cohomologyMatrix -- cohomology groups of a sheaf on P^{n_1}xP^{n_2}, or of (part) of a Tate resolution

If M is a bigraded module over a bigraded polynomial ring representing a sheaf F on P^n₁ x P^n₂, the script returns a block of the cohomology table, represented as a table of "cohomology polynomials" in ℤ[h,k] of the form

in each position {c₁,c₂}for a₁ ≤c₁ ≤b₁ and a₂ ≤c₂ ≤b₂. In case M corresponds to an object in the derived category D^b(P^n₁x P^n₂), then hypercohomology polynomials are returned, with the convention that k stands for k=h ^-1.

The polynomial for {b₁,b₂}sits in the north-east corner, the one corresponding to (a₁,a₂) in the south-west corner.

In the case of a product of more (or fewer) projective spaces, or if a hash table output is desired, use cohomologyHashTable or eulerPolynomialTable instead.

The script computes a sufficient part of the Tate resolution for F, and then calls itself in the version for a Tate resolution. More generally, If T is part of a Tate resolution of F the function returns a matrix of cohomology polynomials corresponding to T.

If T is not a large enough part of the Tate resolution, such as W below, then the function collects only the contribution of T to the cohomology table of the Tate resolution, according to the formula in Corollary 0.2 of Tate Resolutions on Products of Projective Spaces.

i1 : (S,E) = productOfProjectiveSpaces{1,2}

o1 = (S, E)

o1 : Sequence

i2 : M = S^1

      1
o2 = S

o2 : S-module, free

i3 : low = {-3,-3};high={0,0};

i5 : cohomologyMatrix(M,low,high)

o5 = | 2h  h  0 1  |
     | 0   0  0 0  |
     | 0   0  0 0  |
     | 2h3 h3 0 h2 |

                      4                4
o5 : Matrix (ZZ[h, k])  <--- (ZZ[h, k])

As a second example, consider the structure sheaf O_E of a nonsingular cubic contained in (point)xP². The corresponding graded module is

i6 : M = S^1/ideal(x_(0,0), x_(1,0)^3+x_(1,1)^3+x_(1,2)^3)

o6 = cokernel | x_(0,0) x_(1,0)^3+x_(1,1)^3+x_(1,2)^3 |

                            1
o6 : S-module, quotient of S

i7 : low = {-3,-3};high={0,0};

i9 : cohomologyMatrix(M,low,high)

o9 = | h+1 h+1 h+1 h+1 |
     | 3h  3h  3h  3h  |
     | 6h  6h  6h  6h  |
     | 9h  9h  9h  9h  |

                      4                4
o9 : Matrix (ZZ[h, k])  <--- (ZZ[h, k])

and the "1+h" in the Northeast (= upper right) corner signifies that that h⁰(O_E) = h¹(O_E) = 1.