/-
Copyright (c) 2021 Rémy Degenne. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Authors: Rémy Degenne
-/
import algebra.big_operators.intervals
import measure_theory.constructions.pi

/-!
# Independence of sets of sets and measure spaces (σ-algebras)

* A family of sets of sets `π : ι → set (set α)` is independent with respect to a measure `μ` if for
  any finite set of indices `s = {i_1, ..., i_n}`, for any sets `f i_1 ∈ π i_1, ..., f i_n ∈ π i_n`,
  `μ (⋂ i in s, f i) = ∏ i in s, μ (f i) `. It will be used for families of π-systems.
* A family of measurable space structures (i.e. of σ-algebras) is independent with respect to a
  measure `μ` (typically defined on a finer σ-algebra) if the family of sets of measurable sets they
  define is independent. I.e., `m : ι → measurable_space α` is independent with respect to a
  measure `μ` if for any finite set of indices `s = {i_1, ..., i_n}`, for any sets
  `f i_1 ∈ m i_1, ..., f i_n ∈ m i_n`, then `μ (⋂ i in s, f i) = ∏ i in s, μ (f i)`.
* Independence of sets (or events in probabilistic parlance) is defined as independence of the
  measurable space structures they generate: a set `s` generates the measurable space structure with
  measurable sets `∅, s, sᶜ, univ`.
* Independence of functions (or random variables) is also defined as independence of the measurable
  space structures they generate: a function `f` for which we have a measurable space `m` on the
  codomain generates `measurable_space.comap f m`.

## Main statements

* `Indep_sets.Indep`: if π-systems are independent as sets of sets, then the
measurable space structures they generate are independent.
* `indep_sets.indep`: variant with two π-systems.

## Implementation notes

We provide one main definition of independence:
* `Indep_sets`: independence of a family of sets of sets `pi : ι → set (set α)`.
Three other independence notions are defined using `Indep_sets`:
* `Indep`: independence of a family of measurable space structures `m : ι → measurable_space α`,
* `Indep_set`: independence of a family of sets `s : ι → set α`,
* `Indep_fun`: independence of a family of functions. For measurable spaces
  `m : Π (i : ι), measurable_space (β i)`, we consider functions `f : Π (i : ι), α → β i`.

Additionally, we provide four corresponding statements for two measurable space structures (resp.
sets of sets, sets, functions) instead of a family. These properties are denoted by the same names
as for a family, but without a capital letter, for example `indep_fun` is the version of `Indep_fun`
for two functions.

The definition of independence for `Indep_sets` uses finite sets (`finset`). An alternative and
equivalent way of defining independence would have been to use countable sets.
TODO: prove that equivalence.

Most of the definitions and lemma in this file list all variables instead of using the `variables`
keyword at the beginning of a section, for example
`lemma indep.symm {α} {m₁ m₂ : measurable_space α} [measurable_space α] {μ : measure α} ...` .
This is intentional, to be able to control the order of the `measurable_space` variables. Indeed
when defining `μ` in the example above, the measurable space used is the last one defined, here
`[measurable_space α]`, and not `m₁` or `m₂`.

## References

* Williams, David. Probability with martingales. Cambridge university press, 1991.
Part A, Chapter 4.
-/

open measure_theory measurable_space
open_locale big_operators classical measure_theory

namespace probability_theory

section definitions

/-- A family of sets of sets `π : ι → set (set α)` is independent with respect to a measure `μ` if
for any finite set of indices `s = {i_1, ..., i_n}`, for any sets
`f i_1 ∈ π i_1, ..., f i_n ∈ π i_n`, then `μ (⋂ i in s, f i) = ∏ i in s, μ (f i) `.
It will be used for families of pi_systems. -/
def Indep_sets {α ι} [measurable_space α] (π : ι → set (set α)) (μ : measure α . volume_tac) :
  Prop :=
∀ (s : finset ι) {f : ι → set α} (H : ∀ i, i ∈ s → f i ∈ π i), μ (⋂ i ∈ s, f i) = ∏ i in s, μ (f i)

/-- Two sets of sets `s₁, s₂` are independent with respect to a measure `μ` if for any sets
`t₁ ∈ p₁, t₂ ∈ s₂`, then `μ (t₁ ∩ t₂) = μ (t₁) * μ (t₂)` -/
def indep_sets {α} [measurable_space α] (s1 s2 : set (set α)) (μ : measure α . volume_tac) : Prop :=
∀ t1 t2 : set α, t1 ∈ s1 → t2 ∈ s2 → μ (t1 ∩ t2) = μ t1 * μ t2

/-- A family of measurable space structures (i.e. of σ-algebras) is independent with respect to a
measure `μ` (typically defined on a finer σ-algebra) if the family of sets of measurable sets they
define is independent. `m : ι → measurable_space α` is independent with respect to measure `μ` if
for any finite set of indices `s = {i_1, ..., i_n}`, for any sets
`f i_1 ∈ m i_1, ..., f i_n ∈ m i_n`, then `μ (⋂ i in s, f i) = ∏ i in s, μ (f i) `. -/
def Indep {α ι} (m : ι → measurable_space α) [measurable_space α] (μ : measure α . volume_tac) :
  Prop :=
Indep_sets (λ x, {s | measurable_set[m x] s}) μ

/-- Two measurable space structures (or σ-algebras) `m₁, m₂` are independent with respect to a
measure `μ` (defined on a third σ-algebra) if for any sets `t₁ ∈ m₁, t₂ ∈ m₂`,
`μ (t₁ ∩ t₂) = μ (t₁) * μ (t₂)` -/
def indep {α} (m₁ m₂ : measurable_space α) [measurable_space α] (μ : measure α . volume_tac) :
  Prop :=
indep_sets {s | measurable_set[m₁] s} {s | measurable_set[m₂] s} μ

/-- A family of sets is independent if the family of measurable space structures they generate is
independent. For a set `s`, the generated measurable space has measurable sets `∅, s, sᶜ, univ`. -/
def Indep_set {α ι} [measurable_space α] (s : ι → set α) (μ : measure α . volume_tac) : Prop :=
Indep (λ i, generate_from {s i}) μ

/-- Two sets are independent if the two measurable space structures they generate are independent.
For a set `s`, the generated measurable space structure has measurable sets `∅, s, sᶜ, univ`. -/
def indep_set {α} [measurable_space α] (s t : set α) (μ : measure α . volume_tac) : Prop :=
indep (generate_from {s}) (generate_from {t}) μ

/-- A family of functions defined on the same space `α` and taking values in possibly different
spaces, each with a measurable space structure, is independent if the family of measurable space
structures they generate on `α` is independent. For a function `g` with codomain having measurable
space structure `m`, the generated measurable space structure is `measurable_space.comap g m`. -/
def Indep_fun {α ι} [measurable_space α] {β : ι → Type*} (m : Π (x : ι), measurable_space (β x))
  (f : Π (x : ι), α → β x) (μ : measure α . volume_tac) : Prop :=
Indep (λ x, measurable_space.comap (f x) (m x)) μ

/-- Two functions are independent if the two measurable space structures they generate are
independent. For a function `f` with codomain having measurable space structure `m`, the generated
measurable space structure is `measurable_space.comap f m`. -/
def indep_fun {α β γ} [measurable_space α] [mβ : measurable_space β] [mγ : measurable_space γ]
  (f : α → β) (g : α → γ) (μ : measure α . volume_tac) : Prop :=
indep (measurable_space.comap f mβ) (measurable_space.comap g mγ) μ

end definitions

section indep

lemma indep_sets.symm {α} {s₁ s₂ : set (set α)} [measurable_space α] {μ : measure α}
  (h : indep_sets s₁ s₂ μ) :
  indep_sets s₂ s₁ μ :=
by { intros t1 t2 ht1 ht2, rw [set.inter_comm, mul_comm], exact h t2 t1 ht2 ht1, }

lemma indep.symm {α} {m₁ m₂ : measurable_space α} [measurable_space α] {μ : measure α}
  (h : indep m₁ m₂ μ) :
  indep m₂ m₁ μ :=
indep_sets.symm h

lemma indep_sets_of_indep_sets_of_le_left {α} {s₁ s₂ s₃: set (set α)} [measurable_space α]
  {μ : measure α} (h_indep : indep_sets s₁ s₂ μ) (h31 : s₃ ⊆ s₁) :
  indep_sets s₃ s₂ μ :=
λ t1 t2 ht1 ht2, h_indep t1 t2 (set.mem_of_subset_of_mem h31 ht1) ht2

lemma indep_sets_of_indep_sets_of_le_right {α} {s₁ s₂ s₃: set (set α)} [measurable_space α]
  {μ : measure α} (h_indep : indep_sets s₁ s₂ μ) (h32 : s₃ ⊆ s₂) :
  indep_sets s₁ s₃ μ :=
λ t1 t2 ht1 ht2, h_indep t1 t2 ht1 (set.mem_of_subset_of_mem h32 ht2)

lemma indep_of_indep_of_le_left {α} {m₁ m₂ m₃: measurable_space α} [measurable_space α]
  {μ : measure α} (h_indep : indep m₁ m₂ μ) (h31 : m₃ ≤ m₁) :
  indep m₃ m₂ μ :=
λ t1 t2 ht1 ht2, h_indep t1 t2 (h31 _ ht1) ht2

lemma indep_of_indep_of_le_right {α} {m₁ m₂ m₃: measurable_space α} [measurable_space α]
  {μ : measure α} (h_indep : indep m₁ m₂ μ) (h32 : m₃ ≤ m₂) :
  indep m₁ m₃ μ :=
λ t1 t2 ht1 ht2, h_indep t1 t2 ht1 (h32 _ ht2)

lemma indep_sets.union {α} [measurable_space α] {s₁ s₂ s' : set (set α)} {μ : measure α}
  (h₁ : indep_sets s₁ s' μ) (h₂ : indep_sets s₂ s' μ) :
  indep_sets (s₁ ∪ s₂) s' μ :=
begin
  intros t1 t2 ht1 ht2,
  cases (set.mem_union _ _ _).mp ht1 with ht1₁ ht1₂,
  { exact h₁ t1 t2 ht1₁ ht2, },
  { exact h₂ t1 t2 ht1₂ ht2, },
end

@[simp] lemma indep_sets.union_iff {α} [measurable_space α] {s₁ s₂ s' : set (set α)}
  {μ : measure α} :
  indep_sets (s₁ ∪ s₂) s' μ ↔ indep_sets s₁ s' μ ∧ indep_sets s₂ s' μ :=
⟨λ h, ⟨indep_sets_of_indep_sets_of_le_left h (set.subset_union_left s₁ s₂),
    indep_sets_of_indep_sets_of_le_left h (set.subset_union_right s₁ s₂)⟩,
  λ h, indep_sets.union h.left h.right⟩

lemma indep_sets.Union {α ι} [measurable_space α] {s : ι → set (set α)} {s' : set (set α)}
  {μ : measure α} (hyp : ∀ n, indep_sets (s n) s' μ) :
  indep_sets (⋃ n, s n) s' μ :=
begin
  intros t1 t2 ht1 ht2,
  rw set.mem_Union at ht1,
  cases ht1 with n ht1,
  exact hyp n t1 t2 ht1 ht2,
end

lemma indep_sets.inter {α} [measurable_space α] {s₁ s' : set (set α)} (s₂ : set (set α))
  {μ : measure α} (h₁ : indep_sets s₁ s' μ) :
  indep_sets (s₁ ∩ s₂) s' μ :=
λ t1 t2 ht1 ht2, h₁ t1 t2 ((set.mem_inter_iff _ _ _).mp ht1).left ht2

lemma indep_sets.Inter {α ι} [measurable_space α] {s : ι → set (set α)} {s' : set (set α)}
  {μ : measure α} (h : ∃ n, indep_sets (s n) s' μ) :
  indep_sets (⋂ n, s n) s' μ :=
by {intros t1 t2 ht1 ht2, cases h with n h, exact h t1 t2 (set.mem_Inter.mp ht1 n) ht2 }

lemma indep_sets_singleton_iff {α} [measurable_space α] {s t : set α} {μ : measure α} :
  indep_sets {s} {t} μ ↔ μ (s ∩ t) = μ s * μ t :=
⟨λ h, h s t rfl rfl,
  λ h s1 t1 hs1 ht1, by rwa [set.mem_singleton_iff.mp hs1, set.mem_singleton_iff.mp ht1]⟩

end indep

/-! ### Deducing `indep` from `Indep` -/
section from_Indep_to_indep

lemma Indep_sets.indep_sets {α ι} {s : ι → set (set α)} [measurable_space α] {μ : measure α}
  (h_indep : Indep_sets s μ) {i j : ι} (hij : i ≠ j) :
  indep_sets (s i) (s j) μ :=
begin
  intros t₁ t₂ ht₁ ht₂,
  have hf_m : ∀ (x : ι), x ∈ {i, j} → (ite (x=i) t₁ t₂) ∈ s x,
  { intros x hx,
    cases finset.mem_insert.mp hx with hx hx,
    { simp [hx, ht₁], },
    { simp [finset.mem_singleton.mp hx, hij.symm, ht₂], }, },
  have h1 : t₁ = ite (i = i) t₁ t₂, by simp only [if_true, eq_self_iff_true],
  have h2 : t₂ = ite (j = i) t₁ t₂, by simp only [hij.symm, if_false],
  have h_inter : (⋂ (t : ι) (H : t ∈ ({i, j} : finset ι)), ite (t = i) t₁ t₂)
      = (ite (i = i) t₁ t₂) ∩ (ite (j = i) t₁ t₂),
    by simp only [finset.set_bInter_singleton, finset.set_bInter_insert],
  have h_prod : (∏ (t : ι) in ({i, j} : finset ι), μ (ite (t = i) t₁ t₂))
      = μ (ite (i = i) t₁ t₂) * μ (ite (j = i) t₁ t₂),
    by simp only [hij, finset.prod_singleton, finset.prod_insert, not_false_iff,
      finset.mem_singleton],
  rw h1,
  nth_rewrite 1 h2,
  nth_rewrite 3 h2,
  rw [←h_inter, ←h_prod, h_indep {i, j} hf_m],
end

lemma Indep.indep {α ι} {m : ι → measurable_space α} [measurable_space α] {μ : measure α}
  (h_indep : Indep m μ) {i j : ι} (hij : i ≠ j) :
  indep (m i) (m j) μ :=
begin
  change indep_sets ((λ x, measurable_set[m x]) i) ((λ x, measurable_set[m x]) j) μ,
  exact Indep_sets.indep_sets h_indep hij,
end

lemma Indep_fun.indep_fun {α ι : Type*} {m₀ : measurable_space α} {μ : measure α} {β : ι → Type*}
  {m : Π x, measurable_space (β x)} {f : Π i, α → β i} (hf_Indep : Indep_fun m f μ)
  {i j : ι} (hij : i ≠ j) :
  indep_fun (f i) (f j) μ :=
hf_Indep.indep hij

end from_Indep_to_indep

/-!
## π-system lemma

Independence of measurable spaces is equivalent to independence of generating π-systems.
-/

section from_measurable_spaces_to_sets_of_sets
/-! ### Independence of measurable space structures implies independence of generating π-systems -/

lemma Indep.Indep_sets {α ι} [measurable_space α] {μ : measure α} {m : ι → measurable_space α}
  {s : ι → set (set α)} (hms : ∀ n, m n = generate_from (s n))
  (h_indep : Indep m μ) :
  Indep_sets s μ :=
λ S f hfs, h_indep S $ λ x hxS,
  ((hms x).symm ▸ measurable_set_generate_from (hfs x hxS) : measurable_set[m x] (f x))

lemma indep.indep_sets {α} [measurable_space α] {μ : measure α} {s1 s2 : set (set α)}
  (h_indep : indep (generate_from s1) (generate_from s2) μ) :
  indep_sets s1 s2 μ :=
λ t1 t2 ht1 ht2, h_indep t1 t2 (measurable_set_generate_from ht1) (measurable_set_generate_from ht2)

end from_measurable_spaces_to_sets_of_sets

section from_pi_systems_to_measurable_spaces
/-! ### Independence of generating π-systems implies independence of measurable space structures -/

private lemma indep_sets.indep_aux {α} {m2 : measurable_space α}
  {m : measurable_space α} {μ : measure α} [is_probability_measure μ] {p1 p2 : set (set α)}
  (h2 : m2 ≤ m) (hp2 : is_pi_system p2) (hpm2 : m2 = generate_from p2)
  (hyp : indep_sets p1 p2 μ) {t1 t2 : set α} (ht1 : t1 ∈ p1) (ht2m : measurable_set[m2] t2) :
  μ (t1 ∩ t2) = μ t1 * μ t2 :=
begin
  let μ_inter := μ.restrict t1,
  let ν := (μ t1) • μ,
  have h_univ : μ_inter set.univ = ν set.univ,
  by rw [measure.restrict_apply_univ, measure.smul_apply, smul_eq_mul, measure_univ, mul_one],
  haveI : is_finite_measure μ_inter := @restrict.is_finite_measure α _ t1 μ ⟨measure_lt_top μ t1⟩,
  rw [set.inter_comm, ←@measure.restrict_apply α _ μ t1 t2 (h2 t2 ht2m)],
  refine ext_on_measurable_space_of_generate_finite m p2 (λ t ht, _) h2 hpm2 hp2 h_univ ht2m,
  have ht2 : measurable_set[m] t,
  { refine h2 _ _,
    rw hpm2,
    exact measurable_set_generate_from ht, },
  rw [measure.restrict_apply ht2, measure.smul_apply, set.inter_comm],
  exact hyp t1 t ht1 ht,
end

lemma indep_sets.indep {α} {m1 m2 : measurable_space α} {m : measurable_space α}
  {μ : measure α} [is_probability_measure μ] {p1 p2 : set (set α)} (h1 : m1 ≤ m) (h2 : m2 ≤ m)
  (hp1 : is_pi_system p1) (hp2 : is_pi_system p2) (hpm1 : m1 = generate_from p1)
  (hpm2 : m2 = generate_from p2) (hyp : indep_sets p1 p2 μ) :
  indep m1 m2 μ :=
begin
  intros t1 t2 ht1 ht2,
  let μ_inter := μ.restrict t2,
  let ν := (μ t2) • μ,
  have h_univ : μ_inter set.univ = ν set.univ,
  by rw [measure.restrict_apply_univ, measure.smul_apply, smul_eq_mul, measure_univ, mul_one],
  haveI : is_finite_measure μ_inter := @restrict.is_finite_measure α _ t2 μ ⟨measure_lt_top μ t2⟩,
  rw [mul_comm, ←@measure.restrict_apply α _ μ t2 t1 (h1 t1 ht1)],
  refine ext_on_measurable_space_of_generate_finite m p1 (λ t ht, _) h1 hpm1 hp1 h_univ ht1,
  have ht1 : measurable_set[m] t,
  { refine h1 _ _,
    rw hpm1,
    exact measurable_set_generate_from ht, },
  rw [measure.restrict_apply ht1, measure.smul_apply, smul_eq_mul, mul_comm],
  exact indep_sets.indep_aux h2 hp2 hpm2 hyp ht ht2,
end

variables {α ι : Type*} {m0 : measurable_space α} {μ : measure α}

lemma Indep_sets.pi_Union_Inter_singleton {π : ι → set (set α)} {a : ι} {S : finset ι}
  (hp_ind : Indep_sets π μ) (haS : a ∉ S) :
  indep_sets (pi_Union_Inter π {S}) (π a) μ :=
begin
  rintros t1 t2 ⟨s, hs_mem, ft1, hft1_mem, ht1_eq⟩ ht2_mem_pia,
  rw set.mem_singleton_iff at hs_mem,
  subst hs_mem,
  let f := λ n, ite (n = a) t2 (ite (n ∈ s) (ft1 n) set.univ),
  have h_f_mem : ∀ n ∈ insert a s, f n ∈ π n,
  { intros n hn_mem_insert,
    simp_rw f,
    cases (finset.mem_insert.mp hn_mem_insert) with hn_mem hn_mem,
    { simp [hn_mem, ht2_mem_pia], },
    { have hn_ne_a : n ≠ a, by { rintro rfl, exact haS hn_mem, },
      simp [hn_ne_a, hn_mem, hft1_mem n hn_mem], }, },
  have h_f_mem_pi : ∀ n ∈ s, f n ∈ π n, from λ x hxS, h_f_mem x (by simp [hxS]),
  have h_t1 : t1 = ⋂ n ∈ s, f n,
  { suffices h_forall : ∀ n ∈ s, f n = ft1 n,
    { rw ht1_eq,
      congr' with n x,
      congr' with hns y,
      simp only [(h_forall n hns).symm], },
    intros n hnS,
    have hn_ne_a : n ≠ a, by { rintro rfl, exact haS hnS, },
    simp_rw [f, if_pos hnS, if_neg hn_ne_a], },
  have h_μ_t1 : μ t1 = ∏ n in s, μ (f n), by rw [h_t1, ←hp_ind s h_f_mem_pi],
  have h_t2 : t2 = f a, by { simp_rw [f], simp, },
  have h_μ_inter : μ (t1 ∩ t2) = ∏ n in insert a s, μ (f n),
  { have h_t1_inter_t2 : t1 ∩ t2 = ⋂ n ∈ insert a s, f n,
      by rw [h_t1, h_t2, finset.set_bInter_insert, set.inter_comm],
    rw [h_t1_inter_t2, ←hp_ind (insert a s) h_f_mem], },
  rw [h_μ_inter, finset.prod_insert haS, h_t2, mul_comm, h_μ_t1],
end

/-- Auxiliary lemma for `Indep_sets.Indep`. -/
theorem Indep_sets.Indep_aux [is_probability_measure μ] (m : ι → measurable_space α)
  (h_le : ∀ i, m i ≤ m0) (π : ι → set (set α)) (h_pi : ∀ n, is_pi_system (π n))
  (hp_univ : ∀ i, set.univ ∈ π i) (h_generate : ∀ i, m i = generate_from (π i))
  (h_ind : Indep_sets π μ) :
  Indep m μ :=
begin
  refine finset.induction (by simp [measure_univ]) _,
  intros a S ha_notin_S h_rec f hf_m,
  have hf_m_S : ∀ x ∈ S, measurable_set[m x] (f x) := λ x hx, hf_m x (by simp [hx]),
  rw [finset.set_bInter_insert, finset.prod_insert ha_notin_S, ←h_rec hf_m_S],
  let p := pi_Union_Inter π {S},
  set m_p := generate_from p with hS_eq_generate,
  have h_indep : indep m_p (m a) μ,
  { have hp : is_pi_system p := is_pi_system_pi_Union_Inter π h_pi {S} (sup_closed_singleton S),
    have h_le' : ∀ i, generate_from (π i) ≤ m0 := λ i, (h_generate i).symm.trans_le (h_le i),
    have hm_p : m_p ≤ m0 := generate_from_pi_Union_Inter_le π h_le' {S},
    exact indep_sets.indep hm_p (h_le a) hp (h_pi a) hS_eq_generate (h_generate a)
      (h_ind.pi_Union_Inter_singleton ha_notin_S), },
  refine h_indep.symm (f a) (⋂ n ∈ S, f n) (hf_m a (finset.mem_insert_self a S)) _,
  have h_le_p : ∀ i ∈ S, m i ≤ m_p,
  { intros n hn,
    rw [hS_eq_generate, h_generate n],
    exact le_generate_from_pi_Union_Inter {S} hp_univ (set.mem_singleton _) hn, },
  have h_S_f : ∀ i ∈ S, measurable_set[m_p] (f i) := λ i hi, (h_le_p i hi) (f i) (hf_m_S i hi),
  exact S.measurable_set_bInter h_S_f,
end

/-- The measurable space structures generated by independent pi-systems are independent. -/
theorem Indep_sets.Indep [is_probability_measure μ] (m : ι → measurable_space α)
  (h_le : ∀ i, m i ≤ m0) (π : ι → set (set α)) (h_pi : ∀ n, is_pi_system (π n))
  (h_generate : ∀ i, m i = generate_from (π i)) (h_ind : Indep_sets π μ) :
  Indep m μ :=
begin
  -- We want to apply `Indep_sets.Indep_aux`, but `π i` does not contain `univ`, hence we replace
  -- `π` with a new augmented pi-system `π'`, and prove all hypotheses for that pi-system.
  let π' := λ i, insert set.univ (π i),
  have h_subset : ∀ i, π i ⊆ π' i := λ i, set.subset_insert _ _,
  have h_pi' : ∀ n, is_pi_system (π' n) := λ n, (h_pi n).insert_univ,
  have h_univ' : ∀ i, set.univ ∈ π' i, from λ i, set.mem_insert _ _,
  have h_gen' : ∀ i, m i = generate_from (π' i),
  { intros i,
    rw [h_generate i, generate_from_insert_univ (π i)], },
  have h_ind' : Indep_sets π' μ,
  { intros S f hfπ',
    let S' := finset.filter (λ i, f i ≠ set.univ) S,
    have h_mem : ∀ i ∈ S', f i ∈ π i,
    { intros i hi,
      simp_rw [S', finset.mem_filter] at hi,
      cases hfπ' i hi.1,
      { exact absurd h hi.2, },
      { exact h, }, },
    have h_left : (⋂ i ∈ S, f i) = ⋂ i ∈ S', f i,
    { ext1 x,
      simp only [set.mem_Inter, finset.mem_filter, ne.def, and_imp],
      split,
      { exact λ h i hiS hif, h i hiS, },
      { intros h i hiS,
        by_cases hfi_univ : f i = set.univ,
        { rw hfi_univ, exact set.mem_univ _, },
        { exact h i hiS hfi_univ, }, }, },
    have h_right : ∏ i in S, μ (f i) = ∏ i in S', μ (f i),
    { rw ← finset.prod_filter_mul_prod_filter_not S (λ i, f i ≠ set.univ),
      simp only [ne.def, finset.filter_congr_decidable, not_not],
      suffices : ∏ x in finset.filter (λ x, f x = set.univ) S, μ (f x) = 1,
      { rw [this, mul_one], },
      calc ∏ x in finset.filter (λ x, f x = set.univ) S, μ (f x)
          = ∏ x in finset.filter (λ x, f x = set.univ) S, μ set.univ :
            finset.prod_congr rfl (λ x hx, by { rw finset.mem_filter at hx, rw hx.2, })
      ... = ∏ x in finset.filter (λ x, f x = set.univ) S, 1 :
            finset.prod_congr rfl (λ _ _, measure_univ)
      ... = 1 : finset.prod_const_one, },
    rw [h_left, h_right],
    exact h_ind S' h_mem, },
  exact Indep_sets.Indep_aux m h_le π' h_pi' h_univ' h_gen' h_ind',
end

end from_pi_systems_to_measurable_spaces

section indep_set
/-! ### Independence of measurable sets

We prove the following equivalences on `indep_set`, for measurable sets `s, t`.
* `indep_set s t μ ↔ μ (s ∩ t) = μ s * μ t`,
* `indep_set s t μ ↔ indep_sets {s} {t} μ`.
-/

variables {α : Type*} [measurable_space α] {s t : set α} (S T : set (set α))

lemma indep_set_iff_indep_sets_singleton (hs_meas : measurable_set s) (ht_meas : measurable_set t)
  (μ : measure α . volume_tac) [is_probability_measure μ] :
  indep_set s t μ ↔ indep_sets {s} {t} μ :=
⟨indep.indep_sets, λ h, indep_sets.indep
  (generate_from_le (λ u hu, by rwa set.mem_singleton_iff.mp hu))
  (generate_from_le (λ u hu, by rwa set.mem_singleton_iff.mp hu)) (is_pi_system.singleton s)
  (is_pi_system.singleton t) rfl rfl h⟩

lemma indep_set_iff_measure_inter_eq_mul (hs_meas : measurable_set s) (ht_meas : measurable_set t)
  (μ : measure α . volume_tac) [is_probability_measure μ] :
  indep_set s t μ ↔ μ (s ∩ t) = μ s * μ t :=
(indep_set_iff_indep_sets_singleton hs_meas ht_meas μ).trans indep_sets_singleton_iff

lemma indep_sets.indep_set_of_mem (hs : s ∈ S) (ht : t ∈ T) (hs_meas : measurable_set s)
  (ht_meas : measurable_set t) (μ : measure α . volume_tac) [is_probability_measure μ]
  (h_indep : indep_sets S T μ) :
  indep_set s t μ :=
(indep_set_iff_measure_inter_eq_mul hs_meas ht_meas μ).mpr (h_indep s t hs ht)

end indep_set

section indep_fun

/-! ### Independence of random variables

-/

variables {α β β' γ γ' : Type*} {mα : measurable_space α} {μ : measure α} {f : α → β} {g : α → β'}

lemma indep_fun_iff_measure_inter_preimage_eq_mul
  {mβ : measurable_space β} {mβ' : measurable_space β'} :
  indep_fun f g μ
    ↔ ∀ s t, measurable_set s → measurable_set t
      → μ (f ⁻¹' s ∩ g ⁻¹' t) = μ (f ⁻¹' s) * μ (g ⁻¹' t) :=
begin
  split; intro h,
  { refine λ s t hs ht, h (f ⁻¹' s) (g ⁻¹' t) ⟨s, hs, rfl⟩ ⟨t, ht, rfl⟩, },
  { rintros _ _ ⟨s, hs, rfl⟩ ⟨t, ht, rfl⟩, exact h s t hs ht, },
end

lemma Indep_fun_iff_measure_inter_preimage_eq_mul {ι : Type*} {β : ι → Type*}
  (m : Π x, measurable_space (β x)) (f : Π i, α → β i) :
  Indep_fun m f μ
    ↔ ∀ (S : finset ι) {sets : Π i : ι, set (β i)} (H : ∀ i, i ∈ S → measurable_set[m i] (sets i)),
      μ (⋂ i ∈ S, (f i) ⁻¹' (sets i)) = ∏ i in S, μ ((f i) ⁻¹' (sets i)) :=
begin
  refine ⟨λ h S sets h_meas, h _ (λ i hi_mem, ⟨sets i, h_meas i hi_mem, rfl⟩), _⟩,
  intros h S setsα h_meas,
  let setsβ : (Π i : ι, set (β i)) := λ i,
    dite (i ∈ S) (λ hi_mem, (h_meas i hi_mem).some) (λ _, set.univ),
  have h_measβ : ∀ i ∈ S, measurable_set[m i] (setsβ i),
  { intros i hi_mem,
    simp_rw [setsβ, dif_pos hi_mem],
    exact (h_meas i hi_mem).some_spec.1, },
  have h_preim : ∀ i ∈ S, setsα i = (f i) ⁻¹' (setsβ i),
  { intros i hi_mem,
    simp_rw [setsβ, dif_pos hi_mem],
    exact (h_meas i hi_mem).some_spec.2.symm, },
  have h_left_eq : μ (⋂ i ∈ S, setsα i) = μ (⋂ i ∈ S, (f i) ⁻¹' (setsβ i)),
  { congr' with i x,
    simp only [set.mem_Inter],
    split; intros h hi_mem; specialize h hi_mem,
    { rwa h_preim i hi_mem at h, },
    { rwa h_preim i hi_mem, }, },
  have h_right_eq : (∏ i in S, μ (setsα i)) = ∏ i in S, μ ((f i) ⁻¹' (setsβ i)),
  { refine finset.prod_congr rfl (λ i hi_mem, _),
    rw h_preim i hi_mem, },
  rw [h_left_eq, h_right_eq],
  exact h S h_measβ,
end

lemma indep_fun_iff_indep_set_preimage {mβ : measurable_space β} {mβ' : measurable_space β'}
  [is_probability_measure μ] (hf : measurable f) (hg : measurable g) :
  indep_fun f g μ ↔ ∀ s t, measurable_set s → measurable_set t → indep_set (f ⁻¹' s) (g ⁻¹' t) μ :=
begin
  refine indep_fun_iff_measure_inter_preimage_eq_mul.trans _,
  split; intros h s t hs ht; specialize h s t hs ht,
  { rwa indep_set_iff_measure_inter_eq_mul (hf hs) (hg ht) μ, },
  { rwa ← indep_set_iff_measure_inter_eq_mul (hf hs) (hg ht) μ, },
end

lemma indep_fun.ae_eq {mβ : measurable_space β} {f g f' g' : α → β}
  (hfg : indep_fun f g μ) (hf : f =ᵐ[μ] f') (hg : g =ᵐ[μ] g') :
  indep_fun f' g' μ :=
begin
  rintro _ _ ⟨A, hA, rfl⟩ ⟨B, hB, rfl⟩,
  have h1 : f ⁻¹' A =ᵐ[μ] f' ⁻¹' A := hf.fun_comp A,
  have h2 : g ⁻¹' B =ᵐ[μ] g' ⁻¹' B := hg.fun_comp B,
  rw [←measure_congr h1, ←measure_congr h2, ←measure_congr (h1.inter h2)],
  exact hfg _ _ ⟨_, hA, rfl⟩ ⟨_, hB, rfl⟩
end

lemma indep_fun.comp {mβ : measurable_space β} {mβ' : measurable_space β'}
  {mγ : measurable_space γ} {mγ' : measurable_space γ'} {φ : β → γ} {ψ : β' → γ'}
  (hfg : indep_fun f g μ) (hφ : measurable φ) (hψ : measurable ψ) :
  indep_fun (φ ∘ f) (ψ ∘ g) μ :=
begin
  rintro _ _ ⟨A, hA, rfl⟩ ⟨B, hB, rfl⟩,
  apply hfg,
  { exact ⟨φ ⁻¹' A, hφ hA, set.preimage_comp.symm⟩ },
  { exact ⟨ψ ⁻¹' B, hψ hB, set.preimage_comp.symm⟩ }
end

/-- If `f` is a family of mutually independent random variables (`Indep_fun m f μ`) and `S, T` are
two disjoint finite index sets, then the tuple formed by `f i` for `i ∈ S` is independent of the
tuple `(f i)_i` for `i ∈ T`. -/
lemma Indep_fun.indep_fun_finset [is_probability_measure μ]
  {ι : Type*} {β : ι → Type*} {m : Π i, measurable_space (β i)}
  {f : Π i, α → β i} (S T : finset ι) (hST : disjoint S T) (hf_Indep : Indep_fun m f μ)
  (hf_meas : ∀ i, measurable (f i)) :
  indep_fun (λ a (i : S), f i a) (λ a (i : T), f i a) μ :=
begin
  -- We introduce π-systems, build from the π-system of boxes which generates `measurable_space.pi`.
  let πSβ := (set.pi (set.univ : set S) ''
    (set.pi (set.univ : set S) (λ i, {s : set (β i) | measurable_set[m i] s}))),
  let πS := {s : set α | ∃ t ∈ πSβ, (λ a (i : S), f i a) ⁻¹' t = s},
  have hπS_pi : is_pi_system πS := is_pi_system_pi.comap (λ a i, f i a),
  have hπS_gen : measurable_space.pi.comap (λ a (i : S), f i a) = generate_from πS,
  { rw [generate_from_pi.symm, comap_generate_from],
    { congr' with s,
      simp only [set.mem_image, set.mem_set_of_eq, exists_prop], },
    { exact finset.fintype_coe_sort S, }, },
  let πTβ := (set.pi (set.univ : set T) ''
    (set.pi (set.univ : set T) (λ i, {s : set (β i) | measurable_set[m i] s}))),
  let πT := {s : set α | ∃ t ∈ πTβ, (λ a (i : T), f i a) ⁻¹' t = s},
  have hπT_pi : is_pi_system πT := is_pi_system_pi.comap (λ a i, f i a),
  have hπT_gen : measurable_space.pi.comap (λ a (i : T), f i a) = generate_from πT,
  { rw [generate_from_pi.symm, comap_generate_from],
    { congr' with s,
      simp only [set.mem_image, set.mem_set_of_eq, exists_prop], },
    { exact finset.fintype_coe_sort T, }, },

  -- To prove independence, we prove independence of the generating π-systems.
  refine indep_sets.indep (measurable.comap_le (measurable_pi_iff.mpr (λ i, hf_meas i)))
    (measurable.comap_le (measurable_pi_iff.mpr (λ i, hf_meas i))) hπS_pi hπT_pi hπS_gen hπT_gen _,

  rintros _ _ ⟨s, ⟨sets_s, hs1, hs2⟩, rfl⟩ ⟨t, ⟨sets_t, ht1, ht2⟩, rfl⟩,
  simp only [set.mem_univ_pi, set.mem_set_of_eq] at hs1 ht1,
  rw [← hs2, ← ht2],
  let sets_s' : (Π i : ι, set (β i)) := λ i, dite (i ∈ S) (λ hi, sets_s ⟨i, hi⟩) (λ _, set.univ),
  have h_sets_s'_eq : ∀ {i} (hi : i ∈ S), sets_s' i = sets_s ⟨i, hi⟩,
  { intros i hi, simp_rw [sets_s', dif_pos hi], },
  have h_sets_s'_univ : ∀ {i} (hi : i ∈ T), sets_s' i = set.univ,
  { intros i hi, simp_rw [sets_s', dif_neg (finset.disjoint_right.mp hST hi)], },
  let sets_t' : (Π i : ι, set (β i)) := λ i, dite (i ∈ T) (λ hi, sets_t ⟨i, hi⟩) (λ _, set.univ),
  have h_sets_t'_univ : ∀ {i} (hi : i ∈ S), sets_t' i = set.univ,
  { intros i hi, simp_rw [sets_t', dif_neg (finset.disjoint_left.mp hST hi)], },
  have h_meas_s' : ∀ i ∈ S, measurable_set (sets_s' i),
  { intros i hi, rw h_sets_s'_eq hi, exact hs1 _, },
  have h_meas_t' : ∀ i ∈ T, measurable_set (sets_t' i),
  { intros i hi, simp_rw [sets_t', dif_pos hi], exact ht1 _, },
  have h_eq_inter_S : (λ (a : α) (i : ↥S), f ↑i a) ⁻¹' set.pi set.univ sets_s
    = ⋂ i ∈ S, (f i) ⁻¹' (sets_s' i),
  { ext1 x,
    simp only [set.mem_preimage, set.mem_univ_pi, set.mem_Inter],
    split; intro h,
    { intros i hi, rw [h_sets_s'_eq hi], exact h ⟨i, hi⟩, },
    { rintros ⟨i, hi⟩, specialize h i hi, rw [h_sets_s'_eq hi] at h, exact h, }, },
  have h_eq_inter_T : (λ (a : α) (i : ↥T), f ↑i a) ⁻¹' set.pi set.univ sets_t
    = ⋂ i ∈ T, (f i) ⁻¹' (sets_t' i),
  { ext1 x,
    simp only [set.mem_preimage, set.mem_univ_pi, set.mem_Inter],
    split; intro h,
    { intros i hi, simp_rw [sets_t', dif_pos hi], exact h ⟨i, hi⟩, },
    { rintros ⟨i, hi⟩, specialize h i hi, simp_rw [sets_t', dif_pos hi] at h, exact h, }, },
  rw Indep_fun_iff_measure_inter_preimage_eq_mul at hf_Indep,
  rw [h_eq_inter_S, h_eq_inter_T, hf_Indep S h_meas_s', hf_Indep T h_meas_t'],
  have h_Inter_inter : (⋂ i ∈ S, (f i) ⁻¹' (sets_s' i)) ∩ (⋂ i ∈ T, (f i) ⁻¹' (sets_t' i))
    = ⋂ i ∈ (S ∪ T), (f i) ⁻¹' (sets_s' i ∩ sets_t' i),
  { ext1 x,
    simp only [set.mem_inter_eq, set.mem_Inter, set.mem_preimage, finset.mem_union],
    split; intro h,
    { intros i hi,
      cases hi,
      { rw h_sets_t'_univ hi, exact ⟨h.1 i hi, set.mem_univ _⟩, },
      { rw h_sets_s'_univ hi, exact ⟨set.mem_univ _, h.2 i hi⟩, }, },
    { exact ⟨λ i hi, (h i (or.inl hi)).1, λ i hi, (h i (or.inr hi)).2⟩, }, },
  rw [h_Inter_inter, hf_Indep (S ∪ T)],
  swap, { intros i hi_mem,
    rw finset.mem_union at hi_mem,
    cases hi_mem,
    { rw [h_sets_t'_univ hi_mem, set.inter_univ], exact h_meas_s' i hi_mem, },
    { rw [h_sets_s'_univ hi_mem, set.univ_inter], exact h_meas_t' i hi_mem, }, },
  rw finset.prod_union hST,
  congr' 1,
  { refine finset.prod_congr rfl (λ i hi, _),
    rw [h_sets_t'_univ hi, set.inter_univ], },
  { refine finset.prod_congr rfl (λ i hi, _),
    rw [h_sets_s'_univ hi, set.univ_inter], },
end

lemma Indep_fun.indep_fun_prod [is_probability_measure μ]
  {ι : Type*} {β : ι → Type*} {m : Π i, measurable_space (β i)}
  {f : Π i, α → β i} (hf_Indep : Indep_fun m f μ) (hf_meas : ∀ i, measurable (f i))
  (i j k : ι) (hik : i ≠ k) (hjk : j ≠ k) :
  indep_fun (λ a, (f i a, f j a)) (f k) μ :=
begin
  classical,
  have h_right : f k = (λ p : (Π j : ({k} : finset ι), β j), p ⟨k, finset.mem_singleton_self k⟩)
    ∘ (λ a (j : ({k} : finset ι)), f j a) := rfl,
  have h_meas_right : measurable
      (λ p : (Π j : ({k} : finset ι), β j), p ⟨k, finset.mem_singleton_self k⟩),
    from measurable_pi_apply ⟨k, finset.mem_singleton_self k⟩,
  let s : finset ι := {i, j},
  have h_left : (λ ω, (f i ω, f j ω))
    = (λ p : (Π l : s, β l), (p ⟨i, finset.mem_insert_self i _⟩,
        p ⟨j, finset.mem_insert_of_mem (finset.mem_singleton_self _)⟩))
      ∘ (λ a (j : s), f j a),
  { ext1 a,
    simp only [prod.mk.inj_iff],
    split; refl, },
  have h_meas_left : measurable (λ p : (Π l : s, β l), (p ⟨i, finset.mem_insert_self i _⟩,
      p ⟨j, finset.mem_insert_of_mem (finset.mem_singleton_self _)⟩)),
    from measurable.prod (measurable_pi_apply ⟨i, finset.mem_insert_self i {j}⟩)
      (measurable_pi_apply ⟨j, finset.mem_insert_of_mem (finset.mem_singleton_self j)⟩),
  rw [h_left, h_right],
  refine (hf_Indep.indep_fun_finset s {k} _ hf_meas).comp h_meas_left h_meas_right,
  intros x hx,
  simp only [finset.inf_eq_inter, finset.mem_inter, finset.mem_insert, finset.mem_singleton] at hx,
  simp only [finset.bot_eq_empty, finset.not_mem_empty],
  cases hx.1 with hx_eq hx_eq; rw hx_eq at hx,
  { exact hik hx.2, },
  { exact hjk hx.2, },
end

@[to_additive]
lemma Indep_fun.mul [is_probability_measure μ]
  {ι : Type*} {β : Type*} {m : measurable_space β} [has_mul β] [has_measurable_mul₂ β]
  {f : ι → α → β} (hf_Indep : Indep_fun (λ _, m) f μ) (hf_meas : ∀ i, measurable (f i))
  (i j k : ι) (hik : i ≠ k) (hjk : j ≠ k) :
  indep_fun (f i * f j) (f k) μ :=
begin
  have : indep_fun (λ ω, (f i ω, f j ω)) (f k) μ := hf_Indep.indep_fun_prod hf_meas i j k hik hjk,
  change indep_fun ((λ p : β × β, p.fst * p.snd) ∘ (λ ω, (f i ω, f j ω))) (id ∘ (f k)) μ,
  exact indep_fun.comp this (measurable_fst.mul measurable_snd) measurable_id,
end

@[to_additive]
lemma Indep_fun.indep_fun_finset_prod_of_not_mem [is_probability_measure μ]
  {ι : Type*} {β : Type*} {m : measurable_space β} [comm_monoid β] [has_measurable_mul₂ β]
  {f : ι → α → β} (hf_Indep : Indep_fun (λ _, m) f μ) (hf_meas : ∀ i, measurable (f i))
  {s : finset ι} {i : ι} (hi : i ∉ s) :
  indep_fun (∏ j in s, f j) (f i) μ :=
begin
  classical,
  have h_right : f i = (λ p : (Π j : ({i} : finset ι), β), p ⟨i, finset.mem_singleton_self i⟩)
    ∘ (λ a (j : ({i} : finset ι)), f j a) := rfl,
  have h_meas_right : measurable
      (λ p : (Π j : ({i} : finset ι), β), p ⟨i, finset.mem_singleton_self i⟩),
    from measurable_pi_apply ⟨i, finset.mem_singleton_self i⟩,
  have h_left : (∏ j in s, f j) = (λ p : (Π j : s, β), ∏ j, p j) ∘ (λ a (j : s), f j a),
  { ext1 a,
    simp only [function.comp_app],
    have : (∏ (j : ↥s), f ↑j a) = (∏ (j : ↥s), f ↑j) a, by rw finset.prod_apply,
    rw [this, finset.prod_coe_sort], },
  have h_meas_left : measurable (λ p : (Π j : s, β), ∏ j, p j),
    from finset.univ.measurable_prod (λ (j : ↥s) (H : j ∈ finset.univ), measurable_pi_apply j),
  rw [h_left, h_right],
  exact (hf_Indep.indep_fun_finset s {i} (finset.disjoint_singleton_left.mpr hi).symm hf_meas).comp
    h_meas_left h_meas_right,
end

@[to_additive]
lemma Indep_fun.indep_fun_prod_range_succ [is_probability_measure μ]
  {β : Type*} {m : measurable_space β} [comm_monoid β] [has_measurable_mul₂ β]
  {f : ℕ → α → β} (hf_Indep : Indep_fun (λ _, m) f μ) (hf_meas : ∀ i, measurable (f i))
  (n : ℕ) :
  indep_fun (∏ j in finset.range n, f j) (f n) μ :=
hf_Indep.indep_fun_finset_prod_of_not_mem hf_meas finset.not_mem_range_self

end indep_fun

end probability_theory