Cartan eilenberg resolutions (#4248)

experimental/DoubleAndHyperComplexes/src/DoubleAndHyperComplexes.jl

@@ -11,6 +11,7 @@ include("Objects/new_complex_template.jl")
 include("Objects/linear_strands.jl")
 
 include("Morphisms/Types.jl")
+include("Objects/cartan_eilenberg_resolution.jl")
 include("Morphisms/cartan_eilenberg_resolutions.jl")
 include("Morphisms/ext.jl")
 include("Morphisms/simplified_complexes.jl")
@@ -25,3 +26,4 @@ include("Exports.jl")
 
 include("base_change_types.jl")
 include("base_change.jl")
+

experimental/DoubleAndHyperComplexes/src/Morphisms/Types.jl

@@ -142,3 +142,23 @@ end
 underlying_complex(c::SimpleFreeResolution) = c.underlying_complex
 original_module(c::SimpleFreeResolution) = c.M
 
+### Lifting morphisms through projective resolutions
+mutable struct MapLifter{MorphismType} <: HyperComplexMorphismFactory{MorphismType}
+  dom::AbsHyperComplex
+  cod::AbsHyperComplex
+  orig_map::Map
+  start_index::Int
+  offset::Int
+  check::Bool
+
+  function MapLifter(::Type{MorphismType}, dom::AbsHyperComplex, cod::AbsHyperComplex, phi::Map; 
+      start_index::Int=0, offset::Int=0, check::Bool=true
+    ) where {MorphismType}
+    @assert dim(dom) == 1 && dim(cod) == 1 "lifting of maps is only implemented in dimension one"
+    @assert (is_chain_complex(dom) && is_chain_complex(cod)) || (is_cochain_complex(dom) && is_cochain_complex(cod))
+    @assert domain(phi) === dom[start_index]
+    @assert codomain(phi) === cod[start_index + offset]
+    return new{MorphismType}(dom, cod, phi, start_index, offset)
+  end
+end
+

experimental/DoubleAndHyperComplexes/src/Morphisms/cartan_eilenberg_resolutions.jl

@@ -1,22 +1,4 @@
-mutable struct MapLifter{MorphismType} <: HyperComplexMorphismFactory{MorphismType}
-  dom::AbsHyperComplex
-  cod::AbsHyperComplex
-  orig_map::Map
-  start_index::Int
-  offset::Int
-  check::Bool
-
-  function MapLifter(::Type{MorphismType}, dom::AbsHyperComplex, cod::AbsHyperComplex, phi::Map; 
-      start_index::Int=0, offset::Int=0, check::Bool=true
-    ) where {MorphismType}
-    @assert dim(dom) == 1 && dim(cod) == 1 "lifting of maps is only implemented in dimension one"
-    @assert (is_chain_complex(dom) && is_chain_complex(cod)) || (is_cochain_complex(dom) && is_cochain_complex(cod))
-    @assert domain(phi) === dom[start_index]
-    @assert codomain(phi) === cod[start_index + offset]
-    return new{MorphismType}(dom, cod, phi, start_index, offset)
-  end
-end
-
+### Lifting maps through projective resolutions
 is_chain_complex(c::AbsHyperComplex) = (dim(c) == 1 ? direction(c, 1) == (:chain) : error("complex is not one-dimensional"))
 is_cochain_complex(c::AbsHyperComplex) = (dim(c) == 1 ? direction(c, 1) == (:cochain) : error("complex is not one-dimensional"))
 
@@ -57,3 +39,6 @@ function lift_map(dom::AbsHyperComplex{DomChainType}, cod::AbsHyperComplex{CodCh
 end
 
 morphism_type(::Type{T1}, ::Type{T2}) where {T1<:ModuleFP, T2<:ModuleFP} = ModuleFPHom{<:T1, <:T2}
+
+### Cartan Eilenberg resolutions
+

experimental/DoubleAndHyperComplexes/src/Objects/Types.jl

@@ -93,7 +93,7 @@ end
       upper_bounds::Vector=[nothing for i in 1:d],
       lower_bounds::Vector=[nothing for i in 1:d]
     ) where {ChainType, MorphismType}
-    @assert d > 0 "can not create zero or negative dimensional hypercomplex"
+    @assert d >= 0 "can not create negative dimensional hypercomplex"
     chains = Dict{Tuple, ChainType}()
     morphisms = Dict{Tuple, Dict{Int, <:MorphismType}}()
     return new{ChainType, MorphismType}(d, chains, morphisms, 

experimental/DoubleAndHyperComplexes/src/Objects/cartan_eilenberg_resolution.jl

@@ -0,0 +1,212 @@
+#= Cartan Eilenberg resolutions of 1-dimensional complexes
+#
+#  Suppose 
+#
+#    0 ← C₀  ← C₁  ← C₂  ← …
+#
+#  is a bounded below complex. We compute a double complex
+#
+#         0     0     0
+#         ↑     ↑     ↑
+#    0 ← P₀₀ ← P₀₁ ← P₀₂ ← …
+#         ↑     ↑     ↑
+#    0 ← P₁₀ ← P₁₁ ← P₁₂ ← …
+#         ↑     ↑     ↑
+#    0 ← P₂₀ ← P₂₁ ← P₂₂ ← …
+#         ↑     ↑     ↑
+#         ⋮     ⋮     ⋮
+#
+#  whose total complex is quasi-isomorphic to C via some augmentation map 
+#
+#    ε = (εᵢ : P₀ᵢ → Cᵢ)ᵢ
+#
+#  The challenge is that if we were only computing resolutions of the Cᵢ's 
+#  and lifting the maps, then the rows of the resulting diagrams would 
+#  not necessarily form complexes. To accomplish that, we split the original 
+#  complex into short exact sequences
+#
+#    0 ← Bᵢ ← Cᵢ ← Zᵢ ← 0
+#
+#  and apply the Horse shoe lemma to these. Together with the induced maps 
+#  from Bᵢ ↪  Zᵢ₋₁ we get the desired double complex.
+#
+#  If the original complex C is known to be exact, then there is no need 
+#  to compute the resolutions of both Bᵢ and Zᵢ and we can shorten the procedure.
+=#
+### Production of the chains
+struct CEChainFactory{ChainType} <: HyperComplexChainFactory{ChainType}
+  c::AbsHyperComplex
+  is_exact::Bool
+  kernel_resolutions::Dict{Int, <:AbsHyperComplex} # the kernels of  Cᵢ → Cᵢ₋₁
+  boundary_resolutions::Dict{Int, <:AbsHyperComplex} # the boundaries of Cᵢ₊₁ → Cᵢ
+  induced_maps::Dict{Int, <:AbsHyperComplexMorphism} # the induced maps from the free 
+                                                     # resolutions of the boundary and kernel
+
+  function CEChainFactory(c::AbsHyperComplex; is_exact::Bool=false)
+    @assert dim(c) == 1 "complex must be 1-dimensional"
+    #@assert has_lower_bound(c, 1) "complex must be bounded from below"
+    return new{chain_type(c)}(c, is_exact, Dict{Int, AbsHyperComplex}(), Dict{Int, AbsHyperComplex}(), Dict{Int, AbsHyperComplexMorphism}())
+  end
+end
+
+function kernel_resolution(fac::CEChainFactory, i::Int)
+  if !haskey(fac.kernel_resolutions, i)
+    Z, _ = kernel(fac.c, i)
+    fac.kernel_resolutions[i] = free_resolution(SimpleFreeResolution, Z)[1]
+  end
+  return fac.kernel_resolutions[i]
+end
+
+function boundary_resolution(fac::CEChainFactory, i::Int)
+  if !haskey(fac.boundary_resolutions, i)
+    Z, _ = boundary(fac.c, i)
+    fac.boundary_resolutions[i] = free_resolution(SimpleFreeResolution, Z)[1]
+  end
+  return fac.boundary_resolutions[i]
+end
+
+function induced_map(fac::CEChainFactory, i::Int)
+  if !haskey(fac.induced_maps, i)
+    Z, inc = kernel(fac.c, i)
+    B, pr = boundary(fac.c, i)
+    @assert ambient_free_module(Z) === ambient_free_module(B)
+    img_gens = elem_type(Z)[Z(g) for g in ambient_representatives_generators(B)]
+    res_Z = kernel_resolution(fac, i)
+    res_B = boundary_resolution(fac, i)
+    aug_Z = augmentation_map(res_Z)
+    aug_B = augmentation_map(res_B)
+    img_gens = gens(res_B[0])
+    img_gens = aug_B[0].(img_gens)
+    img_gens = elem_type(res_Z[0])[preimage(aug_Z[0], Z(repres(aug_B[0](g)))) for g in gens(res_B[0])]
+    psi = hom(res_B[0], res_Z[0], img_gens; check=true) # TODO: Set to false
+    @assert domain(psi) === boundary_resolution(fac, i)[0]
+    @assert codomain(psi) === kernel_resolution(fac, i)[0]
+    fac.induced_maps[i] = lift_map(boundary_resolution(fac, i), kernel_resolution(fac, i), psi; start_index=0)
+  end
+  return fac.induced_maps[i]
+end
+
+function (fac::CEChainFactory)(self::AbsHyperComplex, I::Tuple)
+  (i, j) = I # i the resolution index, j the index in C
+
+  res_Z = kernel_resolution(fac, j)
+
+  if can_compute_map(fac.c, 1, (j,))
+    if fac.is_exact # Use the next kernel directly
+      res_B = kernel_resolution(fac, j-1)
+      return direct_sum(res_B[i], res_Z[i])[1]
+    else
+      res_B = boundary_resolution(fac, j-1)
+      return direct_sum(res_B[i], res_Z[i])[1]
+    end
+  end
+  # We may assume that the next map can not be computed and is, hence, zero.
+  return res_Z[i]
+end
+
+function can_compute(fac::CEChainFactory, self::AbsHyperComplex, I::Tuple)
+  (i, j) = I
+  can_compute_index(fac.c, (j,)) || return false
+  return i >= 0
+end
+
+### Production of the morphisms 
+struct CEMapFactory{MorphismType} <: HyperComplexMapFactory{MorphismType} end
+
+function (fac::CEMapFactory)(self::AbsHyperComplex, p::Int, I::Tuple)
+  (i, j) = I
+  cfac = chain_factory(self)
+  if p == 1 # vertical upwards maps
+    if can_compute_map(cfac.c, 1, (j,))
+      # both dom and cod are direct sums in this case
+      dom = self[I]
+      cod = self[(i-1, j)]
+      pr1 = canonical_projection(dom, 1)
+      pr2 = canonical_projection(dom, 2)
+      @assert domain(pr1) === domain(pr2) === dom
+      inc1 = canonical_injection(cod, 1)
+      inc2 = canonical_injection(cod, 2)
+      @assert codomain(inc1) === codomain(inc2) === cod
+      res_Z = kernel_resolution(cfac, j)
+      @assert domain(map(res_Z, i)) === codomain(pr2)
+      @assert codomain(map(res_Z, i)) === domain(inc2)
+      res_B = boundary_resolution(cfac, j-1)
+      @assert domain(map(res_B, i)) === codomain(pr1)
+      @assert codomain(map(res_B, i)) === domain(inc1)
+      return compose(pr1, compose(map(res_B, i), inc1)) + compose(pr2, compose(map(res_Z, i), inc2))
+    else
+      res_Z = kernel_resolution(cfac, j)
+      return map(res_Z, i)
+    end
+    error("execution should never reach this point")
+  elseif p == 2 # the horizontal maps
+    dom = self[I]
+    cod = self[(i, j-1)]
+    if can_compute_map(cfac.c, 1, (j-1,))
+      # the codomain is also a direct sum
+      if !cfac.is_exact
+        psi = induced_map(cfac, j-1)
+        phi = psi[i]
+        inc = canonical_injection(cod, 2)
+        pr = canonical_projection(dom, 1)
+        @assert codomain(phi) === domain(inc)
+        @assert codomain(pr) === domain(phi)
+        return compose(pr, compose(phi, inc))
+      else
+        inc = canonical_injection(cod, 2)
+        pr = canonical_projection(dom, 1)
+        return compose(pr, inc)
+      end
+      error("execution should never reach this point")
+    else
+      # the codomain is just the kernel
+      if !cfac.is_exact
+        psi = induced_map(cfac, j-1)
+        phi = psi[i]
+        pr = canonical_projection(dom, 1)
+        return compose(pr, phi)
+      else
+        pr = canonical_projection(dom, 1)
+        return pr
+      end
+      error("execution should never reach this point")
+    end
+    error("execution should never reach this point")
+  end
+  error("direction $p out of bounds")
+end
+
+function can_compute(fac::CEMapFactory, self::AbsHyperComplex, p::Int, I::Tuple)
+  (i, j) = I
+  if p == 1 # vertical maps
+    return i > 0 && can_compute(chain_factory(self).c, j)
+  elseif p == 2 # horizontal maps
+    return i >= 0 && can_compute_map(chain_factory(self).c, j)
+  end
+  return false
+end
+
+### The concrete struct
+@attributes mutable struct CartanEilenbergResolution{ChainType, MorphismType} <: AbsHyperComplex{ChainType, MorphismType} 
+  internal_complex::HyperComplex{ChainType, MorphismType}
+
+  function CartanEilenbergResolution(
+      c::AbsHyperComplex{ChainType, MorphismType};
+      is_exact::Bool=false
+    ) where {ChainType, MorphismType}
+    @assert dim(c) == 1 "complexes must be 1-dimensional"
+    @assert has_lower_bound(c, 1) "complexes must be bounded from below"
+    @assert direction(c, 1) == :chain "resolutions are only implemented for chain complexes"
+    chain_fac = CEChainFactory(c; is_exact)
+    map_fac = CEMapFactory{MorphismType}() # TODO: Do proper type inference here!
+
+    # Assuming d is the dimension of the new complex
+    internal_complex = HyperComplex(2, chain_fac, map_fac, [:chain, :chain]; lower_bounds = Union{Int, Nothing}[0, lower_bound(c, 1)])
+    # Assuming that ChainType and MorphismType are provided by the input
+    return new{ChainType, MorphismType}(internal_complex)
+  end
+end
+
+### Implementing the AbsHyperComplex interface via `underlying_complex`
+underlying_complex(c::CartanEilenbergResolution) = c.internal_complex
+

experimental/DoubleAndHyperComplexes/src/base_change_types.jl

@@ -58,7 +58,13 @@ end
     map_fac = BaseChangeMapFactory(phi, orig)
 
     d = dim(orig)
-    internal_complex = HyperComplex(d, chain_fac, map_fac, [direction(orig, i) for i in 1:d])
+    upper_bounds = Vector{Union{Int, Nothing}}([(has_upper_bound(orig, i) ? upper_bound(orig, i) : nothing) for i in 1:d])
+    lower_bounds = Vector{Union{Int, Nothing}}([(has_lower_bound(orig, i) ? lower_bound(orig, i) : nothing) for i in 1:d])
+    internal_complex = HyperComplex(d, chain_fac, map_fac, 
+                                    [direction(orig, i) for i in 1:d],
+                                    lower_bounds = lower_bounds,
+                                    upper_bounds = upper_bounds
+                                   )
     return new{ModuleFP, ModuleFPHom}(orig, internal_complex)
   end
 end

experimental/DoubleAndHyperComplexes/test/cartan_eilenberg_resolutions.jl

@@ -0,0 +1,25 @@
+@testset "Cartan-Eilenberg resolutions" begin
+  R, (x, y, z, w) = QQ[:x, :y, :z, :w]
+
+  A = R[x y z; y z w]
+
+  R1 = free_module(R, 1)
+  I, inc = sub(R1, [a*R1[1] for a in minors(A, 2)])
+  M = cokernel(inc)
+  R4 = free_module(R, 4)
+  theta = sum(a*g for (a, g) in zip(gens(R), gens(R4)); init=zero(R4))
+  K = koszul_complex(Oscar.KoszulComplex, theta)
+
+  comp = tensor_product(K, Oscar.ZeroDimensionalComplex(M))
+  res = Oscar.CartanEilenbergResolution(comp);
+  tot = total_complex(res);
+  tot_simp = simplify(tot);
+
+  res_M, _ = free_resolution(Oscar.SimpleFreeResolution, M)
+  comp2 = tensor_product(K, res_M)
+  tot2 = total_complex(comp2)
+  tot_simp2 = simplify(tot2);
+
+  @test [ngens(tot_simp[i]) for i in 0:5] == [ngens(tot_simp2[i]) for i in 0:5]
+end
+

experimental/DoubleAndHyperComplexes/test/runtests.jl

@@ -19,3 +19,4 @@ include("linear_strands.jl")
 include("printing.jl")
 include("degree_zero_complex.jl")
 include("base_change.jl")
+include("cartan_eilenberg_resolutions.jl")

src/Modules/ModulesGraded.jl

@@ -414,7 +414,7 @@ function set_grading!(M::FreeMod, W::Vector{<:IntegerUnion})
   M.d = [W[i] * A[1] for i in 1:length(W)]
 end
 
-function degrees(M::FreeMod)
+function degrees(M::FreeMod; check::Bool=true)
   @assert is_graded(M)
   return M.d::Vector{FinGenAbGroupElem}
 end
@@ -437,8 +437,8 @@ julia> degrees_of_generators(F)
  [0]
 ```
 """
-function degrees_of_generators(F::FreeMod)
-  return degrees(F)
+function degrees_of_generators(F::FreeMod; check::Bool=true)
+  return degrees(F; check)
 end
 
 ###############################################################################