Homological Algebra

prune_with_map(M::ModuleFP)

If M is positively graded, return a module N and an isomorphism from N to M such that N is generated by a minimal number of elements.

If M is not (positively) graded, the function still aims at reducing the number of generators.

Examples

julia> R, (w, x, y, z) = graded_polynomial_ring(QQ, [:w, :x, :y, :z]);

julia> Z = R(0)
0

julia> O = R(1)
1

julia> B = [Z Z Z O; w*y w*z-x*y x*z-y^2 Z];

julia> A = transpose(matrix(B))
[0         w*y]
[0   w*z - x*y]
[0   x*z - y^2]
[1           0]

julia> M = graded_cokernel(A)
Graded subquotient of graded submodule of R^2 with 2 generators
  1: e[1]
  2: e[2]
by graded submodule of R^2 with 4 generators
  1: w*y*e[2]
  2: (w*z - x*y)*e[2]
  3: (x*z - y^2)*e[2]
  4: e[1]

julia> N, phi = prune_with_map(M);

julia> N
Graded subquotient of graded submodule of R^1 with 1 generator
  1: e[1]
by graded submodule of R^1 with 3 generators
  1: (-x*z + y^2)*e[1]
  2: (-w*z + x*y)*e[1]
  3: w*y*e[1]

julia> phi(first(gens(N)))
e[2]

presentation(M::ModuleFP; minimal = false)

Return a (free) presentation of M.

Examples

julia> R, (w, x, y, z) = graded_polynomial_ring(QQ, [:w, :x, :y, :z]);

julia> Z = R(0)
0

julia> O = R(1)
1

julia> B = [Z Z Z O; w*y w*z-x*y x*z-y^2 Z];

julia> A = transpose(matrix(B))
[0         w*y]
[0   w*z - x*y]
[0   x*z - y^2]
[1           0]

julia> M = graded_cokernel(A)
Graded subquotient of graded submodule of R^2 with 2 generators
  1: e[1]
  2: e[2]
by graded submodule of R^2 with 4 generators
  1: w*y*e[2]
  2: (w*z - x*y)*e[2]
  3: (x*z - y^2)*e[2]
  4: e[1]

julia> PM1 = presentation(M)
0 <---- M <---- R^2 <---- R^4

julia> PM2 = presentation(M, minimal = true)
0 <---- M <---- R^1 <---- R^3

julia> Rg, (x, y, z) = graded_polynomial_ring(QQ, [:x, :y, :z]);

julia> F = graded_free_module(Rg, [1,2,2]);

julia> p = presentation(F)
0 <---- F <---- F <---- 0

julia> p[-2]
Graded free module Rg^0 of rank 0 over Rg

julia> p[-1]
Graded free module Rg^1([-1]) + Rg^2([-2]) of rank 3 over Rg

julia> p[0]
Graded free module Rg^1([-1]) + Rg^2([-2]) of rank 3 over Rg

julia> p[1]
Graded free module Rg^0 of rank 0 over Rg

julia> map(p,-1)
Homogeneous module homomorphism
  from F
  to 0
defined by
  e[1] -> 0
  e[2] -> 0
  e[3] -> 0

julia> map(p,0)
Homogeneous module homomorphism
  from F
  to F
defined by
  e[1] -> e[1]
  e[2] -> e[2]
  e[3] -> e[3]

julia> map(p,1)
Homogeneous module homomorphism
  from 0
  to F
defined by

julia> F = graded_free_module(Rg, 1);

julia> A = Rg[x; y];

julia> B = Rg[x^2; y^3; z^4];

julia> M = SubquoModule(F, A, B);

julia> P = presentation(M)
0 <---- M <---- Rg^2 <---- Rg^5

julia> P[-2]
Graded free module Rg^0 of rank 0 over Rg

julia> P[-1]
Graded subquotient of graded submodule of F with 2 generators
  1: x*e[1]
  2: y*e[1]
by graded submodule of F with 3 generators
  1: x^2*e[1]
  2: y^3*e[1]
  3: z^4*e[1]

julia> P[0]
Graded free module Rg^2([-1]) of rank 2 over Rg

julia> P[1]
Graded free module Rg^2([-2]) + Rg^1([-3]) + Rg^2([-5]) of rank 5 over Rg

julia> map(P,-1)
Homogeneous module homomorphism
  from M
  to 0
defined by
  x*e[1] -> 0
  y*e[1] -> 0

julia> map(P,0)
Homogeneous module homomorphism
  from Rg^2
  to M
defined by
  e[1] -> x*e[1]
  e[2] -> y*e[1]

julia> map(P,1)
Homogeneous module homomorphism
  from Rg^5
  to Rg^2
defined by
  e[1] -> x*e[1]
  e[2] -> -y*e[1] + x*e[2]
  e[3] -> y^2*e[2]
  e[4] -> z^4*e[1]
  e[5] -> z^4*e[2]

present_as_cokernel(M::SubquoModule, task::Symbol = :none)

Return a subquotient C which is isomorphic to M, and whose generators are the standard unit vectors of its ambient free module.

Additionally,

• return an isomorphism M $\to$ C if task = :with_morphism,
• return and cache an isomorphism M $\to$ C if task = :cache_morphism,
• do none of the above if task = :none (default).

If task = :only_morphism, return only an isomorphism.

Examples

julia> R, (x, y, z) = polynomial_ring(QQ, [:x, :y, :z]);

julia> A = R[x; y];

julia> B = R[x^2; y^3; z^4];

julia> M = SubquoModule(A, B)
Subquotient of submodule with 2 generators
  1: x*e[1]
  2: y*e[1]
by submodule with 3 generators
  1: x^2*e[1]
  2: y^3*e[1]
  3: z^4*e[1]

julia> C = present_as_cokernel(M)
Subquotient of submodule with 2 generators
  1: e[1]
  2: e[2]
by submodule with 5 generators
  1: x*e[1]
  2: -y*e[1] + x*e[2]
  3: y^2*e[2]
  4: z^4*e[1]
  5: z^4*e[2]

julia> Rg, (x, y, z) = graded_polynomial_ring(QQ, [:x, :y, :z]);

julia> F = graded_free_module(Rg, 1);

julia> A = Rg[x; y];

julia> B = Rg[x^2; y^3; z^4];

julia> M = SubquoModule(F, A, B);

julia> present_as_cokernel(M, :with_morphism)
(Graded subquotient of graded submodule of Rg^2 with 2 generators
  1: e[1]
  2: e[2]
by graded submodule of Rg^2 with 5 generators
  1: x*e[1]
  2: -y*e[1] + x*e[2]
  3: y^2*e[2]
  4: z^4*e[1]
  5: z^4*e[2], Hom: graded subquotient of graded submodule of Rg^2 with 2 generators
  1: e[1]
  2: e[2]
by graded submodule of Rg^2 with 5 generators
  1: x*e[1]
  2: -y*e[1] + x*e[2]
  3: y^2*e[2]
  4: z^4*e[1]
  5: z^4*e[2] -> M)

free_resolution(M::SubquoModule{T}; 
    length::Int=0,
    algorithm::Symbol = T <:MPolyRingElem ? :fres : :sres) where {T <: Union{MPolyRingElem, MPolyQuoRingElem}}

Return a free resolution of M.

If length != 0, the free resolution is only computed up to the length-th free module. Current options for algorithm are :fres, :nres, and :mres for modules over polynomial rings and :sres for modules over quotients of polynomial rings.

Examples

julia> R, (x, y, z) = polynomial_ring(QQ, [:x, :y, :z])
(Multivariate polynomial ring in 3 variables over QQ, QQMPolyRingElem[x, y, z])

julia> A = R[x; y]
[x]
[y]

julia> B = R[x^2; x*y; y^2; z^4]
[x^2]
[x*y]
[y^2]
[z^4]

julia> M = SubquoModule(A, B)
Subquotient of submodule with 2 generators
  1: x*e[1]
  2: y*e[1]
by submodule with 4 generators
  1: x^2*e[1]
  2: x*y*e[1]
  3: y^2*e[1]
  4: z^4*e[1]

julia> fr = free_resolution(M, length=1)
Free resolution of M
R^2 <---- R^6
0         1

julia> is_complete(fr)
false

julia> fr[4]
Free module of rank 0 over R

julia> fr
Free resolution of M
R^2 <---- R^6 <---- R^6 <---- R^2 <---- 0
0         1         2         3         4

julia> is_complete(fr)
true

julia> R, (w, x, y, z) = graded_polynomial_ring(QQ, [:w, :x, :y, :z]);

julia> Z = R(0)
0

julia> O = R(1)
1

julia> B = [Z Z Z O; w*y w*z-x*y x*z-y^2 Z];

julia> A = transpose(matrix(B));

julia> M = graded_cokernel(A)
Graded subquotient of graded submodule of R^2 with 2 generators
  1: e[1]
  2: e[2]
by graded submodule of R^2 with 4 generators
  1: w*y*e[2]
  2: (w*z - x*y)*e[2]
  3: (x*z - y^2)*e[2]
  4: e[1]

julia> FM1 = free_resolution(M)
Free resolution of M
R^2 <---- R^7 <---- R^8 <---- R^3 <---- 0
0         1         2         3         4

julia> betti_table(FM1)
degree: 0  1  2  3
------------------
    -1: -  1  -  -
     0: 2  -  -  -
     1: -  3  3  1
     2: -  3  5  2
------------------
 total: 2  7  8  3

julia> matrix(map(FM1, 1))
[1            0]
[0   -x*z + y^2]
[0   -w*z + x*y]
[0          w*y]
[0        x^2*z]
[0        w*x*z]
[0        w^2*z]

julia> FM2 = free_resolution(M, algorithm = :nres)
Free resolution of M
R^2 <---- R^4 <---- R^4 <---- R^2 <---- 0
0         1         2         3         4

julia> betti_table(FM2)
degree: 0  1  2  3
------------------
    -1: -  1  -  -
     0: 2  -  -  -
     1: -  3  -  -
     2: -  -  4  2
------------------
 total: 2  4  4  2

julia> matrix(map(FM2, 1))
[1            0]
[0   -x*z + y^2]
[0   -w*z + x*y]
[0          w*y]

julia> FM3 = free_resolution(M, algorithm = :mres)
Free resolution of M
R^1 <---- R^3 <---- R^4 <---- R^2 <---- 0
0         1         2         3         4

julia> betti_table(FM3)
degree: 0  1  2  3
------------------
     0: 1  -  -  -
     1: -  3  -  -
     2: -  -  4  2
------------------
 total: 1  3  4  2

julia> matrix(map(FM3, 1))
[-x*z + y^2]
[-w*z + x*y]
[       w*y]

Note: Over rings other than polynomial rings or quotients of polynomial rings, the method will default to a lazy, iterative kernel computation.

betti_table(F::FreeResolution)

Given a $\mathbb Z$-graded free resolution F, return the graded Betti numbers of F in form of a Betti table.

Alternatively, use betti.

Examples

julia> R, (w, x, y, z) = graded_polynomial_ring(QQ, [:w, :x, :y, :z]);

julia> I = ideal(R, [x*z, y*z, x*w^2, y*w^2])
Ideal generated by
  x*z
  y*z
  w^2*x
  w^2*y

julia> A, _= quo(R, I)
(Quotient of multivariate polynomial ring by ideal (x*z, y*z, w^2*x, w^2*y), Map: R -> A)

julia> FA  = free_resolution(A)
Free resolution of A
R^1 <---- R^4 <---- R^4 <---- R^1 <---- 0
0         1         2         3         4

julia> betti_table(FA)
degree: 0  1  2  3
------------------
     0: 1  -  -  -
     1: -  2  1  -
     2: -  2  3  1
------------------
 total: 1  4  4  1

julia> R, (x, y) = graded_polynomial_ring(QQ, [:x, :y]);

julia> I = ideal(R, [x, y, x+y]);

julia> M = quotient_ring_as_module(I);

julia> FM = free_resolution(M, algorithm = :nres);

julia> betti_table(FM)
degree: 0  1  2
---------------
     0: 1  2  1
---------------
 total: 1  2  1

minimal_betti_table(F::FreeResolution{T}; check::Bool=true) where {T<:ModuleFP}

Given a graded free resolution F over a standard $\mathbb Z$-graded multivariate polynomial ring with coefficients in a field, return the Betti table of the minimal free resolution arising from F.

Examples

julia> R, (w, x, y, z) = graded_polynomial_ring(QQ, [:w, :x, :y, :z]);

julia> I = ideal(R, [w^2-x*z, w*x-y*z, x^2-w*y, x*y-z^2, y^2-w*z]);

julia> A, _ = quo(R, I)
(Quotient of multivariate polynomial ring by ideal (w^2 - x*z, w*x - y*z, -w*y + x^2, x*y - z^2, -w*z + y^2), Map: R -> A)

julia> FA = free_resolution(A)
Free resolution of A
R^1 <---- R^5 <---- R^6 <---- R^2 <---- 0
0         1         2         3         4

julia> betti_table(FA)
degree: 0  1  2  3
------------------
     0: 1  -  -  -
     1: -  5  5  1
     2: -  -  1  1
------------------
 total: 1  5  6  2

julia> minimal_betti_table(FA)
degree: 0  1  2  3
------------------
     0: 1  -  -  -
     1: -  5  5  -
     2: -  -  -  1
------------------
 total: 1  5  5  1

minimal_betti_table(M::ModuleFP{T}) where {T<:MPolyDecRingElem}
minimal_betti_table(A::MPolyQuoRing{T}) where {T<:MPolyDecRingElem}
minimal_betti_table(I::MPolyIdeal{T}) where {T<:MPolyDecRingElem}

Given a finitely presented graded module M over a standard $\mathbb Z$-graded multivariate polynomial ring with coefficients in a field, return the Betti Table of the minimal free resolution of M. Similarly for A and I.

Examples

julia> R, (w, x, y, z) = graded_polynomial_ring(QQ, [:w, :x, :y, :z]);

julia> I = ideal(R, [w^2-x*z, w*x-y*z, x^2-w*y, x*y-z^2, y^2-w*z]);

julia> A, _ = quo(R, I)
(Quotient of multivariate polynomial ring by ideal (w^2 - x*z, w*x - y*z, -w*y + x^2, x*y - z^2, -w*z + y^2), Map: R -> A)

julia> minimal_betti_table(A)
degree: 0  1  2  3
------------------
     0: 1  -  -  -
     1: -  5  5  -
     2: -  -  -  1
------------------
 total: 1  5  5  1

cm_regularity(M::ModuleFP; check::Bool=true)

Given a finitely presented graded module M over a standard $\mathbb Z$-graded multivariate polynomial ring with coefficients in a field, return the Castelnuovo-Mumford regularity of M.

cm_regularity(I::MPolyIdeal)

Given a (homogeneous) ideal I in a standard $\mathbb Z$-graded multivariate polynomial ring with coefficients in a field, return the Castelnuovo-Mumford regularity of I.

Examples

julia> R, (x, y, z) = graded_polynomial_ring(QQ, [:x, :y, :z]);

julia> F = graded_free_module(R, 1);

julia> M, _ = quo(F, [x^2*F[1], y^2*F[1], z^2*F[1]])
(Graded subquotient of graded submodule of F with 1 generator
  1: e[1]
by graded submodule of F with 3 generators
  1: x^2*e[1]
  2: y^2*e[1]
  3: z^2*e[1], Hom: F -> M)

julia> cm_regularity(M)
3

julia> minimal_betti_table(M)
degree: 0  1  2  3
------------------
     0: 1  -  -  -
     1: -  3  -  -
     2: -  -  3  -
     3: -  -  -  1
------------------
 total: 1  3  3  1 

julia> R, (w, x, y, z) = graded_polynomial_ring(QQ, [:w, :x, :y, :z]);

julia> I = ideal(R, [-x*z+y^2, x*y-w*z, x^2-w*y]);

julia> cm_regularity(I)
2

julia> A, _ = quo(R, I);

julia> minimal_betti_table(A)
degree: 0  1  2
---------------
     0: 1  -  -
     1: -  3  2
---------------
 total: 1  3  2 

homology(C::ComplexOfMorphisms{<:ModuleFP})

Return the homology of C.

Examples

julia> R, (x,) = polynomial_ring(QQ, [:x]);

julia> F = free_module(R, 1);

julia> A, _ = quo(F, [x^4*F[1]]);

julia> B, _ = quo(F, [x^3*F[1]]);

julia> a = hom(A, B, [x^2*B[1]]);

julia> b = hom(B, B, [x^2*B[1]]);

julia> C = ComplexOfMorphisms(ModuleFP, [a, b]);

julia> H = homology(C)
3-element Vector{SubquoModule{QQMPolyRingElem}}:
 Subquotient of submodule with 1 generator
  1: x*e[1]
by submodule with 1 generator
  1: x^4*e[1]
 Subquotient of submodule with 1 generator
  1: x*e[1]
by submodule with 2 generators
  1: x^3*e[1]
  2: x^2*e[1]
 Subquotient of submodule with 1 generator
  1: e[1]
by submodule with 2 generators
  1: x^3*e[1]
  2: x^2*e[1]

homology(C::ComplexOfMorphisms{<:ModuleFP}, i::Int)

Return the i-th homology module of C.

Examples

julia> R, (x,) = polynomial_ring(QQ, [:x]);

julia> F = free_module(R, 1);

julia> A, _ = quo(F, [x^4*F[1]]);

julia> B, _ = quo(F, [x^3*F[1]]);

julia> a = hom(A, B, [x^2*B[1]]);

julia> b = hom(B, B, [x^2*B[1]]);

julia> C = ComplexOfMorphisms(ModuleFP, [a, b]);

julia> H = homology(C, 1)
Subquotient of submodule with 1 generator
  1: x*e[1]
by submodule with 2 generators
  1: x^3*e[1]
  2: x^2*e[1]

hom(M::ModuleFP, N::ModuleFP; algorithm::Symbol=:maps)

Return the module Hom(M,N) as an object of type SubquoModule.

Additionally, if H is that object, return the map which sends an element of H to the corresponding homomorphism M $\to$ N.

The keyword algorithm can be set to :maps for the default algorithm or to :matrices for an alternative based on matrices.

Examples

julia> R, (x, y) = polynomial_ring(QQ, [:x, :y]);

julia> F = FreeMod(R, 2);

julia> V = [x*F[1], y^2*F[2]];

julia> M = quo_object(F, V)
Subquotient of submodule with 2 generators
  1: e[1]
  2: e[2]
by submodule with 2 generators
  1: x*e[1]
  2: y^2*e[2]

julia> H = hom(M, M)[1]
hom of (M, M)

julia> gens(H)
2-element Vector{SubquoModuleElem{QQMPolyRingElem}}:
 (e[1] -> e[1])
 (e[2] -> e[2])

julia> relations(H)
4-element Vector{FreeModElem{QQMPolyRingElem}}:
 x*(e[1] -> e[1])
 y^2*(e[1] -> e[2])
 x*(e[2] -> e[1])
 y^2*(e[2] -> e[2])

element_to_homomorphism(f::ModuleFPElem)

If f is an element of a module created via hom(M,N), for some modules M and N, return the homomorphism M $\to$ N corresponding to f.

Examples

julia> R, (x, y) = polynomial_ring(QQ, [:x, :y]);

julia> F = FreeMod(R, 2);

julia> V = [x*F[1], y^2*F[2]];

julia> M = quo_object(F, V)
Subquotient of submodule with 2 generators
  1: e[1]
  2: e[2]
by submodule with 2 generators
  1: x*e[1]
  2: y^2*e[2]

julia> H = hom(M, M)[1];

julia> gens(H)
2-element Vector{SubquoModuleElem{QQMPolyRingElem}}:
 (e[1] -> e[1])
 (e[2] -> e[2])

julia> relations(H)
4-element Vector{FreeModElem{QQMPolyRingElem}}:
 x*(e[1] -> e[1])
 y^2*(e[1] -> e[2])
 x*(e[2] -> e[1])
 y^2*(e[2] -> e[2])

julia> a = element_to_homomorphism(H[1]+y*H[2])
Module homomorphism
  from M
  to M

julia> matrix(a)
[1   0]
[0   y]

homomorphism_to_element(H::ModuleFP, a::ModuleFPHom)

If the module H is created via hom(M,N), for some modules M and N, and a: M $\to$ N is a homomorphism, then return the element of H corresponding to a.

Examples

julia> R, (x, y) = polynomial_ring(QQ, [:x, :y]);

julia> F = FreeMod(R, 2);

julia> V = [x*F[1], y^2*F[2]];

julia> M = quo_object(F, V)
Subquotient of submodule with 2 generators
  1: e[1]
  2: e[2]
by submodule with 2 generators
  1: x*e[1]
  2: y^2*e[2]

julia> H = hom(M, M)[1];

julia> gens(H)
2-element Vector{SubquoModuleElem{QQMPolyRingElem}}:
 (e[1] -> e[1])
 (e[2] -> e[2])

julia> relations(H)
4-element Vector{FreeModElem{QQMPolyRingElem}}:
 x*(e[1] -> e[1])
 y^2*(e[1] -> e[2])
 x*(e[2] -> e[1])
 y^2*(e[2] -> e[2])

julia> W =  [M[1], y*M[2]];

julia> a = hom(M, M, W);

julia> is_welldefined(a)
true

julia> matrix(a)
[1   0]
[0   y]

julia> m = homomorphism_to_element(H, a)
(e[1] -> e[1]) + y*(e[2] -> e[2])

ext(M::ModuleFP, N::ModuleFP, i::Int)

Return $\text{Ext}^i(M,N)$.

Examples

julia> R, (x, y) = polynomial_ring(QQ, [:x, :y]);

julia> F = FreeMod(R, 1);

julia> V = [x*F[1], y*F[1]];

julia> M = quo_object(F, V)
Subquotient of submodule with 1 generator
  1: e[1]
by submodule with 2 generators
  1: x*e[1]
  2: y*e[1]

julia> ext(M, M, 0)
Subquotient of submodule with 1 generator
  1: (e[1] -> e[1])
by submodule with 2 generators
  1: y*(e[1] -> e[1])
  2: x*(e[1] -> e[1])

julia> ext(M, M, 1)
Subquotient of submodule with 2 generators
  1: (e[1] -> e[1])
  2: (e[2] -> e[1])
by submodule with 4 generators
  1: y*(e[1] -> e[1])
  2: x*(e[1] -> e[1])
  3: y*(e[2] -> e[1])
  4: x*(e[2] -> e[1])

julia> ext(M, M, 2)
Subquotient of submodule with 1 generator
  1: (e[1] -> e[1])
by submodule with 2 generators
  1: y*(e[1] -> e[1])
  2: x*(e[1] -> e[1])

julia> ext(M, M, 3)
Submodule with 0 generators
represented as subquotient with no relations

tensor_product(M::ModuleFP...; task::Symbol = :none)

Given a collection of modules, say, $M_1, \dots, M_n$ over a ring $R$, return $M_1\otimes_R \cdots \otimes_R M_n$.

If task = :map, additionally return the map which sends a tuple $(m_1,\dots, m_n)$ of elements $m_i\in M_i$ to the pure tensor $m_1\otimes\dots\otimes m_n$.

Examples

julia> R, (x, y, z) = polynomial_ring(QQ, [:x, :y, :z]);

julia> F = free_module(R, 1);

julia> A = R[x; y];

julia> B = R[x^2; y^3; z^4];

julia> M = SubquoModule(F, A, B);

julia> gens(M)
2-element Vector{SubquoModuleElem{QQMPolyRingElem}}:
 x*e[1]
 y*e[1]

julia> T, t = tensor_product(M, M; task = :map);

julia> gens(T)
4-element Vector{SubquoModuleElem{QQMPolyRingElem}}:
 x^2*e[1] \otimes e[1]
 x*y*e[1] \otimes e[1]
 x*y*e[1] \otimes e[1]
 y^2*e[1] \otimes e[1]

julia> domain(t)
parent of tuples of type Tuple{SubquoModuleElem{QQMPolyRingElem}, SubquoModuleElem{QQMPolyRingElem}}

julia> t((M[1], M[2]))
x*y*e[1] \otimes e[1]

tor(M::ModuleFP, N::ModuleFP, i::Int)

Return $\text{Tor}_i(M,N)$.

Examples

julia> R, (x, y, z) = polynomial_ring(QQ, [:x, :y, :z]);

julia> A = R[x; y];

julia> B = R[x^2; y^3; z^4];

julia> M = SubquoModule(A, B);

julia> F = free_module(R, 1);

julia> Q, _ = quo(F, [x*F[1]]);

julia> T0 = tor(Q, M, 0)
Subquotient of submodule with 2 generators
  1: x*e[1] \otimes e[1]
  2: y*e[1] \otimes e[1]
by submodule with 4 generators
  1: x^2*e[1] \otimes e[1]
  2: y^3*e[1] \otimes e[1]
  3: z^4*e[1] \otimes e[1]
  4: x*y*e[1] \otimes e[1]

julia> T1 = tor(Q, M, 1)
Subquotient of submodule with 2 generators
  1: x*e[1] \otimes e[1]
  2: x*y*e[1] \otimes e[1]
by submodule with 3 generators
  1: x^2*e[1] \otimes e[1]
  2: y^3*e[1] \otimes e[1]
  3: z^4*e[1] \otimes e[1]

julia> T2 =  tor(Q, M, 2)
Submodule with 0 generators
represented as subquotient with no relations

fitting_ideal(M::ModuleFP{T}, i::Int) where T <: MPolyRingElem

Return the i-th Fitting ideal of M.

Examples

julia> R, (x, y) = polynomial_ring(QQ, [:x, :y]);

julia> F = free_module(R, 2);

julia> o = zero(R)
0

julia> U = matrix([x^3-y^2 o; o x^3-y^2; -x^2 y; -y x])
[x^3 - y^2           0]
[        0   x^3 - y^2]
[     -x^2           y]
[       -y           x]

julia> M = quo_object(F,U)
Subquotient of submodule with 2 generators
  1: e[1]
  2: e[2]
by submodule with 4 generators
  1: (x^3 - y^2)*e[1]
  2: (x^3 - y^2)*e[2]
  3: -x^2*e[1] + y*e[2]
  4: -y*e[1] + x*e[2]

julia> fitting_ideal(M, -1)
Ideal generated by
  0

julia> fitting_ideal(M, 0)
Ideal generated by
  x^3 - y^2

julia> fitting_ideal(M, 1)
Ideal generated by
  y
  x

julia> fitting_ideal(M, 2)
Ideal generated by
  1

is_flat(M::ModuleFP{T}) where T <: MPolyRingElem

Return true if M is flat, false otherwise.

Examples

julia> R, (x, y) = polynomial_ring(QQ, [:x, :y]);

julia> F = free_module(R, 2);

julia> o = zero(R)
0

julia> U = matrix([x^3-y^2 o; o x^3-y^2; -x^2 y; -y x])
[x^3 - y^2           0]
[        0   x^3 - y^2]
[     -x^2           y]
[       -y           x]

julia> M = quo_object(F,U)
Subquotient of submodule with 2 generators
  1: e[1]
  2: e[2]
by submodule with 4 generators
  1: (x^3 - y^2)*e[1]
  2: (x^3 - y^2)*e[2]
  3: -x^2*e[1] + y*e[2]
  4: -y*e[1] + x*e[2]

julia> is_flat(M)
false

non_flat_locus(M::ModuleFP{T}) where T <: MPolyRingElem

Return an ideal of base_ring(M) which defines the non-flat-locus of M in the sense that the localization of M at a prime ideal of base_ring(M) is non-flat iff the prime ideal contains the returned ideal.

Examples

julia> R, (x, y) = polynomial_ring(QQ, [:x, :y]);

julia> F = free_module(R, 2);

julia> o = zero(R)
0

julia> U = matrix([x^3-y^2 o; o x^3-y^2; -x^2 y; -y x])
[x^3 - y^2           0]
[        0   x^3 - y^2]
[     -x^2           y]
[       -y           x]

julia> M = quo_object(F,U)
Subquotient of submodule with 2 generators
  1: e[1]
  2: e[2]
by submodule with 4 generators
  1: (x^3 - y^2)*e[1]
  2: (x^3 - y^2)*e[2]
  3: -x^2*e[1] + y*e[2]
  4: -y*e[1] + x*e[2]

julia> non_flat_locus(M)
Ideal generated by
  x^3 - y^2

is_regular_sequence(V::Vector{T}, M::ModuleFP{T}) where T <: MPolyRingElem

Return true if the elements of V form, in the given order, a regular sequence on M. Return false, otherwise.

Examples

julia> R, (x, y, z) = polynomial_ring(QQ, [:x, :y, :z]);

julia> F = free_module(R, 1);

julia> V =  [x*z-z, x*y-y, x]
3-element Vector{QQMPolyRingElem}:
 x*z - z
 x*y - y
 x

julia> is_regular_sequence(V, F)
false

julia> W = [x*z-z, x, x*y-y]
3-element Vector{QQMPolyRingElem}:
 x*z - z
 x
 x*y - y

julia> is_regular_sequence(W, F)
true

koszul_matrix(V::Vector{T}, p::Int) where T <: MPolyRingElem

If $f_1, \dots, f_r$ are the entries of V in the given order, return the matrix representing the p-th map of the Koszul complex $K(f_1, \dots, f_r)$.

Examples

julia> R, (x, y, z) = polynomial_ring(QQ, [:x, :y, :z]);

julia> V = gens(R)
3-element Vector{QQMPolyRingElem}:
 x
 y
 z

julia> koszul_matrix(V, 3)
[x   y   z]

julia> koszul_matrix(V, 2)
[-y   -z    0]
[ x    0   -z]
[ 0    x    y]

julia> koszul_matrix(V, 1)
[ z]
[-y]
[ x]

koszul_complex(V::Vector{T}) where T <: MPolyRingElem

If $f_1, \dots, f_r$ are the entries of V in the given order, return the Koszul complex $K(f_1, \dots, f_r)$.

Examples

julia> R, (x, y, z) = polynomial_ring(QQ, [:x, :y, :z]);

julia> V = gens(R)
3-element Vector{QQMPolyRingElem}:
 x
 y
 z

julia> K = koszul_complex(V);

julia> matrix(map(K, 2))
[-y   -z    0]
[ x    0   -z]
[ 0    x    y]

julia> Kd = hom(K, free_module(R, 1));

julia> matrix(map(Kd, 1))
[-y    x   0]
[-z    0   x]
[ 0   -z   y]

koszul_homology(V::Vector{T}, M::ModuleFP{T}, p::Int) where T <: MPolyRingElem

If $f_1, \dots, f_r$ are the entries of V in the given order, return the p-th homology module of the complex $K(f_1, \dots, f_r)\otimes_R M$, where $K(f_1, \dots, f_r)$ is the Koszul complex defined by $f_1, \dots, f_r$.

Examples

julia> R, (x, y, z) = polynomial_ring(QQ, [:x, :y, :z]);

julia> F = free_module(R, 1);

julia> V =  [x*y, x*z, y*z]
3-element Vector{QQMPolyRingElem}:
 x*y
 x*z
 y*z

julia> koszul_homology(V, F, 0)
Subquotient of submodule with 1 generator
  1: e[1]^e[2]^e[3]
by submodule with 3 generators
  1: y*z*e[1]^e[2]^e[3]
  2: -x*z*e[1]^e[2]^e[3]
  3: x*y*e[1]^e[2]^e[3]

julia> koszul_homology(V, F, 1)
Subquotient of submodule with 2 generators
  1: y*e[1]^e[3] \otimes e[1] + z*e[2]^e[3] \otimes e[1]
  2: x*e[1]^e[2] \otimes e[1] - z*e[2]^e[3] \otimes e[1]
by submodule with 3 generators
  1: -x*z*e[1]^e[2] \otimes e[1] - y*z*e[1]^e[3] \otimes e[1]
  2: x*y*e[1]^e[2] \otimes e[1] - y*z*e[2]^e[3] \otimes e[1]
  3: x*y*e[1]^e[3] \otimes e[1] + x*z*e[2]^e[3] \otimes e[1]

julia> koszul_homology(V, F, 2)
Subquotient of submodule with 1 generator
  1: x*y*e[1] \otimes e[1] + x*z*e[2] \otimes e[1] + y*z*e[3] \otimes e[1]
by submodule with 1 generator
  1: x*y*e[1] \otimes e[1] + x*z*e[2] \otimes e[1] + y*z*e[3] \otimes e[1]

julia> R, (w, x, y, z) = polynomial_ring(QQ, [:w, :x, :y, :z]);

julia> TC = ideal(R, [x*z-y^2, w*z-x*y, w*y-x^2]);

julia> F = free_module(R, 1);

julia> koszul_homology(gens(TC), F, 0)
Subquotient of submodule with 1 generator
  1: e[1]^e[2]^e[3]
by submodule with 3 generators
  1: (w*y - x^2)*e[1]^e[2]^e[3]
  2: (-w*z + x*y)*e[1]^e[2]^e[3]
  3: (x*z - y^2)*e[1]^e[2]^e[3]

julia> koszul_homology(gens(TC), F, 1)
Subquotient of submodule with 2 generators
  1: z*e[1]^e[2] \otimes e[1] + y*e[1]^e[3] \otimes e[1] + x*e[2]^e[3] \otimes e[1]
  2: y*e[1]^e[2] \otimes e[1] + x*e[1]^e[3] \otimes e[1] + w*e[2]^e[3] \otimes e[1]
by submodule with 3 generators
  1: (-w*z + x*y)*e[1]^e[2] \otimes e[1] + (-w*y + x^2)*e[1]^e[3] \otimes e[1]
  2: (x*z - y^2)*e[1]^e[2] \otimes e[1] + (-w*y + x^2)*e[2]^e[3] \otimes e[1]
  3: (x*z - y^2)*e[1]^e[3] \otimes e[1] + (w*z - x*y)*e[2]^e[3] \otimes e[1]

julia> koszul_homology(gens(TC), F, 2)
Subquotient of submodule with 1 generator
  1: (-x*z + y^2)*e[1] \otimes e[1] + (-w*z + x*y)*e[2] \otimes e[1] + (-w*y + x^2)*e[3] \otimes e[1]
by submodule with 1 generator
  1: (x*z - y^2)*e[1] \otimes e[1] + (w*z - x*y)*e[2] \otimes e[1] + (w*y - x^2)*e[3] \otimes e[1]

depth(I::MPolyIdeal{T}, M::ModuleFP{T}) where T <: MPolyRingElem

Return the depth of I on M.

Examples

julia> R, (w, x, y, z) = polynomial_ring(QQ, [:w, :x, :y, :z]);

julia> TC = ideal(R, [x*z-y^2, w*z-x*y, w*y-x^2]);

julia> dim(TC)
2

julia> F = free_module(R, 1);

julia> U = collect(gen(TC, i)*F[1] for i in 1:ngens(TC));

julia> M, _ = quo(F, U);

julia> I = ideal(R, gens(R))
Ideal generated by
  w
  x
  y
  z

julia> depth(I, M)
2

julia> S, x, y = polynomial_ring(QQ, :x => 1:3, :y => 1:5);

julia> W = [y[1]-x[1]^2,  y[2]-x[2]^2,   y[3]-x[3]^2, y[4]-x[2]*(x[1]-x[3]),  y[5]-(x[1]-x[2])*x[3]];

julia> J = eliminate(ideal(S, W), x);

julia> R, y = polynomial_ring(QQ, :y => 1:5);

julia> W = append!(repeat([zero(R)], 3), gens(R))
8-element Vector{QQMPolyRingElem}:
 0
 0
 0
 y[1]
 y[2]
 y[3]
 y[4]
 y[5]

julia> P = hom(S, R, W);

julia> VP4 = P(J);

julia> dim(VP4)
3

julia> F = free_module(R, 1);

julia> U = collect(gen(VP4, i)*F[1] for i in 1:ngens(VP4));

julia> M, _ = quo(F, U);

julia> I = ideal(R, gens(R))
Ideal generated by
  y[1]
  y[2]
  y[3]
  y[4]
  y[5]

julia> depth(I, M)
1