-- Mathlib/NumberTheory/Padics/PadicNumbers.lean
theorem exi_rat_seq_conv_cauchy : IsCauSeq (padicNorm p) (limSeq f) := fun ε hε ↦ by
  have hε3 : 0 < ε / 3 := div_pos hε (by norm_num)
  let ⟨N, hN⟩ := exi_rat_seq_conv f hε3
  let ⟨N2, hN2⟩ := f.cauchy₂ hε3
  exists max N N2
  intro j hj
  suffices
    padicNormE (limSeq f j - f (max N N2) + (f (max N N2) - limSeq f (max N N2)) : ℚ_[p]) < ε by
    ring_nf at this ⊢
    rw [← padicNormE.eq_padic_norm']
    exact mod_cast this
  apply lt_of_le_of_lt
  · apply padicNormE.add_le
  · rw [← add_thirds ε]
    apply _root_.add_lt_add
    · suffices padicNormE (limSeq f j - f j + (f j - f (max N N2)) : ℚ_[p]) < ε / 3 + ε / 3 by
        simpa only [sub_add_sub_cancel]
      apply lt_of_le_of_lt
      · apply padicNormE.add_le
      · apply _root_.add_lt_add
        · rw [padicNormE.map_sub]
          apply mod_cast hN j
          exact le_of_max_le_left hj
        · exact hN2 _ (le_of_max_le_right hj) _ (le_max_right _ _)
    · apply mod_cast hN (max N N2)
      apply le_max_left

Proof state:
case hbc.h₂
p : ℕ
inst✝ : Fact (Nat.Prime p)
f : CauSeq ℚ_[p] ⇑padicNormE
ε : ℚ
hε : ε > 0
hε3 : 0 < ε / 3
N : ℕ
hN : ∀ i ≥ N, padicNormE (↑f i - ↑(limSeq f i)) < ε / 3
N2 : ℕ
hN2 : ∀ j ≥ N2, ∀ k ≥ N2, padicNormE (↑f j - ↑f k) < ε / 3
j : ℕ
hj : j ≥ N ⊔ N2
⊢ N ⊔ N2 ≥ N

Tactic:
apply le_max_left

Result:
no goals

Proof state:
case hbc.h₁.hbc
p : ℕ
inst✝ : Fact (Nat.Prime p)
f : CauSeq ℚ_[p] ⇑padicNormE
ε : ℚ
hε : ε > 0
hε3 : 0 < ε / 3
N : ℕ
hN : ∀ i ≥ N, padicNormE (↑f i - ↑(limSeq f i)) < ε / 3
N2 : ℕ
hN2 : ∀ j ≥ N2, ∀ k ≥ N2, padicNormE (↑f j - ↑f k) < ε / 3
j : ℕ
hj : j ≥ N ⊔ N2
⊢ padicNormE (↑(limSeq f j) - ↑f j) + padicNormE (↑f j - ↑f (N ⊔ N2)) < ε / 3 + ε / 3

Tactic:
apply _root_.add_lt_add

Result:
case hbc.h₁.hbc.h₁
p : ℕ
inst✝ : Fact (Nat.Prime p)
f : CauSeq ℚ_[p] ⇑padicNormE
ε : ℚ
hε : ε > 0
hε3 : 0 < ε / 3
N : ℕ
hN : ∀ i ≥ N, padicNormE (↑f i - ↑(limSeq f i)) < ε / 3
N2 : ℕ
hN2 : ∀ j ≥ N2, ∀ k ≥ N2, padicNormE (↑f j - ↑f k) < ε / 3
j : ℕ
hj : j ≥ N ⊔ N2
⊢ padicNormE (↑(limSeq f j) - ↑f j) < ε / 3

case hbc.h₁.hbc.h₂
p : ℕ
inst✝ : Fact (Nat.Prime p)
f : CauSeq ℚ_[p] ⇑padicNormE
ε : ℚ
hε : ε > 0
hε3 : 0 < ε / 3
N : ℕ
hN : ∀ i ≥ N, padicNormE (↑f i - ↑(limSeq f i)) < ε / 3
N2 : ℕ
hN2 : ∀ j ≥ N2, ∀ k ≥ N2, padicNormE (↑f j - ↑f k) < ε / 3
j : ℕ
hj : j ≥ N ⊔ N2
⊢ padicNormE (↑f j - ↑f (N ⊔ N2)) < ε / 3

Proof state:
p : ℕ
inst✝ : Fact (Nat.Prime p)
f : CauSeq ℚ_[p] ⇑padicNormE
ε : ℚ
hε : ε > 0
hε3 : 0 < ε / 3
N : ℕ
hN : ∀ i ≥ N, padicNormE (↑f i - ↑(limSeq f i)) < ε / 3
N2 : ℕ
hN2 : ∀ j ≥ N2, ∀ k ≥ N2, padicNormE (↑f j - ↑f k) < ε / 3
j : ℕ
hj : j ≥ N ⊔ N2
this : padicNormE (↑(limSeq f j) - ↑(limSeq f (N ⊔ N2))) < ε
⊢ padicNormE ↑(limSeq f j - limSeq f (N ⊔ N2)) < ε

Tactic:
exact mod_cast this

Result:
no goals

Proof state:
p : ℕ
inst✝ : Fact (Nat.Prime p)
f : CauSeq ℚ_[p] ⇑padicNormE
ε : ℚ
hε : ε > 0
hε3 : 0 < ε / 3
N : ℕ
hN : ∀ i ≥ N, padicNormE (↑f i - ↑(limSeq f i)) < ε / 3
N2 : ℕ
hN2 : ∀ j ≥ N2, ∀ k ≥ N2, padicNormE (↑f j - ↑f k) < ε / 3
⊢ ∃ i, ∀ j ≥ i, padicNorm p (limSeq f j - limSeq f i) < ε

Tactic:
exists max N N2

Result:
p : ℕ
inst✝ : Fact (Nat.Prime p)
f : CauSeq ℚ_[p] ⇑padicNormE
ε : ℚ
hε : ε > 0
hε3 : 0 < ε / 3
N : ℕ
hN : ∀ i ≥ N, padicNormE (↑f i - ↑(limSeq f i)) < ε / 3
N2 : ℕ
hN2 : ∀ j ≥ N2, ∀ k ≥ N2, padicNormE (↑f j - ↑f k) < ε / 3
⊢ ∀ j ≥ N ⊔ N2, padicNorm p (limSeq f j - limSeq f (N ⊔ N2)) < ε