l3lab/ntp-mathlib huggingface dataset:
/-- If `f` is strictly monotone both on `s` and `t`, with `s` to the left of `t` and the center
point belonging to both `s` and `t`, then `f` is strictly monotone on `s ∪ t` -/
protected theorem StrictMonoOn.union {s t : Set α} {c : α} (h₁ : StrictMonoOn f s)
    (h₂ : StrictMonoOn f t) (hs : IsGreatest s c) (ht : IsLeast t c) : StrictMonoOn f (s ∪ t) := by
  have A : ∀ x, x ∈ s ∪ t → x ≤ c → x ∈ s := by
    intro x hx hxc
    cases hx
    · assumption
    rcases eq_or_lt_of_le hxc with (rfl | h'x)
    · exact hs.1
    exact (lt_irrefl _ (h'x.trans_le (ht.2 (by assumption)))).elim
  have B : ∀ x, x ∈ s ∪ t → c ≤ x → x ∈ t := by
    intro x hx hxc
    match hx with
    | Or.inr hx => exact hx
    | Or.inl hx =>
      

Proof state:
α : Type u_1
β : Type u_2
inst✝¹ : LinearOrder α
inst✝ : Preorder β
a : α
f : α → β
s t : Set α
c : α
h₁ : StrictMonoOn f s
h₂ : StrictMonoOn f t
hs : IsGreatest s c
ht : IsLeast t c
A : ∀ x ∈ s ∪ t, x ≤ c → x ∈ s
x : α
hx✝ : x ∈ s ∪ t
hxc : c ≤ x
hx : x ∈ s
⊢ x ∈ t

Next tactic:
rcases eq_or_lt_of_le hxc with (rfl | h'x)
/-- If `f` is strictly monotone both on `s` and `t`, with `s` to the left of `t` and the center
point belonging to both `s` and `t`, then `f` is strictly monotone on `s ∪ t` -/
protected theorem StrictMonoOn.union {s t : Set α} {c : α} (h₁ : StrictMonoOn f s)
    (h₂ : StrictMonoOn f t) (hs : IsGreatest s c) (ht : IsLeast t c) : StrictMonoOn f (s ∪ t) := by
  have A : ∀ x, x ∈ s ∪ t → x ≤ c → x ∈ s := by
    intro x hx hxc
    cases hx
    · assumption
    rcases eq_or_lt_of_le hxc with (rfl | h'x)
    · exact hs.1
    exact (lt_irrefl _ (h'x.trans_le (ht.2 (by assumption)))).elim
  have B : ∀ x, x ∈ s ∪ t → c ≤ x → x ∈ t := by
    intro x hx hxc
    match hx with
    | Or.inr hx => exact hx
    | Or.inl hx =>
      rcases eq_or_lt_of_le hxc with (rfl | h'x)
      · 

Proof state:
case inl
α : Type u_1
β : Type u_2
inst✝¹ : LinearOrder α
inst✝ : Preorder β
a : α
f : α → β
s t : Set α
c : α
h₁ : StrictMonoOn f s
h₂ : StrictMonoOn f t
hs : IsGreatest s c
ht : IsLeast t c
A : ∀ x ∈ s ∪ t, x ≤ c → x ∈ s
hx✝ : c ∈ s ∪ t
hxc : c ≤ c
hx : c ∈ s
⊢ c ∈ t

Next tactic:
exact ht.1
/-- If `f` is strictly monotone both on `s` and `t`, with `s` to the left of `t` and the center
point belonging to both `s` and `t`, then `f` is strictly monotone on `s ∪ t` -/
protected theorem StrictMonoOn.union {s t : Set α} {c : α} (h₁ : StrictMonoOn f s)
    (h₂ : StrictMonoOn f t) (hs : IsGreatest s c) (ht : IsLeast t c) : StrictMonoOn f (s ∪ t) := by
  have A : ∀ x, x ∈ s ∪ t → x ≤ c → x ∈ s := by
    intro x hx hxc
    cases hx
    · assumption
    rcases eq_or_lt_of_le hxc with (rfl | h'x)
    · exact hs.1
    exact (lt_irrefl _ (h'x.trans_le (ht.2 (by assumption)))).elim
  have B : ∀ x, x ∈ s ∪ t → c ≤ x → x ∈ t := by
    intro x hx hxc
    match hx with
    | Or.inr hx => exact hx
    | Or.inl hx =>
      rcases eq_or_lt_of_le hxc with (rfl | h'x)
      · exact ht.1
      

Proof state:
case inr
α : Type u_1
β : Type u_2
inst✝¹ : LinearOrder α
inst✝ : Preorder β
a : α
f : α → β
s t : Set α
c : α
h₁ : StrictMonoOn f s
h₂ : StrictMonoOn f t
hs : IsGreatest s c
ht : IsLeast t c
A : ∀ x ∈ s ∪ t, x ≤ c → x ∈ s
x : α
hx✝ : x ∈ s ∪ t
hxc : c ≤ x
hx : x ∈ s
h'x : c < x
⊢ x ∈ t

Next tactic:
exact (lt_irrefl _ (h'x.trans_le (hs.2 hx))).elim
/-- If `f` is strictly monotone both on `s` and `t`, with `s` to the left of `t` and the center
point belonging to both `s` and `t`, then `f` is strictly monotone on `s ∪ t` -/
protected theorem StrictMonoOn.union {s t : Set α} {c : α} (h₁ : StrictMonoOn f s)
    (h₂ : StrictMonoOn f t) (hs : IsGreatest s c) (ht : IsLeast t c) : StrictMonoOn f (s ∪ t) := by
  have A : ∀ x, x ∈ s ∪ t → x ≤ c → x ∈ s := by
    intro x hx hxc
    cases hx
    · assumption
    rcases eq_or_lt_of_le hxc with (rfl | h'x)
    · exact hs.1
    exact (lt_irrefl _ (h'x.trans_le (ht.2 (by assumption)))).elim
  have B : ∀ x, x ∈ s ∪ t → c ≤ x → x ∈ t := by
    intro x hx hxc
    match hx with
    | Or.inr hx => exact hx
    | Or.inl hx =>
      rcases eq_or_lt_of_le hxc with (rfl | h'x)
      · exact ht.1
      exact (lt_irrefl _ (h'x.trans_le (hs.2 hx))).elim
  

Proof state:
α : Type u_1
β : Type u_2
inst✝¹ : LinearOrder α
inst✝ : Preorder β
a : α
f : α → β
s t : Set α
c : α
h₁ : StrictMonoOn f s
h₂ : StrictMonoOn f t
hs : IsGreatest s c
ht : IsLeast t c
A : ∀ x ∈ s ∪ t, x ≤ c → x ∈ s
B : ∀ x ∈ s ∪ t, c ≤ x → x ∈ t
⊢ StrictMonoOn f (s ∪ t)

Next tactic:
intro x hx y hy hxy
/-- If `f` is strictly monotone both on `s` and `t`, with `s` to the left of `t` and the center
point belonging to both `s` and `t`, then `f` is strictly monotone on `s ∪ t` -/
protected theorem StrictMonoOn.union {s t : Set α} {c : α} (h₁ : StrictMonoOn f s)
    (h₂ : StrictMonoOn f t) (hs : IsGreatest s c) (ht : IsLeast t c) : StrictMonoOn f (s ∪ t) := by
  have A : ∀ x, x ∈ s ∪ t → x ≤ c → x ∈ s := by
    intro x hx hxc
    cases hx
    · assumption
    rcases eq_or_lt_of_le hxc with (rfl | h'x)
    · exact hs.1
    exact (lt_irrefl _ (h'x.trans_le (ht.2 (by assumption)))).elim
  have B : ∀ x, x ∈ s ∪ t → c ≤ x → x ∈ t := by
    intro x hx hxc
    match hx with
    | Or.inr hx => exact hx
    | Or.inl hx =>
      rcases eq_or_lt_of_le hxc with (rfl | h'x)
      · exact ht.1
      exact (lt_irrefl _ (h'x.trans_le (hs.2 hx))).elim
  intro x hx y hy hxy
  

Proof state:
α : Type u_1
β : Type u_2
inst✝¹ : LinearOrder α
inst✝ : Preorder β
a : α
f : α → β
s t : Set α
c : α
h₁ : StrictMonoOn f s
h₂ : StrictMonoOn f t
hs : IsGreatest s c
ht : IsLeast t c
A : ∀ x ∈ s ∪ t, x ≤ c → x ∈ s
B : ∀ x ∈ s ∪ t, c ≤ x → x ∈ t
x : α
hx : x ∈ s ∪ t
y : α
hy : y ∈ s ∪ t
hxy : x < y
⊢ f x < f y

Next tactic:
rcases lt_or_le x c with (hxc | hcx)
/-- If `f` is strictly monotone both on `s` and `t`, with `s` to the left of `t` and the center
point belonging to both `s` and `t`, then `f` is strictly monotone on `s ∪ t` -/
protected theorem StrictMonoOn.union {s t : Set α} {c : α} (h₁ : StrictMonoOn f s)
    (h₂ : StrictMonoOn f t) (hs : IsGreatest s c) (ht : IsLeast t c) : StrictMonoOn f (s ∪ t) := by
  have A : ∀ x, x ∈ s ∪ t → x ≤ c → x ∈ s := by
    intro x hx hxc
    cases hx
    · assumption
    rcases eq_or_lt_of_le hxc with (rfl | h'x)
    · exact hs.1
    exact (lt_irrefl _ (h'x.trans_le (ht.2 (by assumption)))).elim
  have B : ∀ x, x ∈ s ∪ t → c ≤ x → x ∈ t := by
    intro x hx hxc
    match hx with
    | Or.inr hx => exact hx
    | Or.inl hx =>
      rcases eq_or_lt_of_le hxc with (rfl | h'x)
      · exact ht.1
      exact (lt_irrefl _ (h'x.trans_le (hs.2 hx))).elim
  intro x hx y hy hxy
  rcases lt_or_le x c with (hxc | hcx)
  · 

Proof state:
case inl
α : Type u_1
β : Type u_2
inst✝¹ : LinearOrder α
inst✝ : Preorder β
a : α
f : α → β
s t : Set α
c : α
h₁ : StrictMonoOn f s
h₂ : StrictMonoOn f t
hs : IsGreatest s c
ht : IsLeast t c
A : ∀ x ∈ s ∪ t, x ≤ c → x ∈ s
B : ∀ x ∈ s ∪ t, c ≤ x → x ∈ t
x : α
hx : x ∈ s ∪ t
y : α
hy : y ∈ s ∪ t
hxy : x < y
hxc : x < c
⊢ f x < f y

Next tactic:
have xs : x ∈ s := A _ hx hxc.le
/-- If `f` is strictly monotone both on `s` and `t`, with `s` to the left of `t` and the center
point belonging to both `s` and `t`, then `f` is strictly monotone on `s ∪ t` -/
protected theorem StrictMonoOn.union {s t : Set α} {c : α} (h₁ : StrictMonoOn f s)
    (h₂ : StrictMonoOn f t) (hs : IsGreatest s c) (ht : IsLeast t c) : StrictMonoOn f (s ∪ t) := by
  have A : ∀ x, x ∈ s ∪ t → x ≤ c → x ∈ s := by
    intro x hx hxc
    cases hx
    · assumption
    rcases eq_or_lt_of_le hxc with (rfl | h'x)
    · exact hs.1
    exact (lt_irrefl _ (h'x.trans_le (ht.2 (by assumption)))).elim
  have B : ∀ x, x ∈ s ∪ t → c ≤ x → x ∈ t := by
    intro x hx hxc
    match hx with
    | Or.inr hx => exact hx
    | Or.inl hx =>
      rcases eq_or_lt_of_le hxc with (rfl | h'x)
      · exact ht.1
      exact (lt_irrefl _ (h'x.trans_le (hs.2 hx))).elim
  intro x hx y hy hxy
  rcases lt_or_le x c with (hxc | hcx)
  · have xs : x ∈ s := A _ hx hxc.le
    

Proof state:
case inl
α : Type u_1
β : Type u_2
inst✝¹ : LinearOrder α
inst✝ : Preorder β
a : α
f : α → β
s t : Set α
c : α
h₁ : StrictMonoOn f s
h₂ : StrictMonoOn f t
hs : IsGreatest s c
ht : IsLeast t c
A : ∀ x ∈ s ∪ t, x ≤ c → x ∈ s
B : ∀ x ∈ s ∪ t, c ≤ x → x ∈ t
x : α
hx : x ∈ s ∪ t
y : α
hy : y ∈ s ∪ t
hxy : x < y
hxc : x < c
xs : x ∈ s
⊢ f x < f y

Next tactic:
rcases lt_or_le y c with (hyc | hcy)
/-- If `f` is strictly monotone both on `s` and `t`, with `s` to the left of `t` and the center
point belonging to both `s` and `t`, then `f` is strictly monotone on `s ∪ t` -/
protected theorem StrictMonoOn.union {s t : Set α} {c : α} (h₁ : StrictMonoOn f s)
    (h₂ : StrictMonoOn f t) (hs : IsGreatest s c) (ht : IsLeast t c) : StrictMonoOn f (s ∪ t) := by
  have A : ∀ x, x ∈ s ∪ t → x ≤ c → x ∈ s := by
    intro x hx hxc
    cases hx
    · assumption
    rcases eq_or_lt_of_le hxc with (rfl | h'x)
    · exact hs.1
    exact (lt_irrefl _ (h'x.trans_le (ht.2 (by assumption)))).elim
  have B : ∀ x, x ∈ s ∪ t → c ≤ x → x ∈ t := by
    intro x hx hxc
    match hx with
    | Or.inr hx => exact hx
    | Or.inl hx =>
      rcases eq_or_lt_of_le hxc with (rfl | h'x)
      · exact ht.1
      exact (lt_irrefl _ (h'x.trans_le (hs.2 hx))).elim
  intro x hx y hy hxy
  rcases lt_or_le x c with (hxc | hcx)
  · have xs : x ∈ s := A _ hx hxc.le
    rcases lt_or_le y c with (hyc | hcy)
    · 

Proof state:
case inl.inl
α : Type u_1
β : Type u_2
inst✝¹ : LinearOrder α
inst✝ : Preorder β
a : α
f : α → β
s t : Set α
c : α
h₁ : StrictMonoOn f s
h₂ : StrictMonoOn f t
hs : IsGreatest s c
ht : IsLeast t c
A : ∀ x ∈ s ∪ t, x ≤ c → x ∈ s
B : ∀ x ∈ s ∪ t, c ≤ x → x ∈ t
x : α
hx : x ∈ s ∪ t
y : α
hy : y ∈ s ∪ t
hxy : x < y
hxc : x < c
xs : x ∈ s
hyc : y < c
⊢ f x < f y

Next tactic:
exact h₁ xs (A _ hy hyc.le) hxy
/-- If `f` is strictly monotone both on `s` and `t`, with `s` to the left of `t` and the center
point belonging to both `s` and `t`, then `f` is strictly monotone on `s ∪ t` -/
protected theorem StrictMonoOn.union {s t : Set α} {c : α} (h₁ : StrictMonoOn f s)
    (h₂ : StrictMonoOn f t) (hs : IsGreatest s c) (ht : IsLeast t c) : StrictMonoOn f (s ∪ t) := by
  have A : ∀ x, x ∈ s ∪ t → x ≤ c → x ∈ s := by
    intro x hx hxc
    cases hx
    · assumption
    rcases eq_or_lt_of_le hxc with (rfl | h'x)
    · exact hs.1
    exact (lt_irrefl _ (h'x.trans_le (ht.2 (by assumption)))).elim
  have B : ∀ x, x ∈ s ∪ t → c ≤ x → x ∈ t := by
    intro x hx hxc
    match hx with
    | Or.inr hx => exact hx
    | Or.inl hx =>
      rcases eq_or_lt_of_le hxc with (rfl | h'x)
      · exact ht.1
      exact (lt_irrefl _ (h'x.trans_le (hs.2 hx))).elim
  intro x hx y hy hxy
  rcases lt_or_le x c with (hxc | hcx)
  · have xs : x ∈ s := A _ hx hxc.le
    rcases lt_or_le y c with (hyc | hcy)
    · exact h₁ xs (A _ hy hyc.le) hxy
    · 

Proof state:
case inl.inr
α : Type u_1
β : Type u_2
inst✝¹ : LinearOrder α
inst✝ : Preorder β
a : α
f : α → β
s t : Set α
c : α
h₁ : StrictMonoOn f s
h₂ : StrictMonoOn f t
hs : IsGreatest s c
ht : IsLeast t c
A : ∀ x ∈ s ∪ t, x ≤ c → x ∈ s
B : ∀ x ∈ s ∪ t, c ≤ x → x ∈ t
x : α
hx : x ∈ s ∪ t
y : α
hy : y ∈ s ∪ t
hxy : x < y
hxc : x < c
xs : x ∈ s
hcy : c ≤ y
⊢ f x < f y

Next tactic:
exact (h₁ xs hs.1 hxc).trans_le (h₂.monotoneOn ht.1 (B _ hy hcy) hcy)
/-- If `f` is strictly monotone both on `s` and `t`, with `s` to the left of `t` and the center
point belonging to both `s` and `t`, then `f` is strictly monotone on `s ∪ t` -/
protected theorem StrictMonoOn.union {s t : Set α} {c : α} (h₁ : StrictMonoOn f s)
    (h₂ : StrictMonoOn f t) (hs : IsGreatest s c) (ht : IsLeast t c) : StrictMonoOn f (s ∪ t) := by
  have A : ∀ x, x ∈ s ∪ t → x ≤ c → x ∈ s := by
    intro x hx hxc
    cases hx
    · assumption
    rcases eq_or_lt_of_le hxc with (rfl | h'x)
    · exact hs.1
    exact (lt_irrefl _ (h'x.trans_le (ht.2 (by assumption)))).elim
  have B : ∀ x, x ∈ s ∪ t → c ≤ x → x ∈ t := by
    intro x hx hxc
    match hx with
    | Or.inr hx => exact hx
    | Or.inl hx =>
      rcases eq_or_lt_of_le hxc with (rfl | h'x)
      · exact ht.1
      exact (lt_irrefl _ (h'x.trans_le (hs.2 hx))).elim
  intro x hx y hy hxy
  rcases lt_or_le x c with (hxc | hcx)
  · have xs : x ∈ s := A _ hx hxc.le
    rcases lt_or_le y c with (hyc | hcy)
    · exact h₁ xs (A _ hy hyc.le) hxy
    · exact (h₁ xs hs.1 hxc).trans_le (h₂.monotoneOn ht.1 (B _ hy hcy) hcy)
  · 

Proof state:
case inr
α : Type u_1
β : Type u_2
inst✝¹ : LinearOrder α
inst✝ : Preorder β
a : α
f : α → β
s t : Set α
c : α
h₁ : StrictMonoOn f s
h₂ : StrictMonoOn f t
hs : IsGreatest s c
ht : IsLeast t c
A : ∀ x ∈ s ∪ t, x ≤ c → x ∈ s
B : ∀ x ∈ s ∪ t, c ≤ x → x ∈ t
x : α
hx : x ∈ s ∪ t
y : α
hy : y ∈ s ∪ t
hxy : x < y
hcx : c ≤ x
⊢ f x < f y

Next tactic:
have xt : x ∈ t := B _ hx hcx
/-- If `f` is strictly monotone both on `s` and `t`, with `s` to the left of `t` and the center
point belonging to both `s` and `t`, then `f` is strictly monotone on `s ∪ t` -/
protected theorem StrictMonoOn.union {s t : Set α} {c : α} (h₁ : StrictMonoOn f s)
    (h₂ : StrictMonoOn f t) (hs : IsGreatest s c) (ht : IsLeast t c) : StrictMonoOn f (s ∪ t) := by
  have A : ∀ x, x ∈ s ∪ t → x ≤ c → x ∈ s := by
    intro x hx hxc
    cases hx
    · assumption
    rcases eq_or_lt_of_le hxc with (rfl | h'x)
    · exact hs.1
    exact (lt_irrefl _ (h'x.trans_le (ht.2 (by assumption)))).elim
  have B : ∀ x, x ∈ s ∪ t → c ≤ x → x ∈ t := by
    intro x hx hxc
    match hx with
    | Or.inr hx => exact hx
    | Or.inl hx =>
      rcases eq_or_lt_of_le hxc with (rfl | h'x)
      · exact ht.1
      exact (lt_irrefl _ (h'x.trans_le (hs.2 hx))).elim
  intro x hx y hy hxy
  rcases lt_or_le x c with (hxc | hcx)
  · have xs : x ∈ s := A _ hx hxc.le
    rcases lt_or_le y c with (hyc | hcy)
    · exact h₁ xs (A _ hy hyc.le) hxy
    · exact (h₁ xs hs.1 hxc).trans_le (h₂.monotoneOn ht.1 (B _ hy hcy) hcy)
  · have xt : x ∈ t := B _ hx hcx
    

Proof state:
case inr
α : Type u_1
β : Type u_2
inst✝¹ : LinearOrder α
inst✝ : Preorder β
a : α
f : α → β
s t : Set α
c : α
h₁ : StrictMonoOn f s
h₂ : StrictMonoOn f t
hs : IsGreatest s c
ht : IsLeast t c
A : ∀ x ∈ s ∪ t, x ≤ c → x ∈ s
B : ∀ x ∈ s ∪ t, c ≤ x → x ∈ t
x : α
hx : x ∈ s ∪ t
y : α
hy : y ∈ s ∪ t
hxy : x < y
hcx : c ≤ x
xt : x ∈ t
⊢ f x < f y

Next tactic:
have yt : y ∈ t := B _ hy (hcx.trans hxy.le)
/-- If `f` is strictly monotone both on `s` and `t`, with `s` to the left of `t` and the center
point belonging to both `s` and `t`, then `f` is strictly monotone on `s ∪ t` -/
protected theorem StrictMonoOn.union {s t : Set α} {c : α} (h₁ : StrictMonoOn f s)
    (h₂ : StrictMonoOn f t) (hs : IsGreatest s c) (ht : IsLeast t c) : StrictMonoOn f (s ∪ t) := by
  have A : ∀ x, x ∈ s ∪ t → x ≤ c → x ∈ s := by
    intro x hx hxc
    cases hx
    · assumption
    rcases eq_or_lt_of_le hxc with (rfl | h'x)
    · exact hs.1
    exact (lt_irrefl _ (h'x.trans_le (ht.2 (by assumption)))).elim
  have B : ∀ x, x ∈ s ∪ t → c ≤ x → x ∈ t := by
    intro x hx hxc
    match hx with
    | Or.inr hx => exact hx
    | Or.inl hx =>
      rcases eq_or_lt_of_le hxc with (rfl | h'x)
      · exact ht.1
      exact (lt_irrefl _ (h'x.trans_le (hs.2 hx))).elim
  intro x hx y hy hxy
  rcases lt_or_le x c with (hxc | hcx)
  · have xs : x ∈ s := A _ hx hxc.le
    rcases lt_or_le y c with (hyc | hcy)
    · exact h₁ xs (A _ hy hyc.le) hxy
    · exact (h₁ xs hs.1 hxc).trans_le (h₂.monotoneOn ht.1 (B _ hy hcy) hcy)
  · have xt : x ∈ t := B _ hx hcx
    have yt : y ∈ t := B _ hy (hcx.trans hxy.le)
    

Proof state:
case inr
α : Type u_1
β : Type u_2
inst✝¹ : LinearOrder α
inst✝ : Preorder β
a : α
f : α → β
s t : Set α
c : α
h₁ : StrictMonoOn f s
h₂ : StrictMonoOn f t
hs : IsGreatest s c
ht : IsLeast t c
A : ∀ x ∈ s ∪ t, x ≤ c → x ∈ s
B : ∀ x ∈ s ∪ t, c ≤ x → x ∈ t
x : α
hx : x ∈ s ∪ t
y : α
hy : y ∈ s ∪ t
hxy : x < y
hcx : c ≤ x
xt : x ∈ t
yt : y ∈ t
⊢ f x < f y

Next tactic:
exact h₂ xt yt hxy
/-- If `f` is strictly monotone both on `(-∞, a]` and `[a, ∞)`, then it is strictly monotone on the
whole line. -/
protected theorem StrictMonoOn.Iic_union_Ici (h₁ : StrictMonoOn f (Iic a))
    (h₂ : StrictMonoOn f (Ici a)) : StrictMono f := by
  

Proof state:
α : Type u_1
β : Type u_2
inst✝¹ : LinearOrder α
inst✝ : Preorder β
a : α
f : α → β
h₁ : StrictMonoOn f (Iic a)
h₂ : StrictMonoOn f (Ici a)
⊢ StrictMono f

Next tactic:
rw [← strictMonoOn_univ, ← @Iic_union_Ici _ _ a]
/-- If `f` is strictly monotone both on `(-∞, a]` and `[a, ∞)`, then it is strictly monotone on the
whole line. -/
protected theorem StrictMonoOn.Iic_union_Ici (h₁ : StrictMonoOn f (Iic a))
    (h₂ : StrictMonoOn f (Ici a)) : StrictMono f := by
  rw [← strictMonoOn_univ, ← @Iic_union_Ici _ _ a]
  

Proof state:
α : Type u_1
β : Type u_2
inst✝¹ : LinearOrder α
inst✝ : Preorder β
a : α
f : α → β
h₁ : StrictMonoOn f (Iic a)
h₂ : StrictMonoOn f (Ici a)
⊢ StrictMonoOn f (Iic a ∪ Ici a)

Next tactic:
exact StrictMonoOn.union h₁ h₂ isGreatest_Iic isLeast_Ici
/-- If `f` is monotone both on `s` and `t`, with `s` to the left of `t` and the center
point belonging to both `s` and `t`, then `f` is monotone on `s ∪ t` -/
protected theorem MonotoneOn.union_right {s t : Set α} {c : α} (h₁ : MonotoneOn f s)
    (h₂ : MonotoneOn f t) (hs : IsGreatest s c) (ht : IsLeast t c) : MonotoneOn f (s ∪ t) := by
  

Proof state:
α : Type u_1
β : Type u_2
inst✝¹ : LinearOrder α
inst✝ : Preorder β
a : α
f : α → β
s t : Set α
c : α
h₁ : MonotoneOn f s
h₂ : MonotoneOn f t
hs : IsGreatest s c
ht : IsLeast t c
⊢ MonotoneOn f (s ∪ t)

Next tactic:
have A : ∀ x, x ∈ s ∪ t → x ≤ c → x ∈ s := by
    intro x hx hxc
    cases hx
    · assumption
    rcases eq_or_lt_of_le hxc with (rfl | h'x)
    · exact hs.1
    exact (lt_irrefl _ (h'x.trans_le (ht.2 (by assumption)))).elim
/-- If `f` is monotone both on `s` and `t`, with `s` to the left of `t` and the center
point belonging to both `s` and `t`, then `f` is monotone on `s ∪ t` -/
protected theorem MonotoneOn.union_right {s t : Set α} {c : α} (h₁ : MonotoneOn f s)
    (h₂ : MonotoneOn f t) (hs : IsGreatest s c) (ht : IsLeast t c) : MonotoneOn f (s ∪ t) := by
  have A : ∀ x, x ∈ s ∪ t → x ≤ c → x ∈ s := by
    

Proof state:
α : Type u_1
β : Type u_2
inst✝¹ : LinearOrder α
inst✝ : Preorder β
a : α
f : α → β
s t : Set α
c : α
h₁ : MonotoneOn f s
h₂ : MonotoneOn f t
hs : IsGreatest s c
ht : IsLeast t c
⊢ ∀ x ∈ s ∪ t, x ≤ c → x ∈ s

Next tactic:
intro x hx hxc
/-- If `f` is monotone both on `s` and `t`, with `s` to the left of `t` and the center
point belonging to both `s` and `t`, then `f` is monotone on `s ∪ t` -/
protected theorem MonotoneOn.union_right {s t : Set α} {c : α} (h₁ : MonotoneOn f s)
    (h₂ : MonotoneOn f t) (hs : IsGreatest s c) (ht : IsLeast t c) : MonotoneOn f (s ∪ t) := by
  have A : ∀ x, x ∈ s ∪ t → x ≤ c → x ∈ s := by
    intro x hx hxc
    

Proof state:
α : Type u_1
β : Type u_2
inst✝¹ : LinearOrder α
inst✝ : Preorder β
a : α
f : α → β
s t : Set α
c : α
h₁ : MonotoneOn f s
h₂ : MonotoneOn f t
hs : IsGreatest s c
ht : IsLeast t c
x : α
hx : x ∈ s ∪ t
hxc : x ≤ c
⊢ x ∈ s

Next tactic:
cases hx
/-- If `f` is monotone both on `s` and `t`, with `s` to the left of `t` and the center
point belonging to both `s` and `t`, then `f` is monotone on `s ∪ t` -/
protected theorem MonotoneOn.union_right {s t : Set α} {c : α} (h₁ : MonotoneOn f s)
    (h₂ : MonotoneOn f t) (hs : IsGreatest s c) (ht : IsLeast t c) : MonotoneOn f (s ∪ t) := by
  have A : ∀ x, x ∈ s ∪ t → x ≤ c → x ∈ s := by
    intro x hx hxc
    cases hx
    · 

Proof state:
case inl
α : Type u_1
β : Type u_2
inst✝¹ : LinearOrder α
inst✝ : Preorder β
a : α
f : α → β
s t : Set α
c : α
h₁ : MonotoneOn f s
h₂ : MonotoneOn f t
hs : IsGreatest s c
ht : IsLeast t c
x : α
hxc : x ≤ c
h✝ : x ∈ s
⊢ x ∈ s

Next tactic:
assumption
/-- If `f` is monotone both on `s` and `t`, with `s` to the left of `t` and the center
point belonging to both `s` and `t`, then `f` is monotone on `s ∪ t` -/
protected theorem MonotoneOn.union_right {s t : Set α} {c : α} (h₁ : MonotoneOn f s)
    (h₂ : MonotoneOn f t) (hs : IsGreatest s c) (ht : IsLeast t c) : MonotoneOn f (s ∪ t) := by
  have A : ∀ x, x ∈ s ∪ t → x ≤ c → x ∈ s := by
    intro x hx hxc
    cases hx
    · assumption
    

Proof state:
case inr
α : Type u_1
β : Type u_2
inst✝¹ : LinearOrder α
inst✝ : Preorder β
a : α
f : α → β
s t : Set α
c : α
h₁ : MonotoneOn f s
h₂ : MonotoneOn f t
hs : IsGreatest s c
ht : IsLeast t c
x : α
hxc : x ≤ c
h✝ : x ∈ t
⊢ x ∈ s

Next tactic:
rcases eq_or_lt_of_le hxc with (rfl | h'x)
/-- If `f` is monotone both on `s` and `t`, with `s` to the left of `t` and the center
point belonging to both `s` and `t`, then `f` is monotone on `s ∪ t` -/
protected theorem MonotoneOn.union_right {s t : Set α} {c : α} (h₁ : MonotoneOn f s)
    (h₂ : MonotoneOn f t) (hs : IsGreatest s c) (ht : IsLeast t c) : MonotoneOn f (s ∪ t) := by
  have A : ∀ x, x ∈ s ∪ t → x ≤ c → x ∈ s := by
    intro x hx hxc
    cases hx
    · assumption
    rcases eq_or_lt_of_le hxc with (rfl | h'x)
    · 

Proof state:
case inr.inl
α : Type u_1
β : Type u_2
inst✝¹ : LinearOrder α
inst✝ : Preorder β
a : α
f : α → β
s t : Set α
h₁ : MonotoneOn f s
h₂ : MonotoneOn f t
x : α
h✝ : x ∈ t
hs : IsGreatest s x
ht : IsLeast t x
hxc : x ≤ x
⊢ x ∈ s

Next tactic:
exact hs.1
/-- If `f` is monotone both on `s` and `t`, with `s` to the left of `t` and the center
point belonging to both `s` and `t`, then `f` is monotone on `s ∪ t` -/
protected theorem MonotoneOn.union_right {s t : Set α} {c : α} (h₁ : MonotoneOn f s)
    (h₂ : MonotoneOn f t) (hs : IsGreatest s c) (ht : IsLeast t c) : MonotoneOn f (s ∪ t) := by
  have A : ∀ x, x ∈ s ∪ t → x ≤ c → x ∈ s := by
    intro x hx hxc
    cases hx
    · assumption
    rcases eq_or_lt_of_le hxc with (rfl | h'x)
    · exact hs.1
    

Proof state:
case inr.inr
α : Type u_1
β : Type u_2
inst✝¹ : LinearOrder α
inst✝ : Preorder β
a : α
f : α → β
s t : Set α
c : α
h₁ : MonotoneOn f s
h₂ : MonotoneOn f t
hs : IsGreatest s c
ht : IsLeast t c
x : α
hxc : x ≤ c
h✝ : x ∈ t
h'x : x < c
⊢ x ∈ s

Next tactic:
exact (lt_irrefl _ (h'x.trans_le (ht.2 (by assumption)))).elim
/-- If `f` is monotone both on `s` and `t`, with `s` to the left of `t` and the center
point belonging to both `s` and `t`, then `f` is monotone on `s ∪ t` -/
protected theorem MonotoneOn.union_right {s t : Set α} {c : α} (h₁ : MonotoneOn f s)
    (h₂ : MonotoneOn f t) (hs : IsGreatest s c) (ht : IsLeast t c) : MonotoneOn f (s ∪ t) := by
  have A : ∀ x, x ∈ s ∪ t → x ≤ c → x ∈ s := by
    intro x hx hxc
    cases hx
    · assumption
    rcases eq_or_lt_of_le hxc with (rfl | h'x)
    · exact hs.1
    exact (lt_irrefl _ (h'x.trans_le (ht.2 (by 

Proof state:
α : Type u_1
β : Type u_2
inst✝¹ : LinearOrder α
inst✝ : Preorder β
a : α
f : α → β
s t : Set α
c : α
h₁ : MonotoneOn f s
h₂ : MonotoneOn f t
hs : IsGreatest s c
ht : IsLeast t c
x : α
hxc : x ≤ c
h✝ : x ∈ t
h'x : x < c
⊢ x ∈ t

Next tactic:
assumption
/-- If `f` is monotone both on `s` and `t`, with `s` to the left of `t` and the center
point belonging to both `s` and `t`, then `f` is monotone on `s ∪ t` -/
protected theorem MonotoneOn.union_right {s t : Set α} {c : α} (h₁ : MonotoneOn f s)
    (h₂ : MonotoneOn f t) (hs : IsGreatest s c) (ht : IsLeast t c) : MonotoneOn f (s ∪ t) := by
  have A : ∀ x, x ∈ s ∪ t → x ≤ c → x ∈ s := by
    intro x hx hxc
    cases hx
    · assumption
    rcases eq_or_lt_of_le hxc with (rfl | h'x)
    · exact hs.1
    exact (lt_irrefl _ (h'x.trans_le (ht.2 (by assumption)))).elim
  

Proof state:
α : Type u_1
β : Type u_2
inst✝¹ : LinearOrder α
inst✝ : Preorder β
a : α
f : α → β
s t : Set α
c : α
h₁ : MonotoneOn f s
h₂ : MonotoneOn f t
hs : IsGreatest s c
ht : IsLeast t c
A : ∀ x ∈ s ∪ t, x ≤ c → x ∈ s
⊢ MonotoneOn f (s ∪ t)

Next tactic:
have B : ∀ x, x ∈ s ∪ t → c ≤ x → x ∈ t := by
    intro x hx hxc
    match hx with
    | Or.inr hx => exact hx
    | Or.inl hx =>
      rcases eq_or_lt_of_le hxc with (rfl | h'x)
      · exact ht.1
      exact (lt_irrefl _ (h'x.trans_le (hs.2 hx))).elim