The input for this method is a module M over a multigraded polynomial ring whose local cohomology modules can be presented by monomial matrices. If an integer i is also included in the input, quasidegreesLocalCohomology(i,M) computes the quasidegree set of the i-th local cohomology module, supported at the maximal irrelevant ideal, of M. If an integer is excluded from the input, then quasidegreesLocalCohomology(M) computes the quasidegree set of Hm0(M)⊕…⊕Hmd-1(M). The quasidegrees of local cohomology are indexed by a list of pairs (v,F) where v is a vector and F is a list of vectors. The pair (v,F) indexes the plane v+spanCF. The quasidegree set of the local cohomology modules is the union of all such planes that the pairs (v,F) index.
If the input is an ideal I in a multigraded polynomial ring R, then the method executes for the module R/I where R is the ring of I.
A synonym for this function is qlc.
The first example computes the quasidegree set of Hm0(R/I)⊕Hm1(R/I) where I is the toric ideal associated to the matrix A.
i1 : A = matrix{{1,1,1,1},{0,1,5,11}} o1 = | 1 1 1 1 | | 0 1 5 11 | 2 4 o1 : Matrix ZZ <--- ZZ |
i2 : R = QQ[a..d] o2 = R o2 : PolynomialRing |
i3 : R = toGradedRing(A,R) o3 = R o3 : PolynomialRing |
i4 : I = toricIdeal(A,R) 2 2 5 3 2 2 3 4 5 4 o4 = ideal (b*c - a d, c - b d , a c - b d, b - a c) o4 : Ideal of R |
i5 : M = R^1/I o5 = cokernel | bc2-a2d c5-b3d2 a2c3-b4d b5-a4c | 1 o5 : R-module, quotient of R |
i6 : quasidegreesLocalCohomology M o6 = {{| 2 |, {}}, {| 3 |, {}}, {| 3 |, {}}, {| 4 |, {}}} | 4 | | 4 | | 9 | | 9 | o6 : List |
The above example gives that the quasidegrees of the non-top local cohomology of M are (4,9), (3,9), (2,4), and (3,4). We can see that these all come from the first local cohomology module.
i7 : quasidegreesLocalCohomology(1,M) o7 = {{| 2 |, {}}, {| 3 |, {}}, {| 3 |, {}}, {| 4 |, {}}} | 4 | | 4 | | 9 | | 9 | o7 : List |
The next example shows a module whose quasidegree set of its second local cohomology module at the irrelevant ideal, is a line.
i8 : A = matrix{{1,1,1,1,1},{0,0,1,1,0},{0,1,1,0,-2}} o8 = | 1 1 1 1 1 | | 0 0 1 1 0 | | 0 1 1 0 -2 | 3 5 o8 : Matrix ZZ <--- ZZ |
i9 : R = QQ[a..e] o9 = R o9 : PolynomialRing |
i10 : R = toGradedRing(A,R) o10 = R o10 : PolynomialRing |
i11 : I = toricIdeal(A,R) 2 2 2 3 2 o11 = ideal (a*c - b*d, a*d - c e, a d - b*c*e, a - b e) o11 : Ideal of R |
i12 : M = R^1/I o12 = cokernel | ac-bd ad2-c2e a2d-bce a3-b2e | 1 o12 : R-module, quotient of R |
i13 : quasidegreesLocalCohomology(2,M) o13 = {{| 0 |, {| 1 |}}} | 0 | | 0 | | 1 | | -2 | o13 : List |
The above example gives that the quasidegrees of the second local cohomology module of M at the irrelevant ideal is the complex parameterized line (0,0,1)+t•(1,0,-2).