# Documentation

Mathlib.Algebra.Homology.ComplexShape

# Shapes of homological complexes #

We define a structure ComplexShape ι for describing the shapes of homological complexes indexed by a type ι. This is intended to capture chain complexes and cochain complexes, indexed by either ℕ or ℤ, as well as more exotic examples.

Rather than insisting that the indexing type has a succ function specifying where differentials should go, inside c : ComplexShape we have c.Rel : ι → ι → Prop→ ι → Prop→ Prop, and when we define HomologicalComplex we only allow nonzero differentials d i j from i to j if c.Rel i j. Further, we require that { j // c.Rel i j } and { i // c.Rel i j } are subsingletons. This means that the shape consists of some union of lines, rays, intervals, and circles.

Convenience functions c.next and c.prev provide these related elements when they exist, and return their input otherwise.

This design aims to avoid certain problems arising from dependent type theory. In particular we never have to ensure morphisms d i : X i ⟶ X (succ i)⟶ X (succ i) compose as expected (which would often require rewriting by equations in the indexing type). Instead such identities become separate proof obligations when verifying that a complex we've constructed is of the desired shape.

If α is an AddRightCancelSemigroup, then we define up α : ComplexShape α, the shape appropriate for cohomology, so d : X i ⟶ X j⟶ X j is nonzero only when j = i + 1, as well as down α : ComplexShape α, appropriate for homology, so d : X i ⟶ X j⟶ X j is nonzero only when i = j + 1. (Later we'll introduce CochainComplex and ChainComplex as abbreviations for HomologicalComplex with one of these shapes baked in.)

theorem ComplexShape.ext {ι : Type u_1} (x : ) (y : ) (Rel : x.Rel = y.Rel) :
x = y
theorem ComplexShape.ext_iff {ι : Type u_1} (x : ) (y : ) :
x = y x.Rel = y.Rel
structure ComplexShape (ι : Type u_1) :
Type u_1
• Nonzero differentials X i ⟶ X j⟶ X j shall be allowed on homological complexes when Rel i j holds.

Rel : ιιProp
• There is at most one nonzero differential from X i.

next_eq : ∀ {i j j' : ι}, Rel i jRel i j'j = j'
• There is at most one nonzero differential to X j.

prev_eq : ∀ {i i' j : ι}, Rel i jRel i' ji = i'

A c : ComplexShape ι describes the shape of a chain complex, with chain groups indexed by ι. Typically ι will be ℕ, ℤ, or Fin n.

There is a relation Rel : ι → ι → Prop→ ι → Prop→ Prop, and we will only allow a non-zero differential from i to j when Rel i j.

There are axioms which imply { j // c.Rel i j } and { i // c.Rel i j } are subsingletons. This means that the shape consists of some union of lines, rays, intervals, and circles.

Below we define c.next and c.prev which provide these related elements.

Instances For
@[simp]
theorem ComplexShape.refl_Rel (ι : Type u_1) (i : ι) (j : ι) :
= (i = j)
def ComplexShape.refl (ι : Type u_1) :

The complex shape where only differentials from each X.i to itself are allowed.

This is mostly only useful so we can describe the relation of "related in k steps" below.

Equations
• = { Rel := fun i j => i = j, next_eq := (_ : ∀ {i j j' : ι}, i = ji = j'j = j'), prev_eq := (_ : ∀ {i i' j : ι}, i = ji' = ji = i') }
@[simp]
theorem ComplexShape.symm_Rel {ι : Type u_1} (c : ) (i : ι) (j : ι) :
=
def ComplexShape.symm {ι : Type u_1} (c : ) :

The reverse of a ComplexShape.

Equations
• One or more equations did not get rendered due to their size.
@[simp]
theorem ComplexShape.symm_symm {ι : Type u_1} (c : ) :
def ComplexShape.trans {ι : Type u_1} (c₁ : ) (c₂ : ) :

The "composition" of two ComplexShapes.

We need this to define "related in k steps" later.

Equations
• One or more equations did not get rendered due to their size.
instance ComplexShape.subsingleton_next {ι : Type u_1} (c : ) (i : ι) :
Subsingleton { j // }
Equations
instance ComplexShape.subsingleton_prev {ι : Type u_1} (c : ) (j : ι) :
Subsingleton { i // }
Equations
def ComplexShape.next {ι : Type u_1} (c : ) (i : ι) :
ι

An arbitary choice of index j such that Rel i j, if such exists. Returns i otherwise.

Equations
• = if h : j, then else i
def ComplexShape.prev {ι : Type u_1} (c : ) (j : ι) :
ι

An arbitary choice of index i such that Rel i j, if such exists. Returns j otherwise.

Equations
• = if h : i, then else j
theorem ComplexShape.next_eq' {ι : Type u_1} (c : ) {i : ι} {j : ι} (h : ) :
= j
theorem ComplexShape.prev_eq' {ι : Type u_1} (c : ) {i : ι} {j : ι} (h : ) :
= i
@[simp]
theorem ComplexShape.up'_Rel {α : Type u_1} [inst : ] (a : α) (i : α) (j : α) :
= (i + a = j)
def ComplexShape.up' {α : Type u_1} [inst : ] (a : α) :

The ComplexShape allowing differentials from X i to X (i+a). (For example when a = 1, a cohomology theory indexed by ℕ or ℤ)

Equations
• = { Rel := fun i j => i + a = j, next_eq := (_ : ∀ {i j j' : α}, i + a = ji + a = j'j = j'), prev_eq := (_ : ∀ {i i' j : α}, i + a = ji' + a = ji = i') }
@[simp]
theorem ComplexShape.down'_Rel {α : Type u_1} [inst : ] (a : α) (i : α) (j : α) :
= (j + a = i)
def ComplexShape.down' {α : Type u_1} [inst : ] (a : α) :

The ComplexShape allowing differentials from X (j+a) to X j. (For example when a = 1, a homology theory indexed by ℕ or ℤ)

Equations
• = { Rel := fun i j => j + a = i, next_eq := (_ : ∀ {i j j' : α}, j + a = ij' + a = ij = j'), prev_eq := (_ : ∀ {i i' j : α}, j + a = ij + a = i'i = i') }
theorem ComplexShape.down'_mk {α : Type u_1} [inst : ] (a : α) (i : α) (j : α) (h : j + a = i) :
@[simp]
theorem ComplexShape.up_Rel (α : Type u_1) [inst : ] [inst : One α] (i : α) (j : α) :
= (i + 1 = j)
def ComplexShape.up (α : Type u_1) [inst : ] [inst : One α] :

The ComplexShape appropriate for cohomology, so d : X i ⟶ X j⟶ X j only when j = i + 1.

Equations
@[simp]
theorem ComplexShape.down_Rel (α : Type u_1) [inst : ] [inst : One α] (i : α) (j : α) :
= (j + 1 = i)
def ComplexShape.down (α : Type u_1) [inst : ] [inst : One α] :

The ComplexShape appropriate for homology, so d : X i ⟶ X j⟶ X j only when i = j + 1.

Equations
theorem ComplexShape.down_mk {α : Type u_1} [inst : ] [inst : One α] (i : α) (j : α) (h : j + 1 = i) :