# cohomologyHashTable -- cohomology groups of a sheaf on a product of projective spaces, or of (part) of a Tate resolution

## Synopsis

• Usage:
H=cohomologyHashTable(M,low,high)
H=cohomologyHashTable(T,low,high)
• Inputs:
• M, , graded module representing a sheaf on a product of projective spaces
• T, , free complex over the exterior algebra
• low, a list,
• high, a list, two lists representing multi-degrees, the range for computation.
• Outputs:
• H, , values are dimensions of (hyper)cohomology groups

## Description

If M is a multi-graded module representing a coherent sheaf F on $P^n := P^{n_0} x .. x P^{n_{t-1}}$, the script returns a hash table with entries {a,i} => h^i(F(a)) where a is a multi-index, low<=a<=high in the partial order (thus the value is 0 when i is not in the range 0..sum n.) In case T is a Tate resolution corresponding to an object F in D^b(P^n), then the values returned are the dimensions of the hypercohomology groups of twists of F, and the values can be nonzero in a wider range.

In case the number of factors t is 2, the output of cohomologyMatrix is easier to parse.

The script computes a sufficient part of the Tate resolution for F, and then calls itself in the version for a Tate resolution.

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 = {3,3}; i5 : H = cohomologyHashTable(M, low,high);

We can print just the entries representing nonzero cohomology groups:

 i6 : H' = hashTable(select(pairs H, p-> p_1!=0)) o6 = HashTable{{{-2, -3}, 3} => 1} {{-2, 0}, 1} => 1 {{-2, 1}, 1} => 3 {{-2, 2}, 1} => 6 {{-2, 3}, 1} => 10 {{-3, -3}, 3} => 2 {{-3, 0}, 1} => 2 {{-3, 1}, 1} => 6 {{-3, 2}, 1} => 12 {{-3, 3}, 1} => 20 {{0, -3}, 2} => 1 {{0, 0}, 0} => 1 {{0, 1}, 0} => 3 {{0, 2}, 0} => 6 {{0, 3}, 0} => 10 {{1, -3}, 2} => 2 {{1, 0}, 0} => 2 {{1, 1}, 0} => 6 {{1, 2}, 0} => 12 {{1, 3}, 0} => 20 {{2, -3}, 2} => 3 {{2, 0}, 0} => 3 {{2, 1}, 0} => 9 {{2, 2}, 0} => 18 {{2, 3}, 0} => 30 {{3, -3}, 2} => 4 {{3, 0}, 0} => 4 {{3, 1}, 0} => 12 {{3, 2}, 0} => 24 {{3, 3}, 0} => 40 o6 : HashTable

In the case of two factors (t=2), the same information can be read conveniently from a matrix

 i7 : cohomologyMatrix(M, low, high) o7 = | 20h 10h 0 10 20 30 40 | | 12h 6h 0 6 12 18 24 | | 6h 3h 0 3 6 9 12 | | 2h h 0 1 2 3 4 | | 0 0 0 0 0 0 0 | | 0 0 0 0 0 0 0 | | 2h3 h3 0 h2 2h2 3h2 4h2 | 7 7 o7 : Matrix (ZZ[h, k]) <--- (ZZ[h, k])

where the entry in the a= \{a_0,a_1\} place is sum_i h^i(F(a)*h^i \in ZZ[h]. In the case of more factors, the same format is available through the command

 i8 : eulerPolynomialTable H' 3 o8 = HashTable{{-2, -3} => h } {-2, 0} => h {-2, 1} => 3h {-2, 2} => 6h {-2, 3} => 10h 3 {-3, -3} => 2h {-3, 0} => 2h {-3, 1} => 6h {-3, 2} => 12h {-3, 3} => 20h 2 {0, -3} => h {0, 0} => 1 {0, 1} => 3 {0, 2} => 6 {0, 3} => 10 2 {1, -3} => 2h {1, 0} => 2 {1, 1} => 6 {1, 2} => 12 {1, 3} => 20 2 {2, -3} => 3h {2, 0} => 3 {2, 1} => 9 {2, 2} => 18 {2, 3} => 30 2 {3, -3} => 4h {3, 0} => 4 {3, 1} => 12 {3, 2} => 24 {3, 3} => 40 o8 : HashTable

## Caveat

In case of hypercohomology, we write k instead of h^{-1}, and use the cohomology ring ZZ[h,k].

• productOfProjectiveSpaces -- Cox ring of a product of projective spaces and it Koszul dual exterior algebra
• cohomologyMatrix -- cohomology groups of a sheaf on P^{n_1}xP^{n_2}, or of (part) of a Tate resolution
• eulerPolynomialTable -- cohomology groups of a sheaf on a product of projective spaces, or of (part) of a Tate resolution
• cornerComplex -- form the corner complex

## Ways to use cohomologyHashTable :

• "cohomologyHashTable(ChainComplex,List,List)"
• "cohomologyHashTable(Module,List,List)"

## For the programmer

The object cohomologyHashTable is .