Module: Mathlib.Topology.MetricSpace.GromovHausdorff

Goal state before:
pp: X : Type u
inst✝⁵ : MetricSpace X
inst✝⁴ : CompactSpace X
inst✝³ : Nonempty X
Y : Type v
inst✝² : MetricSpace Y
inst✝¹ : CompactSpace Y
inst✝ : Nonempty Y
⊢ hausdorffDist (range (optimalGHInjl X Y)) (range (optimalGHInjr X Y)) = ghDist X Y

Tactic:
by
  inhabit X; inhabit Y
  /- we only need to check the inequality `≤`, as the other one follows from the previous lemma.
       As the Gromov-Hausdorff distance is an infimum, we need to check that the Hausdorff distance
       in the optimal coupling is smaller than the Hausdorff distance of any coupling.
       First, we check this for couplings which already have small Hausdorff distance: in this
       case, the induced "distance" on `X ⊕ Y` belongs to the candidates family introduced in the
       definition of the optimal coupling, and the conclusion follows from the optimality
       of the optimal coupling within this family.
    -/
  have A :
    ∀ p q : NonemptyCompacts ℓ_infty_ℝ,
      ⟦p⟧ = toGHSpace X →
        ⟦q⟧ = toGHSpace Y →
          hausdorffDist (p : Set ℓ_infty_ℝ) q < diam (univ : Set X) + 1 + diam (univ : Set Y) →
            hausdorffDist (range (optimalGHInjl X Y)) (range (optimalGHInjr X Y)) ≤
              hausdorffDist (p : Set ℓ_infty_ℝ) q := by
    intro p q hp hq bound
    rcases eq_toGHSpace_iff.1 hp with ⟨Φ, ⟨Φisom, Φrange⟩⟩
    rcases eq_toGHSpace_iff.1 hq with ⟨Ψ, ⟨Ψisom, Ψrange⟩⟩
    have I : diam (range Φ ∪ range Ψ) ≤ 2 * diam (univ : Set X) + 1 + 2 * diam (univ : Set Y) := by
      rcases exists_mem_of_nonempty X with ⟨xX, _⟩
      have : ∃ y ∈ range Ψ, dist (Φ xX) y < diam (univ : Set X) + 1 + diam (univ : Set Y) := by
        rw [Ψrange]
        have : Φ xX ∈ (p : Set _) := Φrange ▸ (mem_range_self _)
        exact
          exists_dist_lt_of_hausdorffDist_lt this bound
            (hausdorffEDist_ne_top_of_nonempty_of_bounded p.nonempty q.nonempty
              p.isCompact.isBounded q.isCompact.isBounded)
      rcases this with ⟨y, hy, dy⟩
      rcases mem_range.1 hy with ⟨z, hzy⟩
      rw [← hzy] at dy
      have DΦ : diam (range Φ) = diam (univ : Set X) := Φisom.diam_range
      have DΨ : diam (range Ψ) = diam (univ : Set Y) := Ψisom.diam_range
      calc
        diam (range Φ ∪ range Ψ) ≤ diam (range Φ) + dist (Φ xX) (Ψ z) + diam (range Ψ) :=
          diam_union (mem_range_self _) (mem_range_self _)
        _ ≤
            diam (univ : Set X) + (diam (univ : Set X) + 1 + diam (univ : Set Y)) +
              diam (univ : Set Y) := by
          rw [DΦ, DΨ]
          gcongr
        _ = 2 * diam (univ : Set X) + 1 + 2 * diam (univ : Set Y) := by ring
    let f : X ⊕ Y → ℓ_infty_ℝ := fun x =>
      match x with
      | inl y => Φ y
      | inr z => Ψ z
    let F : (X ⊕ Y) × (X ⊕ Y) → ℝ := fun p => dist (f p.1) (f p.2)
    -- check that the induced "distance" is a candidate
    have Fgood : F ∈ candidates X Y := by
      simp only [F, candidates, forall_const,
        dist_eq_zero, Set.mem_ofPred_eq]
      repeat' constructor
      · exact fun x y =>
          calc
            F (inl x, inl y) = dist (Φ x) (Φ y) := rfl
            _ = dist x y := Φisom.dist_eq x y
      · exact fun x y =>
          calc
            F (inr x, inr y) = dist (Ψ x) (Ψ y) := rfl
            _ = dist x y := Ψisom.dist_eq x y
      · exact fun x y => dist_comm _ _
      · exact fun x y z => dist_triangle _ _ _
      · exact fun x y =>
          calc
            F (x, y) ≤ diam (range Φ ∪ range Ψ) := by
              have A : ∀ z : X ⊕ Y, f z ∈ range Φ ∪ range Ψ := by
                intro z
                cases z
                · apply mem_union_left; apply mem_range_self
                · apply mem_union_right; apply mem_range_self
              refine dist_le_diam_of_mem ?_ (A _) (A _)
              rw [Φrange, Ψrange]
              exact (p ⊔ q).isCompact.isBounded
            _ ≤ 2 * diam (univ : Set X) + 1 + 2 * diam (univ : Set Y) := I
    let Fb := candidatesBOfCandidates F Fgood
    have : hausdorffDist (range (optimalGHInjl X Y)) (range (optimalGHInjr X Y)) ≤ HD Fb :=
      hausdorffDist_optimal_le_HD _ _ (candidatesBOfCandidates_mem F Fgood)
    refine le_trans this (le_of_forall_gt_imp_ge_of_dense fun r hr => ?_)
    have I1 : ∀ x : X, (⨅ y, Fb (inl x, inr y)) ≤ r := by
      intro x
      have : f (inl x) ∈ (p : Set _) := Φrange ▸ (mem_range_self _)
      rcases exists_dist_lt_of_hausdorffDist_lt this hr
          (hausdorffEDist_ne_top_of_nonempty_of_bounded p.nonempty q.nonempty p.isCompact.isBounded
            q.isCompact.isBounded) with
        ⟨z, zq, hz⟩
      have : z ∈ range Ψ := by rwa [← Ψrange] at zq
      rcases mem_range.1 this with ⟨y, hy⟩
      calc
        (⨅ y, Fb (inl x, inr y)) ≤ Fb (inl x, inr y) :=
          ciInf_le (by simpa only [add_zero] using HD_below_aux1 0) y
        _ = dist (Φ x) (Ψ y) := rfl
        _ = dist (f (inl x)) z := by rw [hy]
        _ ≤ r := le_of_lt hz
    have I2 : ∀ y : Y, (⨅ x, Fb (inl x, inr y)) ≤ r := by
      intro y
      have : f (inr y) ∈ (q : Set _) := Ψrange ▸ (mem_range_self _)
      rcases exists_dist_lt_of_hausdorffDist_lt' this hr
          (hausdorffEDist_ne_top_of_nonempty_of_bounded p.nonempty q.nonempty p.isCompact.isBounded
            q.isCompact.isBounded) with
        ⟨z, zq, hz⟩
      have : z ∈ range Φ := by rwa [← Φrange] at zq
      rcases mem_range.1 this with ⟨x, hx⟩
      calc
        (⨅ x, Fb (inl x, inr y)) ≤ Fb (inl x, inr y) :=
          ciInf_le (by simpa only [add_zero] using HD_below_aux2 0) x
        _ = dist (Φ x) (Ψ y) := rfl
        _ = dist z (f (inr y)) := by rw [hx]
        _ ≤ r := le_of_lt hz
    simp only [HD, ciSup_le I1, ciSup_le I2, max_le_iff, and_self_iff]
  /- Get the same inequality for any coupling. If the coupling is quite good, the desired
    inequality has been proved above. If it is bad, then the inequality is obvious. -/
  have B :
    ∀ p q : NonemptyCompacts ℓ_infty_ℝ,
      ⟦p⟧ = toGHSpace X →
        ⟦q⟧ = toGHSpace Y →
          hausdorffDist (range (optimalGHInjl X Y)) (range (optimalGHInjr X Y)) ≤
            hausdorffDist (p : Set ℓ_infty_ℝ) q := by
    intro p q hp hq
    by_cases h :
      hausdorffDist (p : Set ℓ_infty_ℝ) q < diam (univ : Set X) + 1 + diam (univ : Set Y)
    · exact A p q hp hq h
    · calc
        hausdorffDist (range (optimalGHInjl X Y)) (range (optimalGHInjr X Y)) ≤
            HD (candidatesBDist X Y) :=
          hausdorffDist_optimal_le_HD _ _ candidatesBDist_mem_candidatesB
        _ ≤ diam (univ : Set X) + 1 + diam (univ : Set Y) := HD_candidatesBDist_le
        _ ≤ hausdorffDist (p : Set ℓ_infty_ℝ) q := not_lt.1 h
  refine le_antisymm ?_ ?_
  · apply le_csInf
    · refine (Set.Nonempty.prod ?_ ?_).image _ <;> exact ⟨_, rfl⟩
    · rintro b ⟨⟨p, q⟩, ⟨hp, hq⟩, rfl⟩
      exact B p q hp hq
  · exact ghDist_le_hausdorffDist (isometry_optimalGHInjl X Y) (isometry_optimalGHInjr X Y)

Goal state after: