-- Mathlib/MeasureTheory/Measure/ContinuousPreimage.lean
theorem tendsto_measure_symmDiff_preimage_nhds_zero
    {l : Filter α} {f : α → C(X, Y)} {g : C(X, Y)} {s : Set Y} (hfg : Tendsto f l (𝓝 g))
    (hf : ∀ᶠ a in l, MeasurePreserving (f a) μ ν) (hg : MeasurePreserving g μ ν)
    (hs : NullMeasurableSet s ν) (hνs : ν s ≠ ∞) :
    Tendsto (fun a ↦ μ ((f a ⁻¹' s) ∆ (g ⁻¹' s))) l (𝓝 0)

Proof state:
α : Type u_1
X : Type u_2
Y : Type u_3
inst✝⁹ : TopologicalSpace X
inst✝⁸ : MeasurableSpace X
inst✝⁷ : BorelSpace X
inst✝⁶ : R1Space X
inst✝⁵ : TopologicalSpace Y
inst✝⁴ : MeasurableSpace Y
inst✝³ : BorelSpace Y
inst✝² : R1Space Y
μ : Measure X
ν : Measure Y
inst✝¹ : μ.InnerRegularCompactLTTop
inst✝ : IsLocallyFiniteMeasure ν
l : Filter α
f : α → C(X, Y)
g : C(X, Y)
s : Set Y
hfg : Tendsto f l (𝓝 g)
hf : ∀ᶠ (a : α) in l, MeasurePreserving (⇑(f a)) μ ν
hg : MeasurePreserving (⇑g) μ ν
hs : NullMeasurableSet s ν
hνs : ν s ≠ ⊤
this : ν.InnerRegularCompactLTTop
ε : ℝ≥0∞
hε : ε > 0
⊢ ∀ᶠ (x : α) in l, μ ((⇑(f x) ⁻¹' s) ∆ (⇑g ⁻¹' s)) ≤ ε

Tactic:
wlog hso : IsOpen s generalizing s ε

Result:
case inr
α : Type u_1
X : Type u_2
Y : Type u_3
inst✝⁹ : TopologicalSpace X
inst✝⁸ : MeasurableSpace X
inst✝⁷ : BorelSpace X
inst✝⁶ : R1Space X
inst✝⁵ : TopologicalSpace Y
inst✝⁴ : MeasurableSpace Y
inst✝³ : BorelSpace Y
inst✝² : R1Space Y
μ : Measure X
ν : Measure Y
inst✝¹ : μ.InnerRegularCompactLTTop
inst✝ : IsLocallyFiniteMeasure ν
l : Filter α
f : α → C(X, Y)
g : C(X, Y)
s : Set Y
hfg : Tendsto f l (𝓝 g)
hf : ∀ᶠ (a : α) in l, MeasurePreserving (⇑(f a)) μ ν
hg : MeasurePreserving (⇑g) μ ν
hs : NullMeasurableSet s ν
hνs : ν s ≠ ⊤
this✝ : ν.InnerRegularCompactLTTop
ε : ℝ≥0∞
hε : ε > 0
this :
  ∀ {s : Set Y},
    NullMeasurableSet s ν → ν s ≠ ⊤ → ∀ ε > 0, IsOpen s → ∀ᶠ (x : α) in l, μ ((⇑(f x) ⁻¹' s) ∆ (⇑g ⁻¹' s)) ≤ ε
hso : ¬IsOpen s
⊢ ∀ᶠ (x : α) in l, μ ((⇑(f x) ⁻¹' s) ∆ (⇑g ⁻¹' s)) ≤ ε

α : Type u_1
X : Type u_2
Y : Type u_3
inst✝⁹ : TopologicalSpace X
inst✝⁸ : MeasurableSpace X
inst✝⁷ : BorelSpace X
inst✝⁶ : R1Space X
inst✝⁵ : TopologicalSpace Y
inst✝⁴ : MeasurableSpace Y
inst✝³ : BorelSpace Y
inst✝² : R1Space Y
μ : Measure X
ν : Measure Y
inst✝¹ : μ.InnerRegularCompactLTTop
inst✝ : IsLocallyFiniteMeasure ν
l : Filter α
f : α → C(X, Y)
g : C(X, Y)
hfg : Tendsto f l (𝓝 g)
hf : ∀ᶠ (a : α) in l, MeasurePreserving (⇑(f a)) μ ν
hg : MeasurePreserving (⇑g) μ ν
this : ν.InnerRegularCompactLTTop
s : Set Y
hs : NullMeasurableSet s ν
hνs : ν s ≠ ⊤
ε : ℝ≥0∞
hε : ε > 0
hso : IsOpen s
⊢ ∀ᶠ (x : α) in l, μ ((⇑(f x) ⁻¹' s) ∆ (⇑g ⁻¹' s)) ≤ ε

Proof state:
case h₁.h₁
α : Type u_1
X : Type u_2
Y : Type u_3
inst✝⁹ : TopologicalSpace X
inst✝⁸ : MeasurableSpace X
inst✝⁷ : BorelSpace X
inst✝⁶ : R1Space X
inst✝⁵ : TopologicalSpace Y
inst✝⁴ : MeasurableSpace Y
inst✝³ : BorelSpace Y
inst✝² : R1Space Y
μ : Measure X
ν : Measure Y
inst✝¹ : μ.InnerRegularCompactLTTop
inst✝ : IsLocallyFiniteMeasure ν
l : Filter α
f : α → C(X, Y)
g : C(X, Y)
s : Set Y
hfg : Tendsto f l (𝓝 g)
hf : ∀ᶠ (a : α) in l, MeasurePreserving (⇑(f a)) μ ν
hg : MeasurePreserving (⇑g) μ ν
hs : NullMeasurableSet s ν
hνs : ν s ≠ ⊤
this✝ : ν.InnerRegularCompactLTTop
ε : ℝ≥0∞
hε : ε > 0
this :
  ∀ {s : Set Y},
    NullMeasurableSet s ν → ν s ≠ ⊤ → ∀ ε > 0, IsOpen s → ∀ᶠ (x : α) in l, μ ((⇑(f x) ⁻¹' s) ∆ (⇑g ⁻¹' s)) ≤ ε
hso : ¬IsOpen s
H : 0 < ε / 3
U : Set Y
hUo : IsOpen U
hU : ν U < ⊤
hmU : NullMeasurableSet U ν
hUs : ν (U ∆ s) ≤ ε / 3
a : α
hfa : MeasurePreserving (⇑(f a)) μ ν
ha : μ ((⇑(f a) ⁻¹' U) ∆ (⇑g ⁻¹' U)) ≤ ε / 3
⊢ μ ((⇑(f a) ⁻¹' s) ∆ (⇑(f a) ⁻¹' U)) ≤ ε / 3

Tactic:
rwa [← preimage_symmDiff, hfa.measure_preimage (hs.symmDiff hmU), symmDiff_comm]

Result:
no goals

Proof state:
α : Type u_1
X : Type u_2
Y : Type u_3
inst✝⁹ : TopologicalSpace X
inst✝⁸ : MeasurableSpace X
inst✝⁷ : BorelSpace X
inst✝⁶ : R1Space X
inst✝⁵ : TopologicalSpace Y
inst✝⁴ : MeasurableSpace Y
inst✝³ : BorelSpace Y
inst✝² : R1Space Y
μ : Measure X
ν : Measure Y
inst✝¹ : μ.InnerRegularCompactLTTop
inst✝ : IsLocallyFiniteMeasure ν
l : Filter α
f : α → C(X, Y)
g : C(X, Y)
s : Set Y
hfg : Tendsto f l (𝓝 g)
hf : ∀ᶠ (a : α) in l, MeasurePreserving (⇑(f a)) μ ν
hg : MeasurePreserving (⇑g) μ ν
hs : NullMeasurableSet s ν
hνs : ν s ≠ ⊤
this✝ : ν.InnerRegularCompactLTTop
ε : ℝ≥0∞
hε : ε > 0
this :
  ∀ {s : Set Y},
    NullMeasurableSet s ν → ν s ≠ ⊤ → ∀ ε > 0, IsOpen s → ∀ᶠ (x : α) in l, μ ((⇑(f x) ⁻¹' s) ∆ (⇑g ⁻¹' s)) ≤ ε
hso : ¬IsOpen s
H : 0 < ε / 3
U : Set Y
hUo : IsOpen U
hU : ν U < ⊤
hmU : NullMeasurableSet U ν
hUs : ν (U ∆ s) ≤ ε / 3
a : α
hfa : MeasurePreserving (⇑(f a)) μ ν
ha : μ ((⇑(f a) ⁻¹' U) ∆ (⇑g ⁻¹' U)) ≤ ε / 3
⊢ μ ((⇑(f a) ⁻¹' s) ∆ (⇑(f a) ⁻¹' U)) + μ ((⇑(f a) ⁻¹' U) ∆ (⇑g ⁻¹' U)) + μ ((⇑g ⁻¹' U) ∆ (⇑g ⁻¹' s)) ≤
    ε / 3 + ε / 3 + ε / 3

Tactic:
gcongr

Result:
case h₁.h₁
α : Type u_1
X : Type u_2
Y : Type u_3
inst✝⁹ : TopologicalSpace X
inst✝⁸ : MeasurableSpace X
inst✝⁷ : BorelSpace X
inst✝⁶ : R1Space X
inst✝⁵ : TopologicalSpace Y
inst✝⁴ : MeasurableSpace Y
inst✝³ : BorelSpace Y
inst✝² : R1Space Y
μ : Measure X
ν : Measure Y
inst✝¹ : μ.InnerRegularCompactLTTop
inst✝ : IsLocallyFiniteMeasure ν
l : Filter α
f : α → C(X, Y)
g : C(X, Y)
s : Set Y
hfg : Tendsto f l (𝓝 g)
hf : ∀ᶠ (a : α) in l, MeasurePreserving (⇑(f a)) μ ν
hg : MeasurePreserving (⇑g) μ ν
hs : NullMeasurableSet s ν
hνs : ν s ≠ ⊤
this✝ : ν.InnerRegularCompactLTTop
ε : ℝ≥0∞
hε : ε > 0
this :
  ∀ {s : Set Y},
    NullMeasurableSet s ν → ν s ≠ ⊤ → ∀ ε > 0, IsOpen s → ∀ᶠ (x : α) in l, μ ((⇑(f x) ⁻¹' s) ∆ (⇑g ⁻¹' s)) ≤ ε
hso : ¬IsOpen s
H : 0 < ε / 3
U : Set Y
hUo : IsOpen U
hU : ν U < ⊤
hmU : NullMeasurableSet U ν
hUs : ν (U ∆ s) ≤ ε / 3
a : α
hfa : MeasurePreserving (⇑(f a)) μ ν
ha : μ ((⇑(f a) ⁻¹' U) ∆ (⇑g ⁻¹' U)) ≤ ε / 3
⊢ μ ((⇑(f a) ⁻¹' s) ∆ (⇑(f a) ⁻¹' U)) ≤ ε / 3

case h₂
α : Type u_1
X : Type u_2
Y : Type u_3
inst✝⁹ : TopologicalSpace X
inst✝⁸ : MeasurableSpace X
inst✝⁷ : BorelSpace X
inst✝⁶ : R1Space X
inst✝⁵ : TopologicalSpace Y
inst✝⁴ : MeasurableSpace Y
inst✝³ : BorelSpace Y
inst✝² : R1Space Y
μ : Measure X
ν : Measure Y
inst✝¹ : μ.InnerRegularCompactLTTop
inst✝ : IsLocallyFiniteMeasure ν
l : Filter α
f : α → C(X, Y)
g : C(X, Y)
s : Set Y
hfg : Tendsto f l (𝓝 g)
hf : ∀ᶠ (a : α) in l, MeasurePreserving (⇑(f a)) μ ν
hg : MeasurePreserving (⇑g) μ ν
hs : NullMeasurableSet s ν
hνs : ν s ≠ ⊤
this✝ : ν.InnerRegularCompactLTTop
ε : ℝ≥0∞
hε : ε > 0
this :
  ∀ {s : Set Y},
    NullMeasurableSet s ν → ν s ≠ ⊤ → ∀ ε > 0, IsOpen s → ∀ᶠ (x : α) in l, μ ((⇑(f x) ⁻¹' s) ∆ (⇑g ⁻¹' s)) ≤ ε
hso : ¬IsOpen s
H : 0 < ε / 3
U : Set Y
hUo : IsOpen U
hU : ν U < ⊤
hmU : NullMeasurableSet U ν
hUs : ν (U ∆ s) ≤ ε / 3
a : α
hfa : MeasurePreserving (⇑(f a)) μ ν
ha : μ ((⇑(f a) ⁻¹' U) ∆ (⇑g ⁻¹' U)) ≤ ε / 3
⊢ μ ((⇑g ⁻¹' U) ∆ (⇑g ⁻¹' s)) ≤ ε / 3

Proof state:
α : Type u_1
X : Type u_2
Y : Type u_3
inst✝⁹ : TopologicalSpace X
inst✝⁸ : MeasurableSpace X
inst✝⁷ : BorelSpace X
inst✝⁶ : R1Space X
inst✝⁵ : TopologicalSpace Y
inst✝⁴ : MeasurableSpace Y
inst✝³ : BorelSpace Y
inst✝² : R1Space Y
μ : Measure X
ν : Measure Y
inst✝¹ : μ.InnerRegularCompactLTTop
inst✝ : IsLocallyFiniteMeasure ν
l : Filter α
f : α → C(X, Y)
g : C(X, Y)
s : Set Y
hfg : Tendsto f l (𝓝 g)
hf : ∀ᶠ (a : α) in l, MeasurePreserving (⇑(f a)) μ ν
hg : MeasurePreserving (⇑g) μ ν
hs : NullMeasurableSet s ν
hνs : ν s ≠ ⊤
this✝ : ν.InnerRegularCompactLTTop
ε : ℝ≥0∞
hε : ε > 0
this :
  ∀ {s : Set Y},
    NullMeasurableSet s ν → ν s ≠ ⊤ → ∀ ε > 0, IsOpen s → ∀ᶠ (x : α) in l, μ ((⇑(f x) ⁻¹' s) ∆ (⇑g ⁻¹' s)) ≤ ε
hso : ¬IsOpen s
H : 0 < ε / 3
U : Set Y
hUo : IsOpen U
hU : ν U < ⊤
hmU : NullMeasurableSet U ν
hUs : ν (U ∆ s) ≤ ε / 3
a : α
hfa : MeasurePreserving (⇑(f a)) μ ν
ha : μ ((⇑(f a) ⁻¹' U) ∆ (⇑g ⁻¹' U)) ≤ ε / 3
⊢ ε / 3 + ε / 3 + ε / 3 = ε

Tactic:
simp

Result:
no goals