-- Mathlib/Analysis/NormedSpace/OperatorNorm/NormedSpace.lean
theorem NormedSpace.equicontinuous_TFAE : List.TFAE
    [ EquicontinuousAt ((↑) ∘ f) 0,
      Equicontinuous ((↑) ∘ f),
      UniformEquicontinuous ((↑) ∘ f),
      ∃ C, ∀ i x, ‖f i x‖ ≤ C * ‖x‖,
      ∃ C ≥ 0, ∀ i x, ‖f i x‖ ≤ C * ‖x‖,
      ∃ C, ∀ i, ‖f i‖ ≤ C,
      ∃ C ≥ 0, ∀ i, ‖f i‖ ≤ C,
      BddAbove (Set.range (‖f ·‖)),
      (⨆ i, (‖f i‖₊ : ENNReal)) < ⊤ ]

Proof state:
case mpr
𝕜 : Type u_1
𝕜₂ : Type u_2
E : Type u_4
F : Type u_5
ι : Type u_8
inst✝⁶ : NontriviallyNormedField 𝕜
inst✝⁵ : NontriviallyNormedField 𝕜₂
σ₁₂ : 𝕜 →+* 𝕜₂
inst✝⁴ : RingHomIsometric σ₁₂
inst✝³ : SeminormedAddCommGroup E
inst✝² : SeminormedAddCommGroup F
inst✝¹ : NormedSpace 𝕜 E
inst✝ : NormedSpace 𝕜₂ F
f : ι → E →SL[σ₁₂] F
tfae_1_to_3 : EquicontinuousAt (DFunLike.coe ∘ f) 0 → UniformEquicontinuous (DFunLike.coe ∘ f)
tfae_3_to_2 : UniformEquicontinuous (DFunLike.coe ∘ f) → Equicontinuous (DFunLike.coe ∘ f)
tfae_2_to_1 : Equicontinuous (DFunLike.coe ∘ f) → EquicontinuousAt (DFunLike.coe ∘ f) 0
tfae_4_iff_5 : (∃ C, ∀ (i : ι) (x : E), ‖(f i) x‖ ≤ C * ‖x‖) ↔ ∃ C ≥ 0, ∀ (i : ι) (x : E), ‖(f i) x‖ ≤ C * ‖x‖
tfae_5_iff_7 : (∃ C ≥ 0, ∀ (i : ι) (x : E), ‖(f i) x‖ ≤ C * ‖x‖) ↔ ∃ C ≥ 0, ∀ (i : ι), ‖f i‖ ≤ C
tfae_7_iff_8 : (∃ C ≥ 0, ∀ (i : ι), ‖f i‖ ≤ C) ↔ BddAbove (Set.range fun x => ‖f x‖)
tfae_6_iff_8 : (∃ C, ∀ (i : ι), ‖f i‖ ≤ C) ↔ BddAbove (Set.range fun x => ‖f x‖)
tfae_8_iff_9 : BddAbove (Set.range fun x => ‖f x‖) ↔ ⨆ i, ↑‖f i‖₊ < ⊤
C : ℝ
hC : ∀ (i : ι) (x : E), ‖(f i) x‖ ≤ C * ‖x‖
⊢ ∃ p, Continuous ⇑p ∧ ∀ (k : ι), (normSeminorm 𝕜₂ F).comp ↑(f k) ≤ p

Tactic:
refine ⟨C.toNNReal • normSeminorm 𝕜 E,
  ((norm_withSeminorms 𝕜 E).continuous_seminorm 0).const_smul C.toNNReal, fun i x ↦ ?_⟩

Result:
case mpr
𝕜 : Type u_1
𝕜₂ : Type u_2
E : Type u_4
F : Type u_5
ι : Type u_8
inst✝⁶ : NontriviallyNormedField 𝕜
inst✝⁵ : NontriviallyNormedField 𝕜₂
σ₁₂ : 𝕜 →+* 𝕜₂
inst✝⁴ : RingHomIsometric σ₁₂
inst✝³ : SeminormedAddCommGroup E
inst✝² : SeminormedAddCommGroup F
inst✝¹ : NormedSpace 𝕜 E
inst✝ : NormedSpace 𝕜₂ F
f : ι → E →SL[σ₁₂] F
tfae_1_to_3 : EquicontinuousAt (DFunLike.coe ∘ f) 0 → UniformEquicontinuous (DFunLike.coe ∘ f)
tfae_3_to_2 : UniformEquicontinuous (DFunLike.coe ∘ f) → Equicontinuous (DFunLike.coe ∘ f)
tfae_2_to_1 : Equicontinuous (DFunLike.coe ∘ f) → EquicontinuousAt (DFunLike.coe ∘ f) 0
tfae_4_iff_5 : (∃ C, ∀ (i : ι) (x : E), ‖(f i) x‖ ≤ C * ‖x‖) ↔ ∃ C ≥ 0, ∀ (i : ι) (x : E), ‖(f i) x‖ ≤ C * ‖x‖
tfae_5_iff_7 : (∃ C ≥ 0, ∀ (i : ι) (x : E), ‖(f i) x‖ ≤ C * ‖x‖) ↔ ∃ C ≥ 0, ∀ (i : ι), ‖f i‖ ≤ C
tfae_7_iff_8 : (∃ C ≥ 0, ∀ (i : ι), ‖f i‖ ≤ C) ↔ BddAbove (Set.range fun x => ‖f x‖)
tfae_6_iff_8 : (∃ C, ∀ (i : ι), ‖f i‖ ≤ C) ↔ BddAbove (Set.range fun x => ‖f x‖)
tfae_8_iff_9 : BddAbove (Set.range fun x => ‖f x‖) ↔ ⨆ i, ↑‖f i‖₊ < ⊤
C : ℝ
hC : ∀ (i : ι) (x : E), ‖(f i) x‖ ≤ C * ‖x‖
i : ι
x : E
⊢ (fun f => ⇑f) ((normSeminorm 𝕜₂ F).comp ↑(f i)) x ≤ (fun f => ⇑f) (C.toNNReal • normSeminorm 𝕜 E) x

Proof state:
case mpr
𝕜 : Type u_1
𝕜₂ : Type u_2
E : Type u_4
F : Type u_5
ι : Type u_8
inst✝⁶ : NontriviallyNormedField 𝕜
inst✝⁵ : NontriviallyNormedField 𝕜₂
σ₁₂ : 𝕜 →+* 𝕜₂
inst✝⁴ : RingHomIsometric σ₁₂
inst✝³ : SeminormedAddCommGroup E
inst✝² : SeminormedAddCommGroup F
inst✝¹ : NormedSpace 𝕜 E
inst✝ : NormedSpace 𝕜₂ F
f : ι → E →SL[σ₁₂] F
tfae_1_to_3 : EquicontinuousAt (DFunLike.coe ∘ f) 0 → UniformEquicontinuous (DFunLike.coe ∘ f)
tfae_3_to_2 : UniformEquicontinuous (DFunLike.coe ∘ f) → Equicontinuous (DFunLike.coe ∘ f)
tfae_2_to_1 : Equicontinuous (DFunLike.coe ∘ f) → EquicontinuousAt (DFunLike.coe ∘ f) 0
tfae_4_iff_5 : (∃ C, ∀ (i : ι) (x : E), ‖(f i) x‖ ≤ C * ‖x‖) ↔ ∃ C ≥ 0, ∀ (i : ι) (x : E), ‖(f i) x‖ ≤ C * ‖x‖
tfae_5_iff_7 : (∃ C ≥ 0, ∀ (i : ι) (x : E), ‖(f i) x‖ ≤ C * ‖x‖) ↔ ∃ C ≥ 0, ∀ (i : ι), ‖f i‖ ≤ C
tfae_7_iff_8 : (∃ C ≥ 0, ∀ (i : ι), ‖f i‖ ≤ C) ↔ BddAbove (Set.range fun x => ‖f x‖)
tfae_6_iff_8 : (∃ C, ∀ (i : ι), ‖f i‖ ≤ C) ↔ BddAbove (Set.range fun x => ‖f x‖)
tfae_8_iff_9 : BddAbove (Set.range fun x => ‖f x‖) ↔ ⨆ i, ↑‖f i‖₊ < ⊤
⊢ (∃ C, ∀ (i : ι) (x : E), ‖(f i) x‖ ≤ C * ‖x‖) → ∃ p, Continuous ⇑p ∧ ∀ (k : ι), (normSeminorm 𝕜₂ F).comp ↑(f k) ≤ p

Tactic:
intro ⟨C, hC⟩

Result:
case mpr
𝕜 : Type u_1
𝕜₂ : Type u_2
E : Type u_4
F : Type u_5
ι : Type u_8
inst✝⁶ : NontriviallyNormedField 𝕜
inst✝⁵ : NontriviallyNormedField 𝕜₂
σ₁₂ : 𝕜 →+* 𝕜₂
inst✝⁴ : RingHomIsometric σ₁₂
inst✝³ : SeminormedAddCommGroup E
inst✝² : SeminormedAddCommGroup F
inst✝¹ : NormedSpace 𝕜 E
inst✝ : NormedSpace 𝕜₂ F
f : ι → E →SL[σ₁₂] F
tfae_1_to_3 : EquicontinuousAt (DFunLike.coe ∘ f) 0 → UniformEquicontinuous (DFunLike.coe ∘ f)
tfae_3_to_2 : UniformEquicontinuous (DFunLike.coe ∘ f) → Equicontinuous (DFunLike.coe ∘ f)
tfae_2_to_1 : Equicontinuous (DFunLike.coe ∘ f) → EquicontinuousAt (DFunLike.coe ∘ f) 0
tfae_4_iff_5 : (∃ C, ∀ (i : ι) (x : E), ‖(f i) x‖ ≤ C * ‖x‖) ↔ ∃ C ≥ 0, ∀ (i : ι) (x : E), ‖(f i) x‖ ≤ C * ‖x‖
tfae_5_iff_7 : (∃ C ≥ 0, ∀ (i : ι) (x : E), ‖(f i) x‖ ≤ C * ‖x‖) ↔ ∃ C ≥ 0, ∀ (i : ι), ‖f i‖ ≤ C
tfae_7_iff_8 : (∃ C ≥ 0, ∀ (i : ι), ‖f i‖ ≤ C) ↔ BddAbove (Set.range fun x => ‖f x‖)
tfae_6_iff_8 : (∃ C, ∀ (i : ι), ‖f i‖ ≤ C) ↔ BddAbove (Set.range fun x => ‖f x‖)
tfae_8_iff_9 : BddAbove (Set.range fun x => ‖f x‖) ↔ ⨆ i, ↑‖f i‖₊ < ⊤
C : ℝ
hC : ∀ (i : ι) (x : E), ‖(f i) x‖ ≤ C * ‖x‖
⊢ ∃ p, Continuous ⇑p ∧ ∀ (k : ι), (normSeminorm 𝕜₂ F).comp ↑(f k) ≤ p