Covering/Vitali: add exists_disjoint_subfamily_covering_enlargement_ball

Author: lua-vr

Mathlib/MeasureTheory/Covering/Vitali.lean

@@ -187,6 +187,43 @@ theorem exists_disjoint_subfamily_covering_enlargement_closedBall
     rcases A b ⟨rb.1, rb.2⟩ with ⟨c, cu, _⟩
     exact ⟨c, cu, by simp only [closedBall_eq_empty.2 h'a, empty_subset]⟩
 
+/- Note: it seems easier to do the analogous proof again than to apply the previous one, because the
+interior of a closed ball may not equal the open ball. -/
+
+/-- Vitali covering theorem, open balls version: given a family `t` of balls, one can
+extract a disjoint subfamily `u ⊆ t` so that all balls in `t` are covered by the τ-times
+dilations of balls in `u`, for some `τ > 3`. -/
+theorem exists_disjoint_subfamily_covering_enlargement_ball
+    [PseudoMetricSpace α] (t : Set ι)
+    (x : ι → α) (r : ι → ℝ) (R : ℝ) (hr : ∀ a ∈ t, r a ≤ R) (τ : ℝ) (hτ : 3 < τ) :
+    ∃ u ⊆ t,
+      (u.PairwiseDisjoint fun a => ball (x a) (r a)) ∧
+        ∀ a ∈ t, ∃ b ∈ u, ball (x a) (r a) ⊆ ball (x b) (τ * r b) := by
+  rcases eq_empty_or_nonempty t with (rfl | _)
+  · exact ⟨∅, Subset.refl _, pairwiseDisjoint_empty, by simp⟩
+  by_cases! ht : ∀ a ∈ t, r a ≤ 0
+  · exact ⟨t, Subset.rfl, fun a ha b _ _ => by
+      simp only [ball_eq_empty.2 (ht a ha), empty_disjoint, Function.onFun],
+      fun a ha => ⟨a, ha, by simp only [ball_eq_empty.2 (ht a ha), empty_subset]⟩⟩
+  let t' := { a ∈ t | 0 < r a }
+  rcases exists_disjoint_subfamily_covering_enlargement (fun a => ball (x a) (r a)) t' r
+      ((τ - 1) / 2) (by linarith) (fun a ha => ha.2.le) R (fun a ha => hr a ha.1) fun a ha =>
+      ⟨x a, mem_ball_self ha.2⟩ with
+    ⟨u, ut', u_disj, hu⟩
+  have A : ∀ a ∈ t', ∃ b ∈ u, ball (x a) (r a) ⊆ ball (x b) (τ * r b) := by
+    intro a ha
+    rcases hu a ha with ⟨b, bu, hb, rb⟩
+    refine ⟨b, bu, ?_⟩
+    have : dist (x a) (x b) < r a + r b := dist_lt_add_of_nonempty_ball_inter_ball hb
+    apply ball_subset_ball'
+    linarith
+  refine ⟨u, ut'.trans fun a ha => ha.1, u_disj, fun a ha => ?_⟩
+  rcases lt_or_ge 0 (r a) with (h'a | h'a)
+  · exact A a ⟨ha, h'a⟩
+  · rcases ht with ⟨b, rb⟩
+    rcases A b ⟨rb.1, rb.2⟩ with ⟨c, cu, _⟩
+    exact ⟨c, cu, by simp only [ball_eq_empty.2 h'a, empty_subset]⟩
+
 /-- The measurable **Vitali covering theorem**.
 
 Assume one is given a family `t` of closed sets with nonempty interior, such that each `a ∈ t` is