Theorem: MeasureTheory.Conservative.measure_mem_forall_ge_image_not_mem_eq_zero
File: Mathlib/Dynamics/Ergodic/Conservative.lean

Proof steps:
state_before: ι : Type u_1
α : Type u_2
inst✝ : MeasurableSpace α
f : α → α
s : Set α
μ : Measure α
hf : Conservative f μ
hs : NullMeasurableSet s μ
n : ℕ
⊢ μ {x | x ∈ s ∧ ∀ m ≥ n, f^[m] x ∉ s} = 0
state_after: ι : Type u_1
α : Type u_2
inst✝ : MeasurableSpace α
f : α → α
s : Set α
μ : Measure α
hf : Conservative f μ
hs : NullMeasurableSet s μ
n : ℕ
H : ¬μ {x | x ∈ s ∧ ∀ m ≥ n, f^[m] x ∉ s} = 0
⊢ False
tactic: by_contra H
state_before: ι : Type u_1
α : Type u_2
inst✝ : MeasurableSpace α
f : α → α
s : Set α
μ : Measure α
hf : Conservative f μ
hs : NullMeasurableSet s μ
n : ℕ
H : ¬μ {x | x ∈ s ∧ ∀ m ≥ n, f^[m] x ∉ s} = 0
⊢ False
state_after: ι : Type u_1
α : Type u_2
inst✝ : MeasurableSpace α
f : α → α
s : Set α
μ : Measure α
hf : Conservative f μ
hs : NullMeasurableSet s μ
n : ℕ
H : ¬μ {x | x ∈ s ∧ ∀ m ≥ n, f^[m] x ∉ s} = 0
this : NullMeasurableSet (s ∩ {x | ∀ m ≥ n, f^[m] x ∉ s}) μ
⊢ False
tactic: have : NullMeasurableSet (s ∩ { x | ∀ m ≥ n, f^[m] x ∉ s }) μ := by
    simp only [setOf_forall, ← compl_setOf]
    exact hs.inter <| .biInter (to_countable _) fun m _ ↦
      (hs.preimage <| hf.toQuasiMeasurePreserving.iterate m).compl
state_before: ι : Type u_1
α : Type u_2
inst✝ : MeasurableSpace α
f : α → α
s : Set α
μ : Measure α
hf : Conservative f μ
hs : NullMeasurableSet s μ
n : ℕ
H : ¬μ {x | x ∈ s ∧ ∀ m ≥ n, f^[m] x ∉ s} = 0
this : NullMeasurableSet (s ∩ {x | ∀ m ≥ n, f^[m] x ∉ s}) μ
⊢ False
state_after: case intro.intro
ι : Type u_1
α : Type u_2
inst✝ : MeasurableSpace α
f : α → α
s : Set α
μ : Measure α
hf : Conservative f μ
hs : NullMeasurableSet s μ
n : ℕ
H : ¬μ {x | x ∈ s ∧ ∀ m ≥ n, f^[m] x ∉ s} = 0
this : NullMeasurableSet (s ∩ {x | ∀ m ≥ n, f^[m] x ∉ s}) μ
m : ℕ
hmn : m > n
hm : μ (s ∩ {x | ∀ m ≥ n, f^[m] x ∉ s} ∩ f^[m] ⁻¹' (s ∩ {x | ∀ m ≥ n, f^[m] x ∉ s})) ≠ 0
⊢ False
tactic: rcases (hf.exists_gt_measure_inter_ne_zero this H) n with ⟨m, hmn, hm⟩
state_before: case intro.intro
ι : Type u_1
α : Type u_2
inst✝ : MeasurableSpace α
f : α → α
s : Set α
μ : Measure α
hf : Conservative f μ
hs : NullMeasurableSet s μ
n : ℕ
H : ¬μ {x | x ∈ s ∧ ∀ m ≥ n, f^[m] x ∉ s} = 0
this : NullMeasurableSet (s ∩ {x | ∀ m ≥ n, f^[m] x ∉ s}) μ
m : ℕ
hmn : m > n
hm : μ (s ∩ {x | ∀ m ≥ n, f^[m] x ∉ s} ∩ f^[m] ⁻¹' (s ∩ {x | ∀ m ≥ n, f^[m] x ∉ s})) ≠ 0
⊢ False
state_after: case intro.intro.intro.intro.intro.intro
ι : Type u_1
α : Type u_2
inst✝ : MeasurableSpace α
f : α → α
s : Set α
μ : Measure α
hf : Conservative f μ
hs : NullMeasurableSet s μ
n : ℕ
H : ¬μ {x | x ∈ s ∧ ∀ m ≥ n, f^[m] x ∉ s} = 0
this : NullMeasurableSet (s ∩ {x | ∀ m ≥ n, f^[m] x ∉ s}) μ
m : ℕ
hmn : m > n
hm : μ (s ∩ {x | ∀ m ≥ n, f^[m] x ∉ s} ∩ f^[m] ⁻¹' (s ∩ {x | ∀ m ≥ n, f^[m] x ∉ s})) ≠ 0
x : α
left✝ : x ∈ s
hxn : x ∈ {x | ∀ m ≥ n, f^[m] x ∉ s}
hxm : f^[m] x ∈ s
⊢ False
tactic: rcases nonempty_of_measure_ne_zero hm with ⟨x, ⟨_, hxn⟩, hxm, -⟩
state_before: case intro.intro.intro.intro.intro.intro
ι : Type u_1
α : Type u_2
inst✝ : MeasurableSpace α
f : α → α
s : Set α
μ : Measure α
hf : Conservative f μ
hs : NullMeasurableSet s μ
n : ℕ
H : ¬μ {x | x ∈ s ∧ ∀ m ≥ n, f^[m] x ∉ s} = 0
this : NullMeasurableSet (s ∩ {x | ∀ m ≥ n, f^[m] x ∉ s}) μ
m : ℕ
hmn : m > n
hm : μ (s ∩ {x | ∀ m ≥ n, f^[m] x ∉ s} ∩ f^[m] ⁻¹' (s ∩ {x | ∀ m ≥ n, f^[m] x ∉ s})) ≠ 0
x : α
left✝ : x ∈ s
hxn : x ∈ {x | ∀ m ≥ n, f^[m] x ∉ s}
hxm : f^[m] x ∈ s
⊢ False
state_after: no goals
tactic: exact hxn m hmn.lt.le hxm