feat(measure_theory/card_measurable_space): cardinality of generated sigma-algebras (#12422)

If a sigma-algebra is generated by a set of sets `s` whose cardinality is at most the continuum,
then the sigma-algebra satisfies the same cardinality bound.



Co-authored-by: Violeta Hernández <vi.hdz.p@gmail.com>

Author: sgouezel

src/measure_theory/card_measurable_space.lean

@@ -0,0 +1,161 @@
+/-
+Copyright (c) 2022 Sébastien Gouëzel. All rights reserved.
+Released under Apache 2.0 license as described in the file LICENSE.
+Authors: Sébastien Gouëzel, Violeta Hernández Palacios
+-/
+import measure_theory.measurable_space_def
+import set_theory.continuum
+import set_theory.cofinality
+
+/-!
+# Cardinal of sigma-algebras
+
+If a sigma-algebra is generated by a set of sets `s`, then the cardinality of the sigma-algebra is
+bounded by `(max (#s) 2) ^ ω`. This is stated in `measurable_space.cardinal_generate_measurable_le`
+and `measurable_space.cardinal_measurable_set_le`.
+
+In particular, if `#s ≤ 𝔠`, then the generated sigma-algebra has cardinality at most `𝔠`, see
+`measurable_space.cardinal_measurable_set_le_continuum`.
+
+For the proof, we rely on an explicit inductive construction of the sigma-algebra generated by
+`s` (instead of the inductive predicate `generate_measurable`). This transfinite inductive
+construction is parameterized by an ordinal `< ω₁`, and the cardinality bound is preserved along
+each step of the construction.
+-/
+
+universe u
+variables {α : Type u}
+
+open_locale cardinal
+open cardinal set
+
+local notation a `<₁` b := (aleph 1 : cardinal.{u}).ord.out.r a b
+local notation `ω₁`:= (aleph 1 : cardinal.{u}).ord.out.α
+
+namespace measurable_space
+
+/-- Transfinite induction construction of the sigma-algebra generated by a set of sets `s`. At each
+step, we add all elements of `s`, the empty set, the complements of already constructed sets, and
+countable unions of already constructed sets. We index this construction by an ordinal `< ω₁`, as
+this will be enough to generate all sets in the sigma-algebra. -/
+def generate_measurable_rec (s : set (set α)) : ω₁ → set (set α)
+| i := let S := ⋃ j : {j // j <₁ i}, generate_measurable_rec j.1 in
+    s ∪ {∅} ∪ compl '' S ∪ (set.range (λ (f : ℕ → S), ⋃ n, (f n).1))
+using_well_founded {dec_tac := `[exact j.2]}
+
+/-- At each step of the inductive construction, the cardinality bound `≤ (max (#s) 2) ^ ω`
+holds. -/
+lemma cardinal_generate_measurable_rec_le (s : set (set α)) (i : ω₁) :
+  #(generate_measurable_rec s i) ≤ (max (#s) 2) ^ omega.{u} :=
+begin
+  apply (aleph 1).ord.out.wo.wf.induction i,
+  assume i IH,
+  have A := omega_le_aleph 1,
+  have B : aleph 1 ≤ (max (#s) 2) ^ omega.{u} :=
+    aleph_one_le_continuum.trans (power_le_power_right (le_max_right _ _)),
+  have C : omega.{u} ≤ (max (#s) 2) ^ omega.{u} := A.trans B,
+  have J : #(⋃ (j : {j // j <₁ i}), generate_measurable_rec s j.1) ≤ (max (#s) 2) ^ omega.{u},
+  { apply (mk_Union_le _).trans,
+    have D : # {j // j <₁ i} ≤ aleph 1 := (mk_subtype_le _).trans (le_of_eq (aleph 1).mk_ord_out),
+    have E : cardinal.sup.{u u}
+      (λ (j : {j // j <₁ i}), #(generate_measurable_rec s j.1)) ≤ (max (#s) 2) ^ omega.{u} :=
+    cardinal.sup_le.2 (λ ⟨j, hj⟩, IH j hj),
+    apply (mul_le_mul' D E).trans,
+    rw mul_eq_max A C,
+    exact max_le B le_rfl },
+  rw [generate_measurable_rec],
+  apply_rules [(mk_union_le _ _).trans, add_le_of_le C, mk_image_le.trans],
+  { exact (le_max_left _ _).trans (self_le_power _ one_lt_omega.le) },
+  { rw [mk_singleton],
+    exact one_lt_omega.le.trans C },
+  { apply mk_range_le.trans,
+    simp only [mk_pi, subtype.val_eq_coe, prod_const, lift_uzero, mk_denumerable, lift_omega],
+    have := @power_le_power_right _ _ omega.{u} J,
+    rwa [← power_mul, omega_mul_omega] at this }
+end
+
+lemma cardinal_Union_generate_measurable_rec_le (s : set (set α)) :
+  #(⋃ i, generate_measurable_rec s i) ≤ (max (#s) 2) ^ omega.{u} :=
+begin
+  apply (mk_Union_le _).trans,
+  rw [(aleph 1).mk_ord_out],
+  refine le_trans (mul_le_mul' aleph_one_le_continuum
+    (cardinal.sup_le.2 (λ i, cardinal_generate_measurable_rec_le s i))) _,
+  have := power_le_power_right (le_max_right (#s) 2),
+  rw mul_eq_max omega_le_continuum (omega_le_continuum.trans this),
+  exact max_le this le_rfl
+end
+
+/-- A set in the sigma-algebra generated by a set of sets `s` is constructed at some step of the
+transfinite induction defining `generate_measurable_rec`.
+The other inclusion is also true, but not proved here as it is not needed for the cardinality
+bounds.-/
+theorem generate_measurable_subset_rec (s : set (set α)) ⦃t : set α⦄
+  (ht : generate_measurable s t) :
+  t ∈ ⋃ i, generate_measurable_rec s i :=
+begin
+  haveI : nonempty ω₁, by simp [← mk_ne_zero_iff, ne_of_gt, (aleph 1).mk_ord_out, aleph_pos 1],
+  inhabit ω₁,
+  induction ht with u hu u hu IH f hf IH,
+  { refine mem_Union.2 ⟨default, _⟩,
+    rw generate_measurable_rec,
+    simp only [hu, mem_union_eq, true_or] },
+  { refine mem_Union.2 ⟨default, _⟩,
+    rw generate_measurable_rec,
+    simp only [union_singleton, mem_union_eq, mem_insert_iff, eq_self_iff_true, true_or] },
+  { rcases mem_Union.1 IH with ⟨i, hi⟩,
+    obtain ⟨j, hj⟩ : ∃ j, i <₁ j := ordinal.has_succ_of_is_limit
+      (by { rw ordinal.type_out, exact ord_aleph_is_limit 1 }) _,
+    apply mem_Union.2 ⟨j, _⟩,
+    rw generate_measurable_rec,
+    have : ∃ a, (a <₁ j) ∧ u ∈ generate_measurable_rec s a := ⟨i, hj, hi⟩,
+    simp [this] },
+  { have : ∀ n, ∃ i, f n ∈ generate_measurable_rec s i := λ n, by simpa using IH n,
+    choose I hI using this,
+    obtain ⟨j, hj⟩ : ∃ j, ∀ k, k ∈ range I → (k <₁ j),
+    { apply ordinal.lt_cof_type,
+      simp only [is_regular_aleph_one.2, mk_singleton, ordinal.type_out],
+      have : #(range I) = lift.{0} (#(range I)), by simp only [lift_uzero],
+      rw this,
+      apply mk_range_le_lift.trans_lt _,
+      simp [omega_lt_aleph_one] },
+    apply mem_Union.2 ⟨j, _⟩,
+    rw generate_measurable_rec,
+    have : ∃ (g : ℕ → (↥⋃ (i : {i // i <₁ j}), generate_measurable_rec s i.1)),
+      (⋃ (n : ℕ), ↑(g n)) = (⋃ n, f n),
+    { refine ⟨λ n, ⟨f n, _⟩, rfl⟩,
+      exact mem_Union.2 ⟨⟨I n, hj (I n) (mem_range_self _)⟩, hI n⟩ },
+    simp [this] }
+end
+
+/-- If a sigma-algebra is generated by a set of sets `s`, then the sigma
+algebra has cardinality at most `(max (#s) 2) ^ ω`. -/
+theorem cardinal_generate_measurable_le (s : set (set α)) :
+  #{t | generate_measurable s t} ≤ (max (#s) 2) ^ omega.{u} :=
+(mk_subtype_le_of_subset (generate_measurable_subset_rec s)).trans
+  (cardinal_Union_generate_measurable_rec_le s)
+
+/-- If a sigma-algebra is generated by a set of sets `s`, then the sigma
+algebra has cardinality at most `(max (#s) 2) ^ ω`. -/
+theorem cardinal_measurable_set_le (s : set (set α)) :
+  #{t | @measurable_set α (generate_from s) t} ≤ (max (#s) 2) ^ omega.{u} :=
+cardinal_generate_measurable_le s
+
+/-- If a sigma-algebra is generated by a set of sets `s` with cardinality at most the continuum,
+then the sigma algebra has the same cardinality bound. -/
+theorem cardinal_generate_measurable_le_continuum {s : set (set α)} (hs : #s ≤ 𝔠) :
+  #{t | generate_measurable s t} ≤ 𝔠 :=
+calc
+#{t | generate_measurable s t}
+    ≤ (max (#s) 2) ^ omega.{u} : cardinal_generate_measurable_le s
+... ≤ 𝔠 ^ omega.{u} :
+  by exact_mod_cast power_le_power_right (max_le hs (nat_lt_continuum 2).le)
+... = 𝔠 : continuum_power_omega
+
+/-- If a sigma-algebra is generated by a set of sets `s` with cardinality at most the continuum,
+then the sigma algebra has the same cardinality bound. -/
+theorem cardinal_measurable_set_le_continuum {s : set (set α)} (hs : #s ≤ 𝔠) :
+  #{t | @measurable_set α (generate_from s) t} ≤ 𝔠 :=
+cardinal_generate_measurable_le_continuum hs
+
+end measurable_space

src/set_theory/cardinal.lean

@@ -460,11 +460,18 @@ lt_of_le_of_ne (zero_le _) zero_ne_one
 lemma zero_power_le (c : cardinal.{u}) : (0 : cardinal.{u}) ^ c ≤ 1 :=
 by { by_cases h : c = 0, rw [h, power_zero], rw [zero_power h], apply zero_le }
 
-theorem power_le_power_left : ∀{a b c : cardinal}, a ≠ 0 → b ≤ c → a ^ b ≤ a ^ c :=
+theorem power_le_power_left : ∀ {a b c : cardinal}, a ≠ 0 → b ≤ c → a ^ b ≤ a ^ c :=
 by rintros ⟨α⟩ ⟨β⟩ ⟨γ⟩ hα ⟨e⟩; exact
   let ⟨a⟩ := mk_ne_zero_iff.1 hα in
   ⟨@embedding.arrow_congr_left _ _ _ ⟨a⟩ e⟩
 
+theorem self_le_power (a : cardinal) {b : cardinal} (hb : 1 ≤ b) : a ≤ a ^ b :=
+begin
+  rcases eq_or_ne a 0 with rfl|ha,
+  { exact zero_le _ },
+  { convert power_le_power_left ha hb, exact power_one.symm }
+end
+
 /-- **Cantor's theorem** -/
 theorem cantor (a : cardinal.{u}) : a < 2 ^ a :=
 begin

src/set_theory/cardinal_ordinal.lean

@@ -232,6 +232,9 @@ aleph'_is_normal.trans $ add_is_normal ordinal.omega
 theorem succ_omega : succ ω = aleph 1 :=
 by rw [← aleph_zero, ← aleph_succ, ordinal.succ_zero]
 
+lemma omega_lt_aleph_one : ω < aleph 1 :=
+by { rw ← succ_omega, exact lt_succ_self _ }
+
 lemma countable_iff_lt_aleph_one {α : Type*} (s : set α) : countable s ↔ #s < aleph 1 :=
 by rw [← succ_omega, lt_succ, mk_set_le_omega]
 
@@ -456,6 +459,10 @@ begin
     { exact le_max_of_le_right (le_of_lt (add_lt_omega (lt_of_not_ge ha) (lt_of_not_ge hb))) } }
 end
 
+theorem add_le_of_le {a b c : cardinal} (hc : ω ≤ c)
+  (h1 : a ≤ c) (h2 : b ≤ c) : a + b ≤ c :=
+(add_le_add h1 h2).trans $ le_of_eq $ add_eq_self hc
+
 theorem add_lt_of_lt {a b c : cardinal} (hc : ω ≤ c)
   (h1 : a < c) (h2 : b < c) : a + b < c :=
 lt_of_le_of_lt (add_le_add (le_max_left a b) (le_max_right a b)) $

src/set_theory/cofinality.lean

@@ -542,6 +542,9 @@ theorem succ_is_regular {c : cardinal.{u}} (h : ω ≤ c) : is_regular (succ c)
     apply typein_lt_type }
 end⟩
 
+theorem is_regular_aleph_one : is_regular (aleph 1) :=
+by { rw ← succ_omega, exact succ_is_regular le_rfl }
+
 theorem aleph'_succ_is_regular {o : ordinal} (h : ordinal.omega ≤ o) : is_regular (aleph' o.succ) :=
 by { rw aleph'_succ, exact succ_is_regular (omega_le_aleph'.2 h) }
 

src/set_theory/continuum.lean

@@ -48,6 +48,9 @@ lemma continuum_pos : 0 < 𝔠 := nat_lt_continuum 0
 
 lemma continuum_ne_zero : 𝔠 ≠ 0 := continuum_pos.ne'
 
+lemma aleph_one_le_continuum : aleph 1 ≤ 𝔠 :=
+by { rw ← succ_omega, exact succ_le.2 omega_lt_continuum }
+
 /-!
 ### Addition
 -/

src/set_theory/ordinal.lean

@@ -474,6 +474,8 @@ instance ordinal.is_equivalent : setoid Well_order :=
 /-- `ordinal.{u}` is the type of well orders in `Type u`, up to order isomorphism. -/
 def ordinal : Type (u + 1) := quotient ordinal.is_equivalent
 
+instance (o : ordinal) : has_well_founded o.out.α := ⟨o.out.r, o.out.wo.wf⟩
+
 namespace ordinal
 
 /-- The order type of a well order is an ordinal. -/