-- Mathlib/Analysis/Normed/Module/FiniteDimension.lean
theorem continuousOn_clm_apply {X : Type*} [TopologicalSpace X] [FiniteDimensional 𝕜 E]
    {f : X → E →L[𝕜] F} {s : Set X} : ContinuousOn f s ↔ ∀ y, ContinuousOn (fun x => f x y) s

Proof state:
𝕜 : Type u
inst✝⁷ : NontriviallyNormedField 𝕜
E : Type v
inst✝⁶ : NormedAddCommGroup E
inst✝⁵ : NormedSpace 𝕜 E
F : Type w
inst✝⁴ : NormedAddCommGroup F
inst✝³ : NormedSpace 𝕜 F
inst✝² : CompleteSpace 𝕜
X : Type u_1
inst✝¹ : TopologicalSpace X
inst✝ : FiniteDimensional 𝕜 E
f : X → E →L[𝕜] F
s : Set X
h : ∀ (y : E), ContinuousOn (fun x => (f x) y) s
d : ℕ := finrank 𝕜 E
hd : d = finrank 𝕜 (Fin d → 𝕜)
e₁ : E ≃L[𝕜] Fin d → 𝕜 := ContinuousLinearEquiv.ofFinrankEq hd
e₂ : (E →L[𝕜] F) ≃L[𝕜] Fin d → F := (e₁.arrowCongr 1).trans (ContinuousLinearEquiv.piRing (Fin d))
⊢ ContinuousOn ((⇑e₂.symm ∘ ⇑e₂) ∘ f) s

Tactic:
exact e₂.symm.continuous.comp_continuousOn (continuousOn_pi.mpr fun i => h _)

Result:
no goals

Proof state:
𝕜 : Type u
inst✝⁷ : NontriviallyNormedField 𝕜
E : Type v
inst✝⁶ : NormedAddCommGroup E
inst✝⁵ : NormedSpace 𝕜 E
F : Type w
inst✝⁴ : NormedAddCommGroup F
inst✝³ : NormedSpace 𝕜 F
inst✝² : CompleteSpace 𝕜
X : Type u_1
inst✝¹ : TopologicalSpace X
inst✝ : FiniteDimensional 𝕜 E
f : X → E →L[𝕜] F
s : Set X
h : ∀ (y : E), ContinuousOn (fun x => (f x) y) s
d : ℕ := finrank 𝕜 E
hd : d = finrank 𝕜 (Fin d → 𝕜)
⊢ ContinuousOn f s

Tactic:
let e₁ : E ≃L[𝕜] Fin d → 𝕜 := ContinuousLinearEquiv.ofFinrankEq hd

Result:
𝕜 : Type u
inst✝⁷ : NontriviallyNormedField 𝕜
E : Type v
inst✝⁶ : NormedAddCommGroup E
inst✝⁵ : NormedSpace 𝕜 E
F : Type w
inst✝⁴ : NormedAddCommGroup F
inst✝³ : NormedSpace 𝕜 F
inst✝² : CompleteSpace 𝕜
X : Type u_1
inst✝¹ : TopologicalSpace X
inst✝ : FiniteDimensional 𝕜 E
f : X → E →L[𝕜] F
s : Set X
h : ∀ (y : E), ContinuousOn (fun x => (f x) y) s
d : ℕ := finrank 𝕜 E
hd : d = finrank 𝕜 (Fin d → 𝕜)
e₁ : E ≃L[𝕜] Fin d → 𝕜 := ContinuousLinearEquiv.ofFinrankEq hd
⊢ ContinuousOn f s

Proof state:
𝕜 : Type u
inst✝⁷ : NontriviallyNormedField 𝕜
E : Type v
inst✝⁶ : NormedAddCommGroup E
inst✝⁵ : NormedSpace 𝕜 E
F : Type w
inst✝⁴ : NormedAddCommGroup F
inst✝³ : NormedSpace 𝕜 F
inst✝² : CompleteSpace 𝕜
X : Type u_1
inst✝¹ : TopologicalSpace X
inst✝ : FiniteDimensional 𝕜 E
f : X → E →L[𝕜] F
s : Set X
h : ∀ (y : E), ContinuousOn (fun x => (f x) y) s
d : ℕ := finrank 𝕜 E
hd : d = finrank 𝕜 (Fin d → 𝕜)
e₁ : E ≃L[𝕜] Fin d → 𝕜 := ContinuousLinearEquiv.ofFinrankEq hd
⊢ ContinuousOn f s

Tactic:
let e₂ : (E →L[𝕜] F) ≃L[𝕜] Fin d → F :=
  (e₁.arrowCongr (1 : F ≃L[𝕜] F)).trans (ContinuousLinearEquiv.piRing (Fin d))

Result:
𝕜 : Type u
inst✝⁷ : NontriviallyNormedField 𝕜
E : Type v
inst✝⁶ : NormedAddCommGroup E
inst✝⁵ : NormedSpace 𝕜 E
F : Type w
inst✝⁴ : NormedAddCommGroup F
inst✝³ : NormedSpace 𝕜 F
inst✝² : CompleteSpace 𝕜
X : Type u_1
inst✝¹ : TopologicalSpace X
inst✝ : FiniteDimensional 𝕜 E
f : X → E →L[𝕜] F
s : Set X
h : ∀ (y : E), ContinuousOn (fun x => (f x) y) s
d : ℕ := finrank 𝕜 E
hd : d = finrank 𝕜 (Fin d → 𝕜)
e₁ : E ≃L[𝕜] Fin d → 𝕜 := ContinuousLinearEquiv.ofFinrankEq hd
e₂ : (E →L[𝕜] F) ≃L[𝕜] Fin d → F := (e₁.arrowCongr 1).trans (ContinuousLinearEquiv.piRing (Fin d))
⊢ ContinuousOn f s

Proof state:
𝕜 : Type u
inst✝⁷ : NontriviallyNormedField 𝕜
E : Type v
inst✝⁶ : NormedAddCommGroup E
inst✝⁵ : NormedSpace 𝕜 E
F : Type w
inst✝⁴ : NormedAddCommGroup F
inst✝³ : NormedSpace 𝕜 F
inst✝² : CompleteSpace 𝕜
X : Type u_1
inst✝¹ : TopologicalSpace X
inst✝ : FiniteDimensional 𝕜 E
f : X → E →L[𝕜] F
s : Set X
h : ∀ (y : E), ContinuousOn (fun x => (f x) y) s
⊢ ContinuousOn f s

Tactic:
let d := finrank 𝕜 E

Result:
𝕜 : Type u
inst✝⁷ : NontriviallyNormedField 𝕜
E : Type v
inst✝⁶ : NormedAddCommGroup E
inst✝⁵ : NormedSpace 𝕜 E
F : Type w
inst✝⁴ : NormedAddCommGroup F
inst✝³ : NormedSpace 𝕜 F
inst✝² : CompleteSpace 𝕜
X : Type u_1
inst✝¹ : TopologicalSpace X
inst✝ : FiniteDimensional 𝕜 E
f : X → E →L[𝕜] F
s : Set X
h : ∀ (y : E), ContinuousOn (fun x => (f x) y) s
d : ℕ := finrank 𝕜 E
⊢ ContinuousOn f s

Proof state:
𝕜 : Type u
inst✝⁷ : NontriviallyNormedField 𝕜
E : Type v
inst✝⁶ : NormedAddCommGroup E
inst✝⁵ : NormedSpace 𝕜 E
F : Type w
inst✝⁴ : NormedAddCommGroup F
inst✝³ : NormedSpace 𝕜 F
inst✝² : CompleteSpace 𝕜
X : Type u_1
inst✝¹ : TopologicalSpace X
inst✝ : FiniteDimensional 𝕜 E
f : X → E →L[𝕜] F
s : Set X
h : ∀ (y : E), ContinuousOn (fun x => (f x) y) s
d : ℕ := finrank 𝕜 E
⊢ ContinuousOn f s

Tactic:
have hd : d = finrank 𝕜 (Fin d → 𝕜) := (finrank_fin_fun 𝕜).symm

Result:
𝕜 : Type u
inst✝⁷ : NontriviallyNormedField 𝕜
E : Type v
inst✝⁶ : NormedAddCommGroup E
inst✝⁵ : NormedSpace 𝕜 E
F : Type w
inst✝⁴ : NormedAddCommGroup F
inst✝³ : NormedSpace 𝕜 F
inst✝² : CompleteSpace 𝕜
X : Type u_1
inst✝¹ : TopologicalSpace X
inst✝ : FiniteDimensional 𝕜 E
f : X → E →L[𝕜] F
s : Set X
h : ∀ (y : E), ContinuousOn (fun x => (f x) y) s
d : ℕ := finrank 𝕜 E
hd : d = finrank 𝕜 (Fin d → 𝕜)
⊢ ContinuousOn f s

Proof state:
𝕜 : Type u
inst✝⁷ : NontriviallyNormedField 𝕜
E : Type v
inst✝⁶ : NormedAddCommGroup E
inst✝⁵ : NormedSpace 𝕜 E
F : Type w
inst✝⁴ : NormedAddCommGroup F
inst✝³ : NormedSpace 𝕜 F
inst✝² : CompleteSpace 𝕜
X : Type u_1
inst✝¹ : TopologicalSpace X
inst✝ : FiniteDimensional 𝕜 E
f : X → E →L[𝕜] F
s : Set X
⊢ ContinuousOn f s ↔ ∀ (y : E), ContinuousOn (fun x => (f x) y) s

Tactic:
refine ⟨fun h y => (ContinuousLinearMap.apply 𝕜 F y).continuous.comp_continuousOn h, fun h => ?_⟩

Result:
𝕜 : Type u
inst✝⁷ : NontriviallyNormedField 𝕜
E : Type v
inst✝⁶ : NormedAddCommGroup E
inst✝⁵ : NormedSpace 𝕜 E
F : Type w
inst✝⁴ : NormedAddCommGroup F
inst✝³ : NormedSpace 𝕜 F
inst✝² : CompleteSpace 𝕜
X : Type u_1
inst✝¹ : TopologicalSpace X
inst✝ : FiniteDimensional 𝕜 E
f : X → E →L[𝕜] F
s : Set X
h : ∀ (y : E), ContinuousOn (fun x => (f x) y) s
⊢ ContinuousOn f s

Proof state:
𝕜 : Type u
inst✝⁷ : NontriviallyNormedField 𝕜
E : Type v
inst✝⁶ : NormedAddCommGroup E
inst✝⁵ : NormedSpace 𝕜 E
F : Type w
inst✝⁴ : NormedAddCommGroup F
inst✝³ : NormedSpace 𝕜 F
inst✝² : CompleteSpace 𝕜
X : Type u_1
inst✝¹ : TopologicalSpace X
inst✝ : FiniteDimensional 𝕜 E
f : X → E →L[𝕜] F
s : Set X
h : ∀ (y : E), ContinuousOn (fun x => (f x) y) s
d : ℕ := finrank 𝕜 E
hd : d = finrank 𝕜 (Fin d → 𝕜)
e₁ : E ≃L[𝕜] Fin d → 𝕜 := ContinuousLinearEquiv.ofFinrankEq hd
e₂ : (E →L[𝕜] F) ≃L[𝕜] Fin d → F := (e₁.arrowCongr 1).trans (ContinuousLinearEquiv.piRing (Fin d))
⊢ ContinuousOn f s

Tactic:
rw [← f.id_comp, ← e₂.symm_comp_self]

Result:
𝕜 : Type u
inst✝⁷ : NontriviallyNormedField 𝕜
E : Type v
inst✝⁶ : NormedAddCommGroup E
inst✝⁵ : NormedSpace 𝕜 E
F : Type w
inst✝⁴ : NormedAddCommGroup F
inst✝³ : NormedSpace 𝕜 F
inst✝² : CompleteSpace 𝕜
X : Type u_1
inst✝¹ : TopologicalSpace X
inst✝ : FiniteDimensional 𝕜 E
f : X → E →L[𝕜] F
s : Set X
h : ∀ (y : E), ContinuousOn (fun x => (f x) y) s
d : ℕ := finrank 𝕜 E
hd : d = finrank 𝕜 (Fin d → 𝕜)
e₁ : E ≃L[𝕜] Fin d → 𝕜 := ContinuousLinearEquiv.ofFinrankEq hd
e₂ : (E →L[𝕜] F) ≃L[𝕜] Fin d → F := (e₁.arrowCongr 1).trans (ContinuousLinearEquiv.piRing (Fin d))
⊢ ContinuousOn ((⇑e₂.symm ∘ ⇑e₂) ∘ f) s