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Mathlib.Algebra.Homology.Embedding.Extend

The extension of a homological complex by an embedding of complex shapes #

Given an embedding e : Embedding c c' of complex shapes, and K : HomologicalComplex C c, we define K.extend e : HomologicalComplex C c', and this leads to a functor e.extendFunctor C : HomologicalComplex C c ⥤ HomologicalComplex C c'.

This construction first appeared in the Liquid Tensor Experiment.

Auxiliary definition for the X field of HomologicalComplex.extend.

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      The isomorphism X K i ≅ K.X j when i = some j.

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          The canonical isomorphism X K.op i ≅ Opposite.op (X K i).

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              Auxiliary definition for the d field of HomologicalComplex.extend.

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                  Auxiliary definition for HomologicalComplex.extendMap.

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                      Given K : HomologicalComplex C c and e : c.Embedding c', this is the extension of K in HomologicalComplex C c': it is zero in the degrees that are not in the image of e.f.

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                          noncomputable def HomologicalComplex.extendXIso {ι : Type u_1} {ι' : Type u_2} {c : ComplexShape ι} {c' : ComplexShape ι'} {C : Type u_3} [CategoryTheory.Category.{v_1, u_3} C] [CategoryTheory.Limits.HasZeroObject C] [CategoryTheory.Limits.HasZeroMorphisms C] (K : HomologicalComplex C c) (e : c.Embedding c') {i' : ι'} {i : ι} (h : e.f i = i') :
                          (K.extend e).X i' K.X i

                          The isomorphism (K.extend e).X i' ≅ K.X i when e.f i = i'.

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                              theorem HomologicalComplex.extend_d_from_eq_zero {ι : Type u_1} {ι' : Type u_2} {c : ComplexShape ι} {c' : ComplexShape ι'} {C : Type u_3} [CategoryTheory.Category.{v_1, u_3} C] [CategoryTheory.Limits.HasZeroObject C] [CategoryTheory.Limits.HasZeroMorphisms C] (K : HomologicalComplex C c) (e : c.Embedding c') (i' j' : ι') (i : ι) (hi : e.f i = i') (hi' : ¬c.Rel i (c.next i)) :
                              (K.extend e).d i' j' = 0
                              theorem HomologicalComplex.extend_d_to_eq_zero {ι : Type u_1} {ι' : Type u_2} {c : ComplexShape ι} {c' : ComplexShape ι'} {C : Type u_3} [CategoryTheory.Category.{v_1, u_3} C] [CategoryTheory.Limits.HasZeroObject C] [CategoryTheory.Limits.HasZeroMorphisms C] (K : HomologicalComplex C c) (e : c.Embedding c') (i' j' : ι') (j : ι) (hj : e.f j = j') (hj' : ¬c.Rel (c.prev j) j) :
                              (K.extend e).d i' j' = 0

                              Given an embedding e : c.Embedding c' of complexes shapes, this is the morphism K.extend e ⟶ L.extend e induced by a morphism K ⟶ L in HomologicalComplex C c.

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                                  theorem HomologicalComplex.extendMap_f_eq_zero {ι : Type u_1} {ι' : Type u_2} {c : ComplexShape ι} {c' : ComplexShape ι'} {C : Type u_3} [CategoryTheory.Category.{v_1, u_3} C] [CategoryTheory.Limits.HasZeroObject C] [CategoryTheory.Limits.HasZeroMorphisms C] {K L : HomologicalComplex C c} (φ : K L) (e : c.Embedding c') (i' : ι') (hi' : ∀ (i : ι), e.f i i') :
                                  (extendMap φ e).f i' = 0

                                  The canonical isomorphism K.op.extend e.op ≅ (K.extend e).op.

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                                      @[simp]
                                      theorem HomologicalComplex.extend_single_d {ι : Type u_1} {ι' : Type u_2} {c : ComplexShape ι} {c' : ComplexShape ι'} {C : Type u_3} [CategoryTheory.Category.{v_1, u_3} C] [CategoryTheory.Limits.HasZeroObject C] [CategoryTheory.Limits.HasZeroMorphisms C] [DecidableEq ι] (e : c.Embedding c') (X : C) (i : ι) (j' k' : ι') :
                                      (((single C c i).obj X).extend e).d j' k' = 0
                                      noncomputable def HomologicalComplex.extendSingleIso {ι : Type u_1} {ι' : Type u_2} {c : ComplexShape ι} {c' : ComplexShape ι'} {C : Type u_3} [CategoryTheory.Category.{v_1, u_3} C] [CategoryTheory.Limits.HasZeroObject C] [CategoryTheory.Limits.HasZeroMorphisms C] [DecidableEq ι] (e : c.Embedding c') (X : C) [DecidableEq ι'] (i : ι) (i' : ι') (h : e.f i = i') :
                                      ((single C c i).obj X).extend e (single C c' i').obj X

                                      The extension of a single complex is a single complex.

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                                          Given an embedding e : c.Embedding c' of complex shapes, this is the functor HomologicalComplex C c ⥤ HomologicalComplex C c' which extend complexes along e: the extended complexes are zero in the degrees that are not in the image of e.f.

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                                              The extension functor attached to an embedding of complex shapes is fully faithful.

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