mathlib-initiative/mathlib-tactics huggingface dataset:
Module: Mathlib.Topology.Order.UpperLowerSetTopology

Goal state before:
pp: α : Type u_1
inst✝² : Preorder α
t₁ t₂ : TopologicalSpace α
inst✝¹ : Topology.IsLowerSet α
inst✝ : IsLower α
s : Set α
hs : IsOpen[t₂] s
⊢ IsOpen[t₁] s

Tactic:
by
  rw [@isOpen_iff_isLowerSet α _ t₁]
  exact IsLower.isLowerSet_of_isOpen hs

Goal state after:
Module: Mathlib.Topology.Order.ScottTopology

Goal state before:
pp: α : Type u_1
inst✝² : Preorder α
s : Set α
inst✝¹ : TopologicalSpace α
inst✝ : IsScott α univ
⊢ IsOpen[inst✝¹] s ↔ Continuous[inst✝¹, _] fun x ↦ x ∈ s

Tactic:
exact isOpen_iff_continuous_mem

Goal state after:
Module: Mathlib.Topology.Order.HullKernel

Goal state before:
pp: case mpr
α : Type u_1
inst✝³ : SemilatticeInf α
T : Set α
inst✝² : OrderTop α
inst✝¹ : TopologicalSpace α
inst✝ : IsLower α
hT : ∀ p ∈ T, InfPrime p
R : Set ↑T
ha : ∃ a, a.Finite ∧ (Subtype.val ⁻¹' ↑(upperClosure a))ᶜ = R
⊢ ∃ a, (hull T a)ᶜ = R

Tactic:
obtain ⟨F, hF⟩ := ha
    lift F to Finset α using hF.1
    use Finset.inf F id
    ext
    simp [hull_finsetInf hT, ← hF.2]

Goal state after:
Module: Mathlib.Topology.Partial

Goal state before:
pp: X : Type u_1
Y : Type u_2
inst✝¹ : TopologicalSpace X
inst✝ : TopologicalSpace Y
f : X →. Y
h : PContinuous f
⊢ IsOpen[inst✝¹] f.Dom

Tactic:
rw [← PFun.preimage_univ]

Goal state after:
X : Type u_1
Y : Type u_2
inst✝¹ : TopologicalSpace X
inst✝ : TopologicalSpace Y
f : X →. Y
h : PContinuous f
⊢ IsOpen[inst✝¹] (f.preimage Set.univ)
Module: Mathlib.Topology.MetricSpace.GromovHausdorff

Goal state before:
pp: x y z : GHSpace
X : Type := x.Rep
⊢ dist x z ≤ dist x y + dist y z

Tactic:
let Y := y.Rep

Goal state after:
x y z : GHSpace
X : Type := x.Rep
Y : Type := y.Rep
⊢ dist x z ≤ dist x y + dist y z
Module: Mathlib.Topology.MetricSpace.GromovHausdorff

Goal state before:
pp: X : Type u
inst✝ : MetricSpace X
p q : NonemptyCompacts X
ha : Isometry Subtype.val
hb : Isometry Subtype.val
A : dist p q = hausdorffDist ↑p ↑q
⊢ dist p.toGHSpace q.toGHSpace ≤ dist p q

Tactic:
have I : ↑p = range ((↑) : p → X) := Subtype.range_coe_subtype.symm

Goal state after:
X : Type u
inst✝ : MetricSpace X
p q : NonemptyCompacts X
ha : Isometry Subtype.val
hb : Isometry Subtype.val
A : dist p q = hausdorffDist ↑p ↑q
I : ↑p = range Subtype.val
⊢ dist p.toGHSpace q.toGHSpace ≤ dist p q
Module: Mathlib.Topology.MetricSpace.GromovHausdorff

Goal state before:
pp: case refine_2
X : Type u
inst✝⁵ : MetricSpace X
inst✝⁴ : CompactSpace X
inst✝³ : Nonempty X
Y : Type v
inst✝² : MetricSpace Y
inst✝¹ : CompactSpace Y
inst✝ : Nonempty Y
s : Set X
Φ : ↑s → Y
ε₁ ε₂ ε₃ : ℝ
hs : ∀ (x : X), ∃ y ∈ s, dist x y ≤ ε₁
hs' : ∀ (x : Y), ∃ y, dist x (Φ y) ≤ ε₃
H : ∀ (x y : ↑s), |dist x y - dist (Φ x) (Φ y)| ≤ ε₂
δ : ℝ
δ0 : 0 < δ
xX : X
h✝ : xX ∈ univ
xs : X
hxs : xs ∈ s
Dxs : dist xX xs ≤ ε₁
sne : s.Nonempty
x✝¹ : Nonempty ↑s := Nonempty.to_subtype sne
this✝⁴ : 0 ≤ ε₂
this✝³ : ∀ (p q : ↑s), |dist p q - dist (Φ p) (Φ q)| ≤ 2 * (ε₂ / 2 + δ)
x✝ : MetricSpace (X ⊕ Y) := glueMetricApprox (fun x ↦ ↑x) (fun x ↦ Φ x) (ε₂ / 2 + δ) ⋯ this✝³
Fl : X → X ⊕ Y := inl
Fr : Y → X ⊕ Y := inr
Il : Isometry Fl
Ir : Isometry Fr
this✝² : ghDist X Y ≤ hausdorffDist (range Fl) (range Fr)
this✝¹ : hausdorffDist (range Fl) (range Fr) ≤ hausdorffDist (range Fl) (Fl '' s) + hausdorffDist (Fl '' s) (range Fr)
this✝ :
  hausdorffDist (Fl '' s) (range Fr) ≤
    hausdorffDist (Fl '' s) (Fr '' range Φ) + hausdorffDist (Fr '' range Φ) (range Fr)
this : hausdorffDist (range Fl) (Fl '' s) ≤ ε₁
x' : X ⊕ Y
hx' : x' ∈ Fr '' range Φ
y : Y
y_in_s' : y ∈ range Φ
yx' : Fr y = x'
⊢ ∃ y ∈ Fl '' s, dist x' y ≤ ε₂ / 2 + δ

Tactic:
rcases mem_range.1 y_in_s' with ⟨x, xy⟩

Goal state after:
case refine_2
X : Type u
inst✝⁵ : MetricSpace X
inst✝⁴ : CompactSpace X
inst✝³ : Nonempty X
Y : Type v
inst✝² : MetricSpace Y
inst✝¹ : CompactSpace Y
inst✝ : Nonempty Y
s : Set X
Φ : ↑s → Y
ε₁ ε₂ ε₃ : ℝ
hs : ∀ (x : X), ∃ y ∈ s, dist x y ≤ ε₁
hs' : ∀ (x : Y), ∃ y, dist x (Φ y) ≤ ε₃
H : ∀ (x y : ↑s), |dist x y - dist (Φ x) (Φ y)| ≤ ε₂
δ : ℝ
δ0 : 0 < δ
xX : X
h✝ : xX ∈ univ
xs : X
hxs : xs ∈ s
Dxs : dist xX xs ≤ ε₁
sne : s.Nonempty
x✝¹ : Nonempty ↑s := Nonempty.to_subtype sne
this✝⁴ : 0 ≤ ε₂
this✝³ : ∀ (p q : ↑s), |dist p q - dist (Φ p) (Φ q)| ≤ 2 * (ε₂ / 2 + δ)
x✝ : MetricSpace (X ⊕ Y) := glueMetricApprox (fun x ↦ ↑x) (fun x ↦ Φ x) (ε₂ / 2 + δ) ⋯ this✝³
Fl : X → X ⊕ Y := inl
Fr : Y → X ⊕ Y := inr
Il : Isometry Fl
Ir : Isometry Fr
this✝² : ghDist X Y ≤ hausdorffDist (range Fl) (range Fr)
this✝¹ : hausdorffDist (range Fl) (range Fr) ≤ hausdorffDist (range Fl) (Fl '' s) + hausdorffDist (Fl '' s) (range Fr)
this✝ :
  hausdorffDist (Fl '' s) (range Fr) ≤
    hausdorffDist (Fl '' s) (Fr '' range Φ) + hausdorffDist (Fr '' range Φ) (range Fr)
this : hausdorffDist (range Fl) (Fl '' s) ≤ ε₁
x' : X ⊕ Y
hx' : x' ∈ Fr '' range Φ
y : Y
y_in_s' : y ∈ range Φ
yx' : Fr y = x'
x : ↑s
xy : Φ x = y
⊢ ∃ y ∈ Fl '' s, dist x' y ≤ ε₂ / 2 + δ
Module: Mathlib.Topology.Separation.PerfectlyNormal

Goal state before:
pp: case refine_1
X : Type u_1
inst✝ : TopologicalSpace X
h✝ : PerfectlyNormalSpace X
s : Set X
hs : IsClosed[inst✝] s
U : ℕ → Set X
ho : ∀ (n : ℕ), IsOpen[inst✝] (U n)
hu : s = ⋂ n, U n
this : ∀ (n : ℕ), Disjoint s (U n)ᶜ
f : ℕ → C(X, ℝ)
hfs : ∀ (n : ℕ), EqOn (⇑(f n)) 0 s
hfu : ∀ (n : ℕ), EqOn (⇑(f n)) 1 (U n)ᶜ
hfr : ∀ (n : ℕ) (x : X), (f n) x ∈ Icc 0 1
hsb : ∀ (x : X) (n : ℕ), ‖(f n) x * (1 / 2 / 2 ^ n)‖ ≤ 1 / 2 / 2 ^ n
hsx : ∀ (x : X), Summable fun n ↦ (f n) x * (1 / 2 / 2 ^ n)
h : C(X, ℝ) := { toFun := fun x ↦ ∑' (n : ℕ), (f n) x * (1 / 2 / 2 ^ n), continuous_toFun := ⋯ }
⊢ s = ⇑h ⁻¹' {0}

Tactic:
ext x
      refine ⟨fun hp => ?_, fun hp => ?_⟩
      · suffices ∀ n, f n x = 0 from by simp [h, this]
        exact fun n => hfs n hp
      · contrapose h
        simp only [preimage, notMem_ofPred_iff, ContinuousMap.coe_mk, mem_singleton_iff]
        apply ne_of_gt
        obtain ⟨i, hi⟩ := mem_iUnion.1 <| compl_iInter _ ▸ mem_compl (hu ▸ h)
        calc
        _ < 1 / 2 / 2 ^ i := by positivity
        _ = f i x * (1 / 2 / 2 ^ i) := by simp [hfu i hi]
        _ ≤ _ := (hsx x).le_tsum i fun j hj => by positivity [(hfr j x).1]

Goal state after:
Module: Mathlib.Topology.MetricSpace.GromovHausdorff

Goal state before:
pp: case h
δ : ℝ
δpos : δ > 0
ε : ℝ := 2 / 5 * δ
εpos : 0 < ε
s : (p : GHSpace) → Set p.Rep
hs : ∀ (p : GHSpace), (s p).Finite ∧ univ ⊆ ⋃ x ∈ s p, ball x ε
N : GHSpace → ℕ := fun p ↦ Nat.card ↑(s p)
E : (p : GHSpace) → ↑(s p) ≃ Fin (Nat.card ↑(s p)) := fun p ↦ Finite.equivFin ↑(s p)
F : GHSpace → (n : ℕ) × (Fin n → Fin n → ℤ) := fun p ↦ ⟨N p, fun a b ↦ ⌊ε⁻¹ * dist ((E p).symm a) ((E p).symm b)⌋⟩
p q : GHSpace
hpq : F p = F q
Npq : N p = N q
Ψ : ↑(s p) → ↑(s q) := fun x ↦ (E q).symm (Fin.cast Npq ((E p) x))
Φ : ↑(s p) → q.Rep := fun x ↦ ↑(Ψ x)
x : q.Rep
this✝ : x ∈ ⋃ y ∈ s q, ball y ε
y : q.Rep
ys : y ∈ s q
hy : x ∈ ball y ε
i : ℕ := ↑((E q) ⟨y, ys⟩)
hi : ↑((E q) ⟨y, ys⟩) < Nat.card ↑(s q) := Fin.is_lt ((E q) ⟨y, ys⟩)
ihi_eq : ⟨i, hi⟩ = (E q) ⟨y, ys⟩
hiq : i < N q
hip : i < N p
z : ↑(s p) := (E p).symm ⟨i, hip⟩
C1 : (E p) z = ⟨i, hip⟩
C2 : Fin.cast Npq ⟨i, hip⟩ = ⟨i, hi⟩
C3 : (E q).symm ⟨i, hi⟩ = ⟨y, ys⟩
this : Φ z = y
⊢ dist x y ≤ ε

Tactic:
exact hy.le

Goal state after:
Module: Mathlib.Topology.Sheaves.MayerVietoris

Goal state before:
pp: case refine_2
T : Type u
inst✝ : TopologicalSpace T
sq : Square (Opens T)
h₄ : sq.X₄ = sq.X₂ ⊔ sq.X₃
h₁ : sq.X₁ = sq.X₂ ⊓ sq.X₃
⊢ ∀ (s : PullbackCone sq.f₂₄ sq.f₃₄), homOfLE ⋯ ≫ sq.f₁₂ = s.fst

Tactic:
intros; apply Subsingleton.elim

Goal state after:
Module: Mathlib.Topology.Sheaves.MayerVietoris

Goal state before:
pp: case refine_3
T : Type u
inst✝ : TopologicalSpace T
sq : Square (Opens T)
h₄ : sq.X₄ = sq.X₂ ⊔ sq.X₃
h₁ : sq.X₁ = sq.X₂ ⊓ sq.X₃
⊢ ∀ (s : PullbackCone sq.f₂₄ sq.f₃₄), homOfLE ⋯ ≫ sq.f₁₃ = s.snd

Tactic:
intros; apply Subsingleton.elim

Goal state after:
Module: Mathlib.Topology.Sheaves.MayerVietoris

Goal state before:
pp: case refine_4
T : Type u
inst✝ : TopologicalSpace T
sq : Square (Opens T)
h₄ : sq.X₄ = sq.X₂ ⊔ sq.X₃
h₁ : sq.X₁ = sq.X₂ ⊓ sq.X₃
⊢ ∀ (s : PullbackCone sq.f₂₄ sq.f₃₄) (m : s.pt ⟶ sq.X₁), m ≫ sq.f₁₂ = s.fst → m ≫ sq.f₁₃ = s.snd → m = homOfLE ⋯

Tactic:
intros; apply Subsingleton.elim

Goal state after:
Module: Mathlib.Topology.Sheaves.Flasque

Goal state before:
pp: X : TopCat
U : Opens ↑X
S : ShortComplex (Sheaf AddCommGrpCat X)
hS : S.ShortExact
inst✝ : S.X₁.IsFlasque
s : ↑(S.X₃.obj.obj (op U))
t : Under S.g s
ht : ∀ (a : Under S.g s), Nonempty (a ⟶ t) → Nonempty (t ⟶ a)
tle : unop (StructuredArrow.right t).fst ≤ U
tcomp :
  (s |_ unop (StructuredArrow.right t).fst) tle =
    (ConcreteCategory.hom (S.g.hom.app (StructuredArrow.right t).fst)) (StructuredArrow.right t).snd
⊢ U ≤ unop (StructuredArrow.right t).fst

Tactic:
intro x hx

Goal state after:
X : TopCat
U : Opens ↑X
S : ShortComplex (Sheaf AddCommGrpCat X)
hS : S.ShortExact
inst✝ : S.X₁.IsFlasque
s : ↑(S.X₃.obj.obj (op U))
t : Under S.g s
ht : ∀ (a : Under S.g s), Nonempty (a ⟶ t) → Nonempty (t ⟶ a)
tle : unop (StructuredArrow.right t).fst ≤ U
tcomp :
  (s |_ unop (StructuredArrow.right t).fst) tle =
    (ConcreteCategory.hom (S.g.hom.app (StructuredArrow.right t).fst)) (StructuredArrow.right t).snd
x : ↑X
hx : x ∈ U
⊢ x ∈ unop (StructuredArrow.right t).fst
Module: Mathlib.Topology.MetricSpace.GromovHausdorffRealized

Goal state before:
pp: case refine_2
X : Type u
Y : Type v
inst✝³ : MetricSpace X
inst✝² : MetricSpace Y
inst✝¹ : CompactSpace X
inst✝ : CompactSpace Y
⊢ Equicontinuous fun x ↦ ⇑↑x

Tactic:
· refine equicontinuous_of_continuity_modulus (fun t => 2 * maxVar X Y * t) ?_ _ ?_
    · have : Tendsto (fun t : ℝ => 2 * (maxVar X Y : ℝ) * t) (𝓝 0) (𝓝 (2 * maxVar X Y * 0)) :=
        tendsto_const_nhds.mul tendsto_id
      simpa using this
    · rintro x y ⟨f, hf⟩
      exact (candidates_lipschitz hf).dist_le_mul _ _

Goal state after:
Module: Mathlib.Topology.SmallInductiveDimension

Goal state before:
pp: X : Type u_1
inst✝ : TopologicalSpace X
⊢ smallInductiveDimension X = ⊥ ↔ IsEmpty X

Tactic:
← hasSmallInductiveDimensionLT_zero_iff,

Goal state after:
X : Type u_1
inst✝ : TopologicalSpace X
⊢ smallInductiveDimension X = ⊥ ↔ HasSmallInductiveDimensionLT X 0
Module: Mathlib.Topology.Subpath

Goal state before:
pp: case zero
X : Type u_1
inst✝ : TopologicalSpace X
a b : X
n : ℕ
p : Fin (0 + 1) → X
F G : (k : Fin 0) → Path (p k.castSucc) (p k.succ)
H : (k : Fin 0) → (F k).Homotopy (G k)
⊢ (concat p F).Homotopy (concat p G)

Tactic:
concat_zero,

Goal state after:
case zero
X : Type u_1
inst✝ : TopologicalSpace X
a b : X
n : ℕ
p : Fin (0 + 1) → X
F G : (k : Fin 0) → Path (p k.castSucc) (p k.succ)
H : (k : Fin 0) → (F k).Homotopy (G k)
⊢ (Path.refl (p 0)).Homotopy (concat p G)
Module: Mathlib.Topology.Sion

Goal state before:
pp: E : Type u_1
F : Type u_2
β : Type u_3
inst✝¹¹ : LinearOrder β
Y : Set F
f : E → F → β
inst✝¹⁰ : TopologicalSpace E
inst✝⁹ : AddCommGroup E
inst✝⁸ : Module ℝ E
inst✝⁷ : IsTopologicalAddGroup E
inst✝⁶ : ContinuousSMul ℝ E
inst✝⁵ : TopologicalSpace F
inst✝⁴ : AddCommGroup F
inst✝³ : Module ℝ F
cY : Convex ℝ Y
inst✝² : DenselyOrdered β
inst✝¹ : IsTopologicalAddGroup F
inst✝ : ContinuousSMul ℝ F
s✝ : Set F
t : β
b : F
s : Set F
a✝ : b ∉ s
hs : s.Finite
X : Set E
ne_X : X.Nonempty
kX : IsCompact X
hfy : ∀ y ∈ Y, LowerSemicontinuousOn (fun x ↦ f x y) X
hfy' : ∀ y ∈ Y, QuasiconvexOn ℝ X fun x ↦ f x y
hfx : ∀ x ∈ X, UpperSemicontinuousOn (fun y ↦ f x y) Y
hfx' : ∀ x ∈ X, QuasiconcaveOn ℝ Y fun y ↦ f x y
cX : Convex ℝ X
hsY : insert b s ⊆ Y
ht : ∀ x ∈ X, ∃ y ∈ insert b s, t < f x y
hb : b ∈ Y
X' : Set E := X ∩ (fun x ↦ f x b) ⁻¹' Iic t
hrec : s ⊆ Y → (∀ x ∈ X', ∃ y ∈ s, t < f x y) → ∃ y0 ∈ Y, ∀ x ∈ X', t < f x y0
X'_ne : X'.Nonempty
hX'X : X' ⊆ X
kX' : IsCompact X'
cX' : Convex ℝ X'
⊢ ∀ x ∈ X', ∃ y ∈ s, t < f x y

Tactic:
intro x hx

Goal state after:
E : Type u_1
F : Type u_2
β : Type u_3
inst✝¹¹ : LinearOrder β
Y : Set F
f : E → F → β
inst✝¹⁰ : TopologicalSpace E
inst✝⁹ : AddCommGroup E
inst✝⁸ : Module ℝ E
inst✝⁷ : IsTopologicalAddGroup E
inst✝⁶ : ContinuousSMul ℝ E
inst✝⁵ : TopologicalSpace F
inst✝⁴ : AddCommGroup F
inst✝³ : Module ℝ F
cY : Convex ℝ Y
inst✝² : DenselyOrdered β
inst✝¹ : IsTopologicalAddGroup F
inst✝ : ContinuousSMul ℝ F
s✝ : Set F
t : β
b : F
s : Set F
a✝ : b ∉ s
hs : s.Finite
X : Set E
ne_X : X.Nonempty
kX : IsCompact X
hfy : ∀ y ∈ Y, LowerSemicontinuousOn (fun x ↦ f x y) X
hfy' : ∀ y ∈ Y, QuasiconvexOn ℝ X fun x ↦ f x y
hfx : ∀ x ∈ X, UpperSemicontinuousOn (fun y ↦ f x y) Y
hfx' : ∀ x ∈ X, QuasiconcaveOn ℝ Y fun y ↦ f x y
cX : Convex ℝ X
hsY : insert b s ⊆ Y
ht : ∀ x ∈ X, ∃ y ∈ insert b s, t < f x y
hb : b ∈ Y
X' : Set E := X ∩ (fun x ↦ f x b) ⁻¹' Iic t
hrec : s ⊆ Y → (∀ x ∈ X', ∃ y ∈ s, t < f x y) → ∃ y0 ∈ Y, ∀ x ∈ X', t < f x y0
X'_ne : X'.Nonempty
hX'X : X' ⊆ X
kX' : IsCompact X'
cX' : Convex ℝ X'
x : E
hx : x ∈ X'
⊢ ∃ y ∈ s, t < f x y
Module: Mathlib.Topology.UniformSpace.Uniformizable

Goal state before:
pp: case refine_1
X : Type u_1
inst✝ : UniformSpace X
u : Set X
x : X
uc s : SetRel X X
huc : IsOpen uc
ucu : uc ∈ 𝓤 X
hn : ball x (s ○ uc ○ s) ⊆ u
hs : s ∈ 𝓤 X
ds : SetRel X X
hdsu : ds ∈ 𝓤 X
hdso : IsOpen ds
hdsd : ds ○ ds ⊆ s
ho : IsOpen (ds ○ uc ○ ds)
hsub : ball x (ds ○ (ds ○ uc ○ ds) ○ ds) ⊆ u
this : ds.IsRefl
⊢ closure (ball x uc) ⊆ ball x (ds ○ uc ○ ds)

Tactic:
refine closure_ball_subset.trans (ball_mono ?_ x)
    rw [closure_eq_inter_uniformity]
    exact iInter₂_subset_of_subset ds hdsu (by simp [comp_assoc])

Goal state after:
Module: Mathlib.Topology.UniformSpace.Uniformizable

Goal state before:
pp: case refine_2
X : Type u_1
inst✝ : UniformSpace X
u : Set X
x : X
uc s : SetRel X X
huc : IsOpen uc
ucu : uc ∈ 𝓤 X
hn : ball x (s ○ uc ○ s) ⊆ u
hs : s ∈ 𝓤 X
ds : SetRel X X
hdsu : ds ∈ 𝓤 X
hdso : IsOpen ds
hdsd : ds ○ ds ⊆ s
ho : IsOpen (ds ○ uc ○ ds)
hsub : ball x (ds ○ (ds ○ uc ○ ds) ○ ds) ⊆ u
this : ds.IsRefl
⊢ closure (ball x (ds ○ uc ○ ds)) ⊆ ball x (ds ○ (ds ○ uc ○ ds) ○ ds)

Tactic:
refine closure_ball_subset.trans (ball_mono ?_ x)
    rw [closure_eq_inter_uniformity]
    exact iInter₂_subset_of_subset ds hdsu (by simp [comp_assoc])

Goal state after:
Module: Mathlib.Topology.Sion

Goal state before:
pp: E : Type u_1
F : Type u_2
β : Type u_3
inst✝¹¹ : CompleteLinearOrder β
inst✝¹⁰ : DenselyOrdered β
X : Set E
Y : Set F
f : E → F → β
inst✝⁹ : TopologicalSpace E
inst✝⁸ : AddCommGroup E
inst✝⁷ : Module ℝ E
inst✝⁶ : IsTopologicalAddGroup E
inst✝⁵ : ContinuousSMul ℝ E
ne_X : X.Nonempty
cX : Convex ℝ X
kX : IsCompact X
hfy : ∀ y ∈ Y, LowerSemicontinuousOn (fun x ↦ f x y) X
hfy' : ∀ y ∈ Y, QuasiconvexOn ℝ X fun x ↦ f x y
inst✝⁴ : TopologicalSpace F
inst✝³ : AddCommGroup F
inst✝² : Module ℝ F
inst✝¹ : IsTopologicalAddGroup F
inst✝ : ContinuousSMul ℝ F
cY : Convex ℝ Y
ne_Y : Y.Nonempty
kY : IsCompact Y
hfx : ∀ x ∈ X, UpperSemicontinuousOn (fun y ↦ f x y) Y
hfx' : ∀ x ∈ X, QuasiconcaveOn ℝ Y fun y ↦ f x y
hlsc : LowerSemicontinuousOn (fun x ↦ ⨆ y ∈ Y, f x y) X
husc : UpperSemicontinuousOn (fun y ↦ ⨅ x ∈ X, f x y) Y
⊢ ∃ a ∈ X, ∃ b ∈ Y, IsSaddlePointOn X Y f a b

Tactic:
obtain ⟨a, ha, ha'⟩ := LowerSemicontinuousOn.exists_isMinOn
   ne_X kX hlsc

Goal state after:
E : Type u_1
F : Type u_2
β : Type u_3
inst✝¹¹ : CompleteLinearOrder β
inst✝¹⁰ : DenselyOrdered β
X : Set E
Y : Set F
f : E → F → β
inst✝⁹ : TopologicalSpace E
inst✝⁸ : AddCommGroup E
inst✝⁷ : Module ℝ E
inst✝⁶ : IsTopologicalAddGroup E
inst✝⁵ : ContinuousSMul ℝ E
ne_X : X.Nonempty
cX : Convex ℝ X
kX : IsCompact X
hfy : ∀ y ∈ Y, LowerSemicontinuousOn (fun x ↦ f x y) X
hfy' : ∀ y ∈ Y, QuasiconvexOn ℝ X fun x ↦ f x y
inst✝⁴ : TopologicalSpace F
inst✝³ : AddCommGroup F
inst✝² : Module ℝ F
inst✝¹ : IsTopologicalAddGroup F
inst✝ : ContinuousSMul ℝ F
cY : Convex ℝ Y
ne_Y : Y.Nonempty
kY : IsCompact Y
hfx : ∀ x ∈ X, UpperSemicontinuousOn (fun y ↦ f x y) Y
hfx' : ∀ x ∈ X, QuasiconcaveOn ℝ Y fun y ↦ f x y
hlsc : LowerSemicontinuousOn (fun x ↦ ⨆ y ∈ Y, f x y) X
husc : UpperSemicontinuousOn (fun y ↦ ⨅ x ∈ X, f x y) Y
a : E
ha : a ∈ X
ha' : IsMinOn (fun x ↦ ⨆ y ∈ Y, f x y) X a
⊢ ∃ a ∈ X, ∃ b ∈ Y, IsSaddlePointOn X Y f a b