-- Mathlib/Topology/Order/IsLUB.lean
theorem exists_seq_strictAnti_strictMono_tendsto [DenselyOrdered α] [FirstCountableTopology α]
    {x y : α} (h : x < y) :
    ∃ u v : ℕ → α, StrictAnti u ∧ StrictMono v ∧ (∀ k, u k ∈ Ioo x y) ∧ (∀ l, v l ∈ Ioo x y) ∧
      (∀ k l, u k < v l) ∧ Tendsto u atTop (𝓝 x) ∧ Tendsto v atTop (𝓝 y)

Proof state:
α : Type u_1
inst✝⁴ : TopologicalSpace α
inst✝³ : LinearOrder α
inst✝² : OrderTopology α
inst✝¹ : DenselyOrdered α
inst✝ : FirstCountableTopology α
x y : α
h : x < y
⊢ ∃ u v,
    StrictAnti u ∧
      StrictMono v ∧
        (∀ (k : ℕ), u k ∈ Ioo x y) ∧
          (∀ (l : ℕ), v l ∈ Ioo x y) ∧ (∀ (k l : ℕ), u k < v l) ∧ Tendsto u atTop (𝓝 x) ∧ Tendsto v atTop (𝓝 y)

Tactic:
rcases exists_seq_strictAnti_tendsto' h with ⟨u, hu_anti, hu_mem, hux⟩

Result:
case intro.intro.intro
α : Type u_1
inst✝⁴ : TopologicalSpace α
inst✝³ : LinearOrder α
inst✝² : OrderTopology α
inst✝¹ : DenselyOrdered α
inst✝ : FirstCountableTopology α
x y : α
h : x < y
u : ℕ → α
hu_anti : StrictAnti u
hu_mem : ∀ (n : ℕ), u n ∈ Ioo x y
hux : Tendsto u atTop (𝓝 x)
⊢ ∃ u v,
    StrictAnti u ∧
      StrictMono v ∧
        (∀ (k : ℕ), u k ∈ Ioo x y) ∧
          (∀ (l : ℕ), v l ∈ Ioo x y) ∧ (∀ (k l : ℕ), u k < v l) ∧ Tendsto u atTop (𝓝 x) ∧ Tendsto v atTop (𝓝 y)

Proof state:
case intro.intro.intro
α : Type u_1
inst✝⁴ : TopologicalSpace α
inst✝³ : LinearOrder α
inst✝² : OrderTopology α
inst✝¹ : DenselyOrdered α
inst✝ : FirstCountableTopology α
x y : α
h : x < y
u : ℕ → α
hu_anti : StrictAnti u
hu_mem : ∀ (n : ℕ), u n ∈ Ioo x y
hux : Tendsto u atTop (𝓝 x)
⊢ ∃ u v,
    StrictAnti u ∧
      StrictMono v ∧
        (∀ (k : ℕ), u k ∈ Ioo x y) ∧
          (∀ (l : ℕ), v l ∈ Ioo x y) ∧ (∀ (k l : ℕ), u k < v l) ∧ Tendsto u atTop (𝓝 x) ∧ Tendsto v atTop (𝓝 y)

Tactic:
rcases exists_seq_strictMono_tendsto' (hu_mem 0).2 with ⟨v, hv_mono, hv_mem, hvy⟩

Result:
case intro.intro.intro.intro.intro.intro
α : Type u_1
inst✝⁴ : TopologicalSpace α
inst✝³ : LinearOrder α
inst✝² : OrderTopology α
inst✝¹ : DenselyOrdered α
inst✝ : FirstCountableTopology α
x y : α
h : x < y
u : ℕ → α
hu_anti : StrictAnti u
hu_mem : ∀ (n : ℕ), u n ∈ Ioo x y
hux : Tendsto u atTop (𝓝 x)
v : ℕ → α
hv_mono : StrictMono v
hv_mem : ∀ (n : ℕ), v n ∈ Ioo (u 0) y
hvy : Tendsto v atTop (𝓝 y)
⊢ ∃ u v,
    StrictAnti u ∧
      StrictMono v ∧
        (∀ (k : ℕ), u k ∈ Ioo x y) ∧
          (∀ (l : ℕ), v l ∈ Ioo x y) ∧ (∀ (k l : ℕ), u k < v l) ∧ Tendsto u atTop (𝓝 x) ∧ Tendsto v atTop (𝓝 y)

Proof state:
case intro.intro.intro.intro.intro.intro
α : Type u_1
inst✝⁴ : TopologicalSpace α
inst✝³ : LinearOrder α
inst✝² : OrderTopology α
inst✝¹ : DenselyOrdered α
inst✝ : FirstCountableTopology α
x y : α
h : x < y
u : ℕ → α
hu_anti : StrictAnti u
hu_mem : ∀ (n : ℕ), u n ∈ Ioo x y
hux : Tendsto u atTop (𝓝 x)
v : ℕ → α
hv_mono : StrictMono v
hv_mem : ∀ (n : ℕ), v n ∈ Ioo (u 0) y
hvy : Tendsto v atTop (𝓝 y)
⊢ ∃ u v,
    StrictAnti u ∧
      StrictMono v ∧
        (∀ (k : ℕ), u k ∈ Ioo x y) ∧
          (∀ (l : ℕ), v l ∈ Ioo x y) ∧ (∀ (k l : ℕ), u k < v l) ∧ Tendsto u atTop (𝓝 x) ∧ Tendsto v atTop (𝓝 y)

Tactic:
exact
  ⟨u, v, hu_anti, hv_mono, hu_mem, fun l => ⟨(hu_mem 0).1.trans (hv_mem l).1, (hv_mem l).2⟩,
    fun k l => (hu_anti.antitone (zero_le k)).trans_lt (hv_mem l).1, hux, hvy⟩

Result:
no goals