-- Mathlib/MeasureTheory/Measure/EverywherePos.lean
/-- Any compact set contained in `s \ μ.everywherePosSubset s` has zero measure. -/
lemma measure_eq_zero_of_subset_diff_everywherePosSubset
    (hk : IsCompact k) (h'k : k ⊆ s \ μ.everywherePosSubset s) : μ k = 0

Proof state:
α : Type u_1
inst✝¹ : TopologicalSpace α
inst✝ : MeasurableSpace α
μ : Measure α
s k : Set α
hk : IsCompact k
h'k : k ⊆ s \ μ.everywherePosSubset s
x : α
hx : x ∈ k
⊢ ∃ u ∈ 𝓝[s] x, μ u = 0

Tactic:
simpa [everywherePosSubset, (h'k hx).1] using (h'k hx).2

Result:
no goals

Proof state:
case hmono
α : Type u_1
inst✝¹ : TopologicalSpace α
inst✝ : MeasurableSpace α
μ : Measure α
s k : Set α
hk : IsCompact k
h'k : k ⊆ s \ μ.everywherePosSubset s
⊢ ∀ ⦃s t : Set α⦄, s ⊆ t → μ t = 0 → μ s = 0

Tactic:
exact fun s t hst ht ↦ measure_mono_null hst ht

Result:
no goals

Proof state:
case he
α : Type u_1
inst✝¹ : TopologicalSpace α
inst✝ : MeasurableSpace α
μ : Measure α
s k : Set α
hk : IsCompact k
h'k : k ⊆ s \ μ.everywherePosSubset s
⊢ μ ∅ = 0

Tactic:
exact measure_empty

Result:
no goals

Proof state:
case hunion
α : Type u_1
inst✝¹ : TopologicalSpace α
inst✝ : MeasurableSpace α
μ : Measure α
s k : Set α
hk : IsCompact k
h'k : k ⊆ s \ μ.everywherePosSubset s
⊢ ∀ ⦃s t : Set α⦄, μ s = 0 → μ t = 0 → μ (s ∪ t) = 0

Tactic:
exact fun s t hs ht ↦ measure_union_null hs ht

Result:
no goals

Proof state:
case hnhds
α : Type u_1
inst✝¹ : TopologicalSpace α
inst✝ : MeasurableSpace α
μ : Measure α
s k : Set α
hk : IsCompact k
h'k : k ⊆ s \ μ.everywherePosSubset s
x : α
hx : x ∈ k
⊢ ∃ t ∈ 𝓝[k] x, μ t = 0

Tactic:
obtain ⟨u, ux, hu⟩ : ∃ u ∈ 𝓝[s] x, μ u = 0 := by
  simpa [everywherePosSubset, (h'k hx).1] using (h'k hx).2

Result:
case hnhds.intro.intro
α : Type u_1
inst✝¹ : TopologicalSpace α
inst✝ : MeasurableSpace α
μ : Measure α
s k : Set α
hk : IsCompact k
h'k : k ⊆ s \ μ.everywherePosSubset s
x : α
hx : x ∈ k
u : Set α
ux : u ∈ 𝓝[s] x
hu : μ u = 0
⊢ ∃ t ∈ 𝓝[k] x, μ t = 0

Proof state:
case hnhds
α : Type u_1
inst✝¹ : TopologicalSpace α
inst✝ : MeasurableSpace α
μ : Measure α
s k : Set α
hk : IsCompact k
h'k : k ⊆ s \ μ.everywherePosSubset s
⊢ ∀ x ∈ k, ∃ t ∈ 𝓝[k] x, μ t = 0

Tactic:
intro x hx

Result:
case hnhds
α : Type u_1
inst✝¹ : TopologicalSpace α
inst✝ : MeasurableSpace α
μ : Measure α
s k : Set α
hk : IsCompact k
h'k : k ⊆ s \ μ.everywherePosSubset s
x : α
hx : x ∈ k
⊢ ∃ t ∈ 𝓝[k] x, μ t = 0

Proof state:
α : Type u_1
inst✝¹ : TopologicalSpace α
inst✝ : MeasurableSpace α
μ : Measure α
s k : Set α
hk : IsCompact k
h'k : k ⊆ s \ μ.everywherePosSubset s
⊢ μ k = 0

Tactic:
apply hk.induction_on (p := fun t ↦ μ t = 0)

Result:
case he
α : Type u_1
inst✝¹ : TopologicalSpace α
inst✝ : MeasurableSpace α
μ : Measure α
s k : Set α
hk : IsCompact k
h'k : k ⊆ s \ μ.everywherePosSubset s
⊢ μ ∅ = 0

case hmono
α : Type u_1
inst✝¹ : TopologicalSpace α
inst✝ : MeasurableSpace α
μ : Measure α
s k : Set α
hk : IsCompact k
h'k : k ⊆ s \ μ.everywherePosSubset s
⊢ ∀ ⦃s t : Set α⦄, s ⊆ t → μ t = 0 → μ s = 0

case hunion
α : Type u_1
inst✝¹ : TopologicalSpace α
inst✝ : MeasurableSpace α
μ : Measure α
s k : Set α
hk : IsCompact k
h'k : k ⊆ s \ μ.everywherePosSubset s
⊢ ∀ ⦃s t : Set α⦄, μ s = 0 → μ t = 0 → μ (s ∪ t) = 0

case hnhds
α : Type u_1
inst✝¹ : TopologicalSpace α
inst✝ : MeasurableSpace α
μ : Measure α
s k : Set α
hk : IsCompact k
h'k : k ⊆ s \ μ.everywherePosSubset s
⊢ ∀ x ∈ k, ∃ t ∈ 𝓝[k] x, μ t = 0

Proof state:
case hnhds.intro.intro
α : Type u_1
inst✝¹ : TopologicalSpace α
inst✝ : MeasurableSpace α
μ : Measure α
s k : Set α
hk : IsCompact k
h'k : k ⊆ s \ μ.everywherePosSubset s
x : α
hx : x ∈ k
u : Set α
ux : u ∈ 𝓝[s] x
hu : μ u = 0
⊢ ∃ t ∈ 𝓝[k] x, μ t = 0

Tactic:
exact ⟨u, nhdsWithin_mono x (h'k.trans diff_subset) ux, hu⟩

Result:
no goals