-- Mathlib/Topology/DerivedSet.lean
lemma IsPreconnected.inter_derivedSet_nonempty [T1Space X] {U : Set X} (hs : IsPreconnected U)
    (a b : Set X) (h : U ⊆ a ∪ b) (ha : (U ∩ derivedSet a).Nonempty)
    (hb : (U ∩ derivedSet b).Nonempty) : (U ∩ (derivedSet a ∩ derivedSet b)).Nonempty

Proof state:
case pos.a
X : Type u_1
inst✝¹ : TopologicalSpace X
inst✝ : T1Space X
U : Set X
hs : IsPreconnected U
a b : Set X
h : U ⊆ a ∪ b
ha : (U ∩ derivedSet a).Nonempty
hb : (U ∩ derivedSet b).Nonempty
hu : U.Nontrivial
⊢ (U ∩ derivedSet b).Nonempty

Tactic:
exact hb

Result:
no goals

Proof state:
case pos.a
X : Type u_1
inst✝¹ : TopologicalSpace X
inst✝ : T1Space X
U : Set X
hs : IsPreconnected U
a b : Set X
h : U ⊆ a ∪ b
ha : (U ∩ derivedSet a).Nonempty
hb : (U ∩ derivedSet b).Nonempty
hu : U.Nontrivial
⊢ IsClosed (derivedSet b)

Tactic:
simp

Result:
no goals

Proof state:
X : Type u_1
inst✝¹ : TopologicalSpace X
inst✝ : T1Space X
U : Set X
hs : IsPreconnected U
a b : Set X
h : U ⊆ a ∪ b
ha : (U ∩ derivedSet a).Nonempty
hb : (U ∩ derivedSet b).Nonempty
⊢ (U ∩ (derivedSet a ∩ derivedSet b)).Nonempty

Tactic:
by_cases hu : U.Nontrivial

Result:
case pos
X : Type u_1
inst✝¹ : TopologicalSpace X
inst✝ : T1Space X
U : Set X
hs : IsPreconnected U
a b : Set X
h : U ⊆ a ∪ b
ha : (U ∩ derivedSet a).Nonempty
hb : (U ∩ derivedSet b).Nonempty
hu : U.Nontrivial
⊢ (U ∩ (derivedSet a ∩ derivedSet b)).Nonempty

case neg
X : Type u_1
inst✝¹ : TopologicalSpace X
inst✝ : T1Space X
U : Set X
hs : IsPreconnected U
a b : Set X
h : U ⊆ a ∪ b
ha : (U ∩ derivedSet a).Nonempty
hb : (U ∩ derivedSet b).Nonempty
hu : ¬U.Nontrivial
⊢ (U ∩ (derivedSet a ∩ derivedSet b)).Nonempty

Proof state:
case pos.a
X : Type u_1
inst✝¹ : TopologicalSpace X
inst✝ : T1Space X
U : Set X
hs : IsPreconnected U
a b : Set X
h : U ⊆ a ∪ b
ha : (U ∩ derivedSet a).Nonempty
hb : (U ∩ derivedSet b).Nonempty
hu : U.Nontrivial
⊢ (U ∩ derivedSet a).Nonempty

Tactic:
exact ha

Result:
no goals

Proof state:
case pos
X : Type u_1
inst✝¹ : TopologicalSpace X
inst✝ : T1Space X
U : Set X
hs : IsPreconnected U
a b : Set X
h : U ⊆ a ∪ b
ha : (U ∩ derivedSet a).Nonempty
hb : (U ∩ derivedSet b).Nonempty
hu : U.Nontrivial
⊢ (U ∩ (derivedSet a ∩ derivedSet b)).Nonempty

Tactic:
apply isPreconnected_closed_iff.mp hs

Result:
case pos.a
X : Type u_1
inst✝¹ : TopologicalSpace X
inst✝ : T1Space X
U : Set X
hs : IsPreconnected U
a b : Set X
h : U ⊆ a ∪ b
ha : (U ∩ derivedSet a).Nonempty
hb : (U ∩ derivedSet b).Nonempty
hu : U.Nontrivial
⊢ IsClosed (derivedSet a)

case pos.a
X : Type u_1
inst✝¹ : TopologicalSpace X
inst✝ : T1Space X
U : Set X
hs : IsPreconnected U
a b : Set X
h : U ⊆ a ∪ b
ha : (U ∩ derivedSet a).Nonempty
hb : (U ∩ derivedSet b).Nonempty
hu : U.Nontrivial
⊢ IsClosed (derivedSet b)

case pos.a
X : Type u_1
inst✝¹ : TopologicalSpace X
inst✝ : T1Space X
U : Set X
hs : IsPreconnected U
a b : Set X
h : U ⊆ a ∪ b
ha : (U ∩ derivedSet a).Nonempty
hb : (U ∩ derivedSet b).Nonempty
hu : U.Nontrivial
⊢ U ⊆ derivedSet a ∪ derivedSet b

case pos.a
X : Type u_1
inst✝¹ : TopologicalSpace X
inst✝ : T1Space X
U : Set X
hs : IsPreconnected U
a b : Set X
h : U ⊆ a ∪ b
ha : (U ∩ derivedSet a).Nonempty
hb : (U ∩ derivedSet b).Nonempty
hu : U.Nontrivial
⊢ (U ∩ derivedSet a).Nonempty

case pos.a
X : Type u_1
inst✝¹ : TopologicalSpace X
inst✝ : T1Space X
U : Set X
hs : IsPreconnected U
a b : Set X
h : U ⊆ a ∪ b
ha : (U ∩ derivedSet a).Nonempty
hb : (U ∩ derivedSet b).Nonempty
hu : U.Nontrivial
⊢ (U ∩ derivedSet b).Nonempty

Proof state:
X : Type u_1
inst✝¹ : TopologicalSpace X
inst✝ : T1Space X
U : Set X
hs : IsPreconnected U
a b : Set X
h : U ⊆ a ∪ b
ha : (U ∩ derivedSet a).Nonempty
hb : (U ∩ derivedSet b).Nonempty
hu : U.Nontrivial
⊢ derivedSet U ⊆ derivedSet a ∪ derivedSet b

Tactic:
rw [← derivedSet_union]

Result:
X : Type u_1
inst✝¹ : TopologicalSpace X
inst✝ : T1Space X
U : Set X
hs : IsPreconnected U
a b : Set X
h : U ⊆ a ∪ b
ha : (U ∩ derivedSet a).Nonempty
hb : (U ∩ derivedSet b).Nonempty
hu : U.Nontrivial
⊢ derivedSet U ⊆ derivedSet (a ∪ b)

Proof state:
case neg
X : Type u_1
inst✝¹ : TopologicalSpace X
inst✝ : T1Space X
U : Set X
hs : IsPreconnected U
a b : Set X
h : U ⊆ a ∪ b
ha : (U ∩ derivedSet a).Nonempty
hb : (U ∩ derivedSet b).Nonempty
hu : ¬U.Nontrivial
⊢ (U ∩ (derivedSet a ∩ derivedSet b)).Nonempty

Tactic:
obtain ⟨x, hx⟩ := ha.left.exists_eq_singleton_or_nontrivial.resolve_right hu

Result:
case neg.intro
X : Type u_1
inst✝¹ : TopologicalSpace X
inst✝ : T1Space X
U : Set X
hs : IsPreconnected U
a b : Set X
h : U ⊆ a ∪ b
ha : (U ∩ derivedSet a).Nonempty
hb : (U ∩ derivedSet b).Nonempty
hu : ¬U.Nontrivial
x : X
hx : U = {x}
⊢ (U ∩ (derivedSet a ∩ derivedSet b)).Nonempty

Proof state:
case pos.a
X : Type u_1
inst✝¹ : TopologicalSpace X
inst✝ : T1Space X
U : Set X
hs : IsPreconnected U
a b : Set X
h : U ⊆ a ∪ b
ha : (U ∩ derivedSet a).Nonempty
hb : (U ∩ derivedSet b).Nonempty
hu : U.Nontrivial
⊢ U ⊆ derivedSet a ∪ derivedSet b

Tactic:
trans derivedSet U

Result:
X : Type u_1
inst✝¹ : TopologicalSpace X
inst✝ : T1Space X
U : Set X
hs : IsPreconnected U
a b : Set X
h : U ⊆ a ∪ b
ha : (U ∩ derivedSet a).Nonempty
hb : (U ∩ derivedSet b).Nonempty
hu : U.Nontrivial
⊢ U ⊆ derivedSet U

X : Type u_1
inst✝¹ : TopologicalSpace X
inst✝ : T1Space X
U : Set X
hs : IsPreconnected U
a b : Set X
h : U ⊆ a ∪ b
ha : (U ∩ derivedSet a).Nonempty
hb : (U ∩ derivedSet b).Nonempty
hu : U.Nontrivial
⊢ derivedSet U ⊆ derivedSet a ∪ derivedSet b

Proof state:
case neg.intro
X : Type u_1
inst✝¹ : TopologicalSpace X
inst✝ : T1Space X
U : Set X
hs : IsPreconnected U
a b : Set X
h : U ⊆ a ∪ b
ha : (U ∩ derivedSet a).Nonempty
hb : (U ∩ derivedSet b).Nonempty
hu : ¬U.Nontrivial
x : X
hx : U = {x}
⊢ (U ∩ (derivedSet a ∩ derivedSet b)).Nonempty

Tactic:
simp_all

Result:
no goals

Proof state:
X : Type u_1
inst✝¹ : TopologicalSpace X
inst✝ : T1Space X
U : Set X
hs : IsPreconnected U
a b : Set X
h : U ⊆ a ∪ b
ha : (U ∩ derivedSet a).Nonempty
hb : (U ∩ derivedSet b).Nonempty
hu : U.Nontrivial
⊢ U ⊆ derivedSet U

Tactic:
apply hs.preperfect_of_nontrivial hu

Result:
no goals

Proof state:
case pos.a
X : Type u_1
inst✝¹ : TopologicalSpace X
inst✝ : T1Space X
U : Set X
hs : IsPreconnected U
a b : Set X
h : U ⊆ a ∪ b
ha : (U ∩ derivedSet a).Nonempty
hb : (U ∩ derivedSet b).Nonempty
hu : U.Nontrivial
⊢ IsClosed (derivedSet a)

Tactic:
simp

Result:
no goals

Proof state:
X : Type u_1
inst✝¹ : TopologicalSpace X
inst✝ : T1Space X
U : Set X
hs : IsPreconnected U
a b : Set X
h : U ⊆ a ∪ b
ha : (U ∩ derivedSet a).Nonempty
hb : (U ∩ derivedSet b).Nonempty
hu : U.Nontrivial
⊢ derivedSet U ⊆ derivedSet (a ∪ b)

Tactic:
exact derivedSet_mono _ _ h

Result:
no goals