for the TA2 , with n−1,2 (TA2 )α ∼ = Hprim (X). Figure 2: Hodge Diamond of a Fano Threefold In particular we looked at the Graded Ring Database (see [1]) by Gavin Brown, Alexander Kasprzyk, Miles Reid et al., that contains several thousands of examples of quasi-smooth Fano Threefolds of various codimensions in weighted projective spaces: some months ago, using some birational tricks (namely, the famous Unprojections) we were able to compute the Euler characteristic of all the Fano in codimension 2 and most of the codimension 3. Computing the T 1 (this is enough, being all Fano of index 1) with SINGULAR we are able to confirm these results. In particular we are now able to effectively compute the full Hodge Structure for more than one hundred example of Fano Threefolds (and, at a theorical level, several thousands). What happens next? As we saw, we have been able to identify some part of the Hodge Theory of X by looking to some graded component of a deformation module. A pretty natural question is: where are the others?We are currently thinking about this V• question. One possibility could be to look at the ΘX (α) instead of ’just’ the sheaf ΘX (α), but this whole complex is totally work in progress... References Figure 1: Deformations of affine cones into affine cones Application to Fano Threefolds Up to now everything is very classical, but now we add Hodge Theory to the recipe, and figure out something new. We said above that, amongst the various deformation-components of the T 1-module of A, the degree 0 gives information on X, and in particular takes care of its own deformation. What we want to do now is identify another special component in the TA1 , and find there (part of) the Hodge Theory of X. since in their case the knowledge of H 2,1(X) and H 1,1(X), predicted by the T 1 and the T 2 is enough to determine the whole of the Hodge Structure. Up to now, we described a theorical method to find (part of) the Hodge structure of a smooth projective variety inside some (not too) weird module. The question is: can we compute anything? The answer is positive, thanks to Computer Algebra software such as SINGULAR that are particularly good in computing the TA1 . As an immediate application, we decide to turn our attention to the Fano Threefold case, [1] Gavin Brown and Alexander Kasprzyk. The graded ring database, online, access via http://grdb.lboro.ac.uk/. [2] Michael Schlessinger. On rigid singularities. The Rice University Studies, 19(1):147–162, 1973. [3] Jonathan Wahl. The jacobian algebra of a graded gorenstein singularity. Duke Mathematical Journal, 55(4):843–871, 1987.
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