-- Mathlib/Order/OrderIsoNat.lean
theorem exists_increasing_or_nonincreasing_subseq' (r : α → α → Prop) (f : ℕ → α) :
    ∃ g : ℕ ↪o ℕ,
      (∀ n : ℕ, r (f (g n)) (f (g (n + 1)))) ∨ ∀ m n : ℕ, m < n → ¬r (f (g m)) (f (g n))

Proof state:
α : Type u_1
r : α → α → Prop
f : ℕ → α
bad : Set ℕ := {m | ∀ (n : ℕ), m < n → ¬r (f m) (f n)}
hbad : bad.Finite
m : ℕ
hm : ∀ (n : ℕ), m ≤ n → n ∉ bad
n n' : ℕ
hn1 : n + m < n'
hn2 : r (f (n + m)) (f n')
⊢ n < n + n' - n - m

Tactic:
omega

Result:
no goals

Proof state:
α : Type u_1
r : α → α → Prop
f : ℕ → α
bad : Set ℕ := {m | ∀ (n : ℕ), m < n → ¬r (f m) (f n)}
⊢ ∃ g, (∀ (n : ℕ), r (f (g n)) (f (g (n + 1)))) ∨ ∀ (m n : ℕ), m < n → ¬r (f (g m)) (f (g n))

Tactic:
by_cases hbad : Infinite bad

Result:
case pos
α : Type u_1
r : α → α → Prop
f : ℕ → α
bad : Set ℕ := {m | ∀ (n : ℕ), m < n → ¬r (f m) (f n)}
hbad : Infinite ↑bad
⊢ ∃ g, (∀ (n : ℕ), r (f (g n)) (f (g (n + 1)))) ∨ ∀ (m n : ℕ), m < n → ¬r (f (g m)) (f (g n))

case neg
α : Type u_1
r : α → α → Prop
f : ℕ → α
bad : Set ℕ := {m | ∀ (n : ℕ), m < n → ¬r (f m) (f n)}
hbad : ¬Infinite ↑bad
⊢ ∃ g, (∀ (n : ℕ), r (f (g n)) (f (g (n + 1)))) ∨ ∀ (m n : ℕ), m < n → ¬r (f (g m)) (f (g n))

Proof state:
case neg.intro
α : Type u_1
r : α → α → Prop
f : ℕ → α
bad : Set ℕ := {m | ∀ (n : ℕ), m < n → ¬r (f m) (f n)}
hbad : bad.Finite
m : ℕ
hm : ∀ (n : ℕ), m ≤ n → n ∉ bad
h : ∀ (n : ℕ), ∃ n', n < n' ∧ r (f (n + m)) (f (n' + m))
g' : ℕ → ℕ := Nat.rec m fun n gn => Nat.find ⋯
⊢ ∃ g, (∀ (n : ℕ), r (f (g n)) (f (g (n + 1)))) ∨ ∀ (m n : ℕ), m < n → ¬r (f (g m)) (f (g n))

Tactic:
exact
  ⟨(RelEmbedding.natLT (fun n => g' n + m) fun n =>
        Nat.add_lt_add_right (Nat.find_spec (h (g' n))).1 m).orderEmbeddingOfLTEmbedding,
    Or.intro_left _ fun n => (Nat.find_spec (h (g' n))).2⟩

Result:
no goals

Proof state:
α : Type u_1
r : α → α → Prop
f : ℕ → α
bad : Set ℕ := {m | ∀ (n : ℕ), m < n → ¬r (f m) (f n)}
hbad : bad.Finite
m : ℕ
hm : ∀ (n : ℕ), m ≤ n → n ∉ bad
n : ℕ
h : ∃ x, n + m < x ∧ r (f (n + m)) (f x)
⊢ ∃ n', n < n' ∧ r (f (n + m)) (f (n' + m))

Tactic:
obtain ⟨n', hn1, hn2⟩ := h

Result:
case intro.intro
α : Type u_1
r : α → α → Prop
f : ℕ → α
bad : Set ℕ := {m | ∀ (n : ℕ), m < n → ¬r (f m) (f n)}
hbad : bad.Finite
m : ℕ
hm : ∀ (n : ℕ), m ≤ n → n ∉ bad
n n' : ℕ
hn1 : n + m < n'
hn2 : r (f (n + m)) (f n')
⊢ ∃ n', n < n' ∧ r (f (n + m)) (f (n' + m))

Proof state:
case pos
α : Type u_1
r : α → α → Prop
f : ℕ → α
bad : Set ℕ := {m | ∀ (n : ℕ), m < n → ¬r (f m) (f n)}
hbad : Infinite ↑bad
⊢ ∃ g, (∀ (n : ℕ), r (f (g n)) (f (g (n + 1)))) ∨ ∀ (m n : ℕ), m < n → ¬r (f (g m)) (f (g n))

Tactic:
haveI := hbad

Result:
case pos
α : Type u_1
r : α → α → Prop
f : ℕ → α
bad : Set ℕ := {m | ∀ (n : ℕ), m < n → ¬r (f m) (f n)}
hbad this : Infinite ↑bad
⊢ ∃ g, (∀ (n : ℕ), r (f (g n)) (f (g (n + 1)))) ∨ ∀ (m n : ℕ), m < n → ¬r (f (g m)) (f (g n))

Proof state:
case pos
α : Type u_1
r : α → α → Prop
f : ℕ → α
bad : Set ℕ := {m | ∀ (n : ℕ), m < n → ¬r (f m) (f n)}
hbad this : Infinite ↑bad
m n : ℕ
mn : m < n
h : (Nat.orderEmbeddingOfSet bad) m ∈ Set.range ⇑(Nat.orderEmbeddingOfSet bad)
⊢ ¬r (f ((Nat.orderEmbeddingOfSet bad) m)) (f ((Nat.orderEmbeddingOfSet bad) n))

Tactic:
rw [Nat.orderEmbeddingOfSet_range bad] at h

Result:
case pos
α : Type u_1
r : α → α → Prop
f : ℕ → α
bad : Set ℕ := {m | ∀ (n : ℕ), m < n → ¬r (f m) (f n)}
hbad this : Infinite ↑bad
m n : ℕ
mn : m < n
h : (Nat.orderEmbeddingOfSet bad) m ∈ bad
⊢ ¬r (f ((Nat.orderEmbeddingOfSet bad) m)) (f ((Nat.orderEmbeddingOfSet bad) n))

Proof state:
case pos
α : Type u_1
r : α → α → Prop
f : ℕ → α
bad : Set ℕ := {m | ∀ (n : ℕ), m < n → ¬r (f m) (f n)}
hbad this : Infinite ↑bad
⊢ ∃ g, (∀ (n : ℕ), r (f (g n)) (f (g (n + 1)))) ∨ ∀ (m n : ℕ), m < n → ¬r (f (g m)) (f (g n))

Tactic:
refine ⟨Nat.orderEmbeddingOfSet bad, Or.intro_right _ fun m n mn => ?_⟩

Result:
case pos
α : Type u_1
r : α → α → Prop
f : ℕ → α
bad : Set ℕ := {m | ∀ (n : ℕ), m < n → ¬r (f m) (f n)}
hbad this : Infinite ↑bad
m n : ℕ
mn : m < n
⊢ ¬r (f ((Nat.orderEmbeddingOfSet bad) m)) (f ((Nat.orderEmbeddingOfSet bad) n))

Proof state:
case h.e'_2.h.e'_1
α : Type u_1
r : α → α → Prop
f : ℕ → α
bad : Set ℕ := {m | ∀ (n : ℕ), m < n → ¬r (f m) (f n)}
hbad : bad.Finite
m : ℕ
hm : ∀ (n : ℕ), m ≤ n → n ∉ bad
n n' : ℕ
hn1 : n + m < n'
hn2 : r (f (n + m)) (f n')
⊢ n + n' - n - m + m = n'

Tactic:
omega

Result:
no goals