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HH^ZZ(ProjectiveVariety,CoherentSheaf) -- cohomology of a coherent sheaf on a projective variety



The command computes the i-th cohomology group of F as a vector space over the coefficient field of X. For i>0 this is currently done via local duality, while for i=0 it is computed as a limit of Homs. Eventually there will exist an alternative option for computing sheaf cohomology via the Bernstein-Gelfand-Gelfand correspondence

As examples we compute the Picard numbers, Hodge numbers and dimension of the infinitesimal deformation spaces of various quintic hypersurfaces in projective fourspace (or their Calabi-Yau small resolutions)

We will make computations for quintics V in the family given by $$x_0^5+x_1^5+x_2^5+x_3^5+x_4^5-5\lambda x_0x_1x_2x_3x_4=0$$ for various values of $\lambda$. If $\lambda$ is general (that is, $\lambda$ not a 5-th root of unity, 0 or $\infty$), then the quintic $V$ is smooth, so is a Calabi-Yau threefold, and in that case the Hodge numbers are as follows.

$$h^{1,1}(V)=1, h^{2,1}(V) = h^{1,2}(V) = 101,$$

so the Picard group of V has rank 1 (generated by the hyperplane section) and the moduli space of V (which is unobstructed) has dimension 101:

i1 : Quintic = Proj(QQ[x_0..x_4]/ideal(x_0^5+x_1^5+x_2^5+x_3^5+x_4^5-101*x_0*x_1*x_2*x_3*x_4))

o1 = Quintic

o1 : ProjectiveVariety
i2 : singularLocus(Quintic)

         /QQ[x ..x ]\
         |    0   4 |
o2 = Proj|----------|
         \     1    /

o2 : ProjectiveVariety
i3 : omegaQuintic = cotangentSheaf(Quintic);
i4 : h11 = rank HH^1(omegaQuintic)

o4 = 1
i5 : h12 = rank HH^2(omegaQuintic)

o5 = 101

By Hodge duality this is $h^{2,1}$. Directly $h^{2,1}$ could be computed as

i6 : h21 = rank HH^1(cotangentSheaf(2,Quintic))

o6 = 101

The Hodge numbers of a (smooth) projective variety can also be computed directly using the hh command:

i7 : hh^(2,1)(Quintic)

o7 = 101
i8 : hh^(1,1)(Quintic)

o8 = 1

Using the Hodge number we compute the topological Euler characteristic of V:

i9 : euler(Quintic)

o9 = -200

When $\lambda$ is a 5th root of unity the quintic V is singular. It has 125 ordinary double points (nodes), namely the orbit of the point $(1:\lambda:\lambda:\lambda:\lambda)$ under a natural action of $\ZZ/5^3$. Then $V$ has a projective small resolution $W$ which is a Calabi-Yau threefold (since the action of $\ZZ/5^3$ is transitive on the sets of nodes of $V$, or for instance, just by blowing up one of the $(1,5)$ polarized abelian surfaces $V$ contains). Perhaps the most interesting such 3-fold is the one for the value $\lambda=1$, which is defined over $\QQ$ and is modular (see Schoen's work). To compute the Hodge numbers of the small resolution $W$ of $V$ we proceed as follows:

i10 : SchoensQuintic = Proj(QQ[x_0..x_4]/ideal(x_0^5+x_1^5+x_2^5+x_3^5+x_4^5-5*x_0*x_1*x_2*x_3*x_4))

o10 = SchoensQuintic

o10 : ProjectiveVariety
i11 : Z = singularLocus(SchoensQuintic)

o11 = Z

o11 : ProjectiveVariety
i12 : degree Z

o12 = 125
i13 : II'Z = sheaf module ideal Z

o13 = image | x_3^4-x_0x_1x_2x_4 x_0x_1x_2x_3-x_4^4 x_2^4-x_0x_1x_3x_4 x_1^4-x_0x_2x_3x_4 x_0^4-x_1x_2x_3x_4 x_2^3x_3^3-x_0^2x_1^2x_4^2 x_1^3x_3^3-x_0^2x_2^2x_4^2 x_0^3x_3^3-x_1^2x_2^2x_4^2 x_1^2x_2^2x_3^2-x_0^3x_4^3 x_0^2x_2^2x_3^2-x_1^3x_4^3 x_0^2x_1^2x_3^2-x_2^3x_4^3 x_1^3x_2^3-x_0^2x_3^2x_4^2 x_0^3x_2^3-x_1^2x_3^2x_4^2 x_0^2x_1^2x_2^2-x_3^3x_4^3 x_0^3x_1^3-x_2^2x_3^2x_4^2 |

o13 : coherent sheaf on Proj(QQ[x ..x ]), subsheaf of OO
                                 0   4                  Proj(QQ[x ..x ])
                                                                 0   4

The defect of $W$ (that is, $h^{1,1}(W)-1$) can be computed from the cohomology of the ideal sheaf of the singular locus Z of V twisted by 5 (see Werner's thesis):

i14 : defect = rank HH^1(II'Z(5))

o14 = 24
i15 : h11 = defect + 1

o15 = 25

The number $h^{2,1}(W)$ (the dimension of the moduli space of $W$) can be computed (Clemens-Griffiths, Werner) as $\dim H^0({\mathbf I}_Z(5))/JacobianIdeal(V)_5$.

i16 : quinticsJac = numgens source basis(5,ideal Z)

o16 = 25
i17 : h21 = rank HH^0(II'Z(5)) - quinticsJac

o17 = 0

In other words W is rigid. It has the following topological Euler characteristic.

i18 : chiW = euler(Quintic)+2*degree(Z)

o18 = 50

See also

Ways to use this method: