module.lean

/-
Copyright (c) 2018 Kenny Lau. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Kenny Lau
-/
import algebra.direct_sum.basic
import linear_algebra.dfinsupp

/-!
# Direct sum of modules

The first part of the file provides constructors for direct sums of modules. It provides a
construction of the direct sum using the universal property and proves its uniqueness
(`direct_sum.to_module.unique`).

The second part of the file covers the special case of direct sums of submodules of a fixed module
`M`.  There is a canonical linear map from this direct sum to `M`, and the construction is
of particular importance when this linear map is an equivalence; that is, when the submodules
provide an internal decomposition of `M`.  The property is defined as
`direct_sum.submodule_is_internal`, and its basic consequences are established.

-/

universes u v w u₁


namespace direct_sum
open_locale direct_sum

section general
variables {R : Type u} [semiring R]
variables {ι : Type v} [dec_ι : decidable_eq ι]
include R
variables {M : ι → Type w} [Π i, add_comm_monoid (M i)] [Π i, module R (M i)]

instance : module R (⨁ i, M i) := dfinsupp.module
instance {S : Type*} [semiring S] [Π i, module S (M i)] [Π i, smul_comm_class R S (M i)] :
  smul_comm_class R S (⨁ i, M i) := dfinsupp.smul_comm_class
instance {S : Type*} [semiring S] [has_scalar R S] [Π i, module S (M i)]
  [Π i, is_scalar_tower R S (M i)] :
  is_scalar_tower R S (⨁ i, M i) := dfinsupp.is_scalar_tower

lemma smul_apply (b : R) (v : ⨁ i, M i) (i : ι) :
  (b • v) i = b • (v i) := dfinsupp.smul_apply _ _ _

include dec_ι

variables R ι M
/-- Create the direct sum given a family `M` of `R` modules indexed over `ι`. -/
def lmk : Π s : finset ι, (Π i : (↑s : set ι), M i.val) →ₗ[R] (⨁ i, M i) :=
dfinsupp.lmk

/-- Inclusion of each component into the direct sum. -/
def lof : Π i : ι, M i →ₗ[R] (⨁ i, M i) :=
dfinsupp.lsingle

lemma lof_eq_of (i : ι) (b : M i) : lof R ι M i b = of M i b := rfl

variables {ι M}

lemma single_eq_lof (i : ι) (b : M i) :
  dfinsupp.single i b = lof R ι M i b := rfl

/-- Scalar multiplication commutes with direct sums. -/
theorem mk_smul (s : finset ι) (c : R) (x) : mk M s (c • x) = c • mk M s x :=
(lmk R ι M s).map_smul c x

/-- Scalar multiplication commutes with the inclusion of each component into the direct sum. -/
theorem of_smul (i : ι) (c : R) (x) : of M i (c • x) = c • of M i x :=
(lof R ι M i).map_smul c x

variables {R}
lemma support_smul [Π (i : ι) (x : M i), decidable (x ≠ 0)]
  (c : R) (v : ⨁ i, M i) : (c • v).support ⊆ v.support := dfinsupp.support_smul _ _

variables {N : Type u₁} [add_comm_monoid N] [module R N]
variables (φ : Π i, M i →ₗ[R] N)

variables (R ι N φ)
/-- The linear map constructed using the universal property of the coproduct. -/
def to_module : (⨁ i, M i) →ₗ[R] N :=
dfinsupp.lsum ℕ φ

/-- Coproducts in the categories of modules and additive monoids commute with the forgetful functor
from modules to additive monoids. -/
lemma coe_to_module_eq_coe_to_add_monoid :
  (to_module R ι N φ : (⨁ i, M i) → N) = to_add_monoid (λ i, (φ i).to_add_monoid_hom) :=
rfl

variables {ι N φ}

/-- The map constructed using the universal property gives back the original maps when
restricted to each component. -/
@[simp] lemma to_module_lof (i) (x : M i) : to_module R ι N φ (lof R ι M i x) = φ i x :=
to_add_monoid_of (λ i, (φ i).to_add_monoid_hom) i x

variables (ψ : (⨁ i, M i) →ₗ[R] N)

/-- Every linear map from a direct sum agrees with the one obtained by applying
the universal property to each of its components. -/
theorem to_module.unique (f : ⨁ i, M i) : ψ f = to_module R ι N (λ i, ψ.comp $ lof R ι M i) f :=
to_add_monoid.unique ψ.to_add_monoid_hom f

variables {ψ} {ψ' : (⨁ i, M i) →ₗ[R] N}

/-- Two `linear_map`s out of a direct sum are equal if they agree on the generators.

See note [partially-applied ext lemmas]. -/
@[ext]
theorem linear_map_ext ⦃ψ ψ' : (⨁ i, M i) →ₗ[R] N⦄
  (H : ∀ i, ψ.comp (lof R ι M i) = ψ'.comp (lof R ι M i)) : ψ = ψ' :=
dfinsupp.lhom_ext' H

/--
The inclusion of a subset of the direct summands
into a larger subset of the direct summands, as a linear map.
-/
def lset_to_set (S T : set ι) (H : S ⊆ T) :
  (⨁ (i : S), M i) →ₗ[R] (⨁ (i : T), M i) :=
to_module R _ _ $ λ i, lof R T (λ (i : subtype T), M i) ⟨i, H i.prop⟩

omit dec_ι

variables (ι M)
/-- Given `fintype α`, `linear_equiv_fun_on_fintype R` is the natural `R`-linear equivalence
between `⨁ i, M i` and `Π i, M i`. -/
@[simps apply] def linear_equiv_fun_on_fintype [fintype ι] :
  (⨁ i, M i) ≃ₗ[R] (Π i, M i) :=
{ to_fun := coe_fn,
  map_add' := λ f g, by { ext, simp only [add_apply, pi.add_apply] },
  map_smul' := λ c f, by { ext, simp only [dfinsupp.coe_smul, ring_hom.id_apply] },
  .. dfinsupp.equiv_fun_on_fintype }

variables {ι M}
@[simp] lemma linear_equiv_fun_on_fintype_lof [fintype ι] [decidable_eq ι] (i : ι) (m : M i) :
  (linear_equiv_fun_on_fintype R ι M) (lof R ι M i m) = pi.single i m :=
begin
  ext a,
  change (dfinsupp.equiv_fun_on_fintype (lof R ι M i m)) a = _,
  convert _root_.congr_fun (dfinsupp.equiv_fun_on_fintype_single i m) a,
end

@[simp] lemma linear_equiv_fun_on_fintype_symm_single [fintype ι] [decidable_eq ι]
  (i : ι) (m : M i) :
  (linear_equiv_fun_on_fintype R ι M).symm (pi.single i m) = lof R ι M i m :=
begin
  ext a,
  change (dfinsupp.equiv_fun_on_fintype.symm (pi.single i m)) a = _,
  rw (dfinsupp.equiv_fun_on_fintype_symm_single i m),
  refl
end

@[simp] lemma linear_equiv_fun_on_fintype_symm_coe [fintype ι] (f : ⨁ i, M i) :
  (linear_equiv_fun_on_fintype R ι M).symm f = f :=
by { ext, simp [linear_equiv_fun_on_fintype], }

/-- The natural linear equivalence between `⨁ _ : ι, M` and `M` when `unique ι`. -/
protected def lid (M : Type v) (ι : Type* := punit) [add_comm_monoid M] [module R M]
  [unique ι] :
  (⨁ (_ : ι), M) ≃ₗ[R] M :=
{ .. direct_sum.id M ι,
  .. to_module R ι M (λ i, linear_map.id) }

variables (ι M)
/-- The projection map onto one component, as a linear map. -/
def component (i : ι) : (⨁ i, M i) →ₗ[R] M i :=
dfinsupp.lapply i

variables {ι M}

lemma apply_eq_component (f : ⨁ i, M i) (i : ι) :
  f i = component R ι M i f := rfl

@[ext] lemma ext {f g : ⨁ i, M i}
  (h : ∀ i, component R ι M i f = component R ι M i g) : f = g :=
dfinsupp.ext h

lemma ext_iff {f g : ⨁ i, M i} : f = g ↔
  ∀ i, component R ι M i f = component R ι M i g :=
⟨λ h _, by rw h, ext R⟩

include dec_ι

@[simp] lemma lof_apply (i : ι) (b : M i) : ((lof R ι M i) b) i = b :=
dfinsupp.single_eq_same

@[simp] lemma component.lof_self (i : ι) (b : M i) :
  component R ι M i ((lof R ι M i) b) = b :=
lof_apply R i b

lemma component.of (i j : ι) (b : M j) :
  component R ι M i ((lof R ι M j) b) =
  if h : j = i then eq.rec_on h b else 0 :=
dfinsupp.single_apply

end general

section submodule

section semiring
variables {R : Type u} [semiring R]
variables {ι : Type v} [dec_ι : decidable_eq ι]
include dec_ι
variables {M : Type*} [add_comm_monoid M] [module R M]
variables (A : ι → submodule R M)

/-- The canonical embedding from `⨁ i, A i` to `M`  where `A` is a collection of `submodule R M`
indexed by `ι`-/
def submodule_coe : (⨁ i, A i) →ₗ[R] M := to_module R ι M (λ i, (A i).subtype)

@[simp] lemma submodule_coe_of (i : ι) (x : A i) : submodule_coe A (of (λ i, A i) i x) = x :=
to_add_monoid_of _ _ _


/-- The `direct_sum` formed by a collection of `submodule`s of `M` is said to be internal if the
canonical map `(⨁ i, A i) →ₗ[R] M` is bijective.

For the alternate statement in terms of independence and spanning, see
`direct_sum.submodule_is_internal_iff_independent_and_supr_eq_top`. -/
def submodule_is_internal : Prop := function.bijective (submodule_coe A)

lemma submodule_is_internal.to_add_submonoid :
  submodule_is_internal A ↔ add_submonoid_is_internal (λ i, (A i).to_add_submonoid) :=
iff.rfl

variables {A}

/-- If a direct sum of submodules is internal then the submodules span the module. -/
lemma submodule_is_internal.supr_eq_top (h : submodule_is_internal A) : supr A = ⊤ :=
begin
  rw [submodule.supr_eq_range_dfinsupp_lsum, linear_map.range_eq_top],
  exact function.bijective.surjective h,
end

/-- If a direct sum of submodules is internal then the submodules are independent. -/
lemma submodule_is_internal.independent (h : submodule_is_internal A) :
  complete_lattice.independent A :=
complete_lattice.independent_of_dfinsupp_lsum_injective _ h.injective

end semiring

section ring
variables {R : Type u} [ring R]
variables {ι : Type v} [dec_ι : decidable_eq ι]
include dec_ι
variables {M : Type*} [add_comm_group M] [module R M]

lemma submodule_is_internal.to_add_subgroup (A : ι → submodule R M) :
  submodule_is_internal A ↔ add_subgroup_is_internal (λ i, (A i).to_add_subgroup) :=
iff.rfl

/-- Note that this is not generally true for `[semiring R]`; see
`complete_lattice.independent.dfinsupp_lsum_injective` for details. -/
lemma submodule_is_internal_of_independent_of_supr_eq_top {A : ι → submodule R M}
  (hi : complete_lattice.independent A) (hs : supr A = ⊤) : submodule_is_internal A :=
⟨hi.dfinsupp_lsum_injective, linear_map.range_eq_top.1 $
  (submodule.supr_eq_range_dfinsupp_lsum _).symm.trans hs⟩

/-- `iff` version of `direct_sum.submodule_is_internal_of_independent_of_supr_eq_top`,
`direct_sum.submodule_is_internal.independent`, and `direct_sum.submodule_is_internal.supr_eq_top`.
-/
lemma submodule_is_internal_iff_independent_and_supr_eq_top (A : ι → submodule R M) :
    submodule_is_internal A ↔ complete_lattice.independent A ∧ supr A = ⊤ :=
⟨λ i, ⟨i.independent, i.supr_eq_top⟩,
 and.rec submodule_is_internal_of_independent_of_supr_eq_top⟩

end ring

end submodule

end direct_sum