-- Mathlib/Analysis/Normed/Operator/Compact.lean
theorem isClosed_setOf_isCompactOperator {𝕜₁ 𝕜₂ : Type*} [NontriviallyNormedField 𝕜₁]
    [NormedField 𝕜₂] {σ₁₂ : 𝕜₁ →+* 𝕜₂} {M₁ M₂ : Type*} [SeminormedAddCommGroup M₁]
    [AddCommGroup M₂] [NormedSpace 𝕜₁ M₁] [Module 𝕜₂ M₂] [UniformSpace M₂] [UniformAddGroup M₂]
    [ContinuousConstSMul 𝕜₂ M₂] [T2Space M₂] [CompleteSpace M₂] :
    IsClosed { f : M₁ →SL[σ₁₂] M₂ | IsCompactOperator f }

Proof state:
𝕜₁ : Type u_1
𝕜₂ : Type u_2
inst✝¹⁰ : NontriviallyNormedField 𝕜₁
inst✝⁹ : NormedField 𝕜₂
σ₁₂ : 𝕜₁ →+* 𝕜₂
M₁ : Type u_3
M₂ : Type u_4
inst✝⁸ : SeminormedAddCommGroup M₁
inst✝⁷ : AddCommGroup M₂
inst✝⁶ : NormedSpace 𝕜₁ M₁
inst✝⁵ : Module 𝕜₂ M₂
inst✝⁴ : UniformSpace M₂
inst✝³ : UniformAddGroup M₂
inst✝² : ContinuousConstSMul 𝕜₂ M₂
inst✝¹ : T2Space M₂
inst✝ : CompleteSpace M₂
u : M₁ →SL[σ₁₂] M₂
hu : ∀ U ∈ 𝓝 0, ∃ y ∈ {f | IsCompactOperator ⇑f}, y - u ∈ U
⊢ ∀ U ∈ 𝓝 0, ∃ t, t.Finite ∧ ⇑u '' closedBall 0 1 ⊆ ⋃ y ∈ t, y +ᵥ U

Tactic:
intro U hU

Result:
𝕜₁ : Type u_1
𝕜₂ : Type u_2
inst✝¹⁰ : NontriviallyNormedField 𝕜₁
inst✝⁹ : NormedField 𝕜₂
σ₁₂ : 𝕜₁ →+* 𝕜₂
M₁ : Type u_3
M₂ : Type u_4
inst✝⁸ : SeminormedAddCommGroup M₁
inst✝⁷ : AddCommGroup M₂
inst✝⁶ : NormedSpace 𝕜₁ M₁
inst✝⁵ : Module 𝕜₂ M₂
inst✝⁴ : UniformSpace M₂
inst✝³ : UniformAddGroup M₂
inst✝² : ContinuousConstSMul 𝕜₂ M₂
inst✝¹ : T2Space M₂
inst✝ : CompleteSpace M₂
u : M₁ →SL[σ₁₂] M₂
hu : ∀ U ∈ 𝓝 0, ∃ y ∈ {f | IsCompactOperator ⇑f}, y - u ∈ U
U : Set M₂
hU : U ∈ 𝓝 0
⊢ ∃ t, t.Finite ∧ ⇑u '' closedBall 0 1 ⊆ ⋃ y ∈ t, y +ᵥ U

Proof state:
𝕜₁ : Type u_1
𝕜₂ : Type u_2
inst✝¹⁰ : NontriviallyNormedField 𝕜₁
inst✝⁹ : NormedField 𝕜₂
σ₁₂ : 𝕜₁ →+* 𝕜₂
M₁ : Type u_3
M₂ : Type u_4
inst✝⁸ : SeminormedAddCommGroup M₁
inst✝⁷ : AddCommGroup M₂
inst✝⁶ : NormedSpace 𝕜₁ M₁
inst✝⁵ : Module 𝕜₂ M₂
inst✝⁴ : UniformSpace M₂
inst✝³ : UniformAddGroup M₂
inst✝² : ContinuousConstSMul 𝕜₂ M₂
inst✝¹ : T2Space M₂
inst✝ : CompleteSpace M₂
u : M₁ →SL[σ₁₂] M₂
hu : ∀ U ∈ 𝓝 0, ∃ y ∈ {f | IsCompactOperator ⇑f}, y - u ∈ U
⊢ u ∈ {f | IsCompactOperator ⇑f}

Tactic:
suffices TotallyBounded (u '' Metric.closedBall 0 1) by
  change IsCompactOperator (u : M₁ →ₛₗ[σ₁₂] M₂)
  rw [isCompactOperator_iff_isCompact_closure_image_closedBall (u : M₁ →ₛₗ[σ₁₂] M₂) zero_lt_one]
  exact isCompact_of_totallyBounded_isClosed this.closure isClosed_closure

Result:
𝕜₁ : Type u_1
𝕜₂ : Type u_2
inst✝¹⁰ : NontriviallyNormedField 𝕜₁
inst✝⁹ : NormedField 𝕜₂
σ₁₂ : 𝕜₁ →+* 𝕜₂
M₁ : Type u_3
M₂ : Type u_4
inst✝⁸ : SeminormedAddCommGroup M₁
inst✝⁷ : AddCommGroup M₂
inst✝⁶ : NormedSpace 𝕜₁ M₁
inst✝⁵ : Module 𝕜₂ M₂
inst✝⁴ : UniformSpace M₂
inst✝³ : UniformAddGroup M₂
inst✝² : ContinuousConstSMul 𝕜₂ M₂
inst✝¹ : T2Space M₂
inst✝ : CompleteSpace M₂
u : M₁ →SL[σ₁₂] M₂
hu : ∀ U ∈ 𝓝 0, ∃ y ∈ {f | IsCompactOperator ⇑f}, y - u ∈ U
⊢ TotallyBounded (⇑u '' closedBall 0 1)

Proof state:
case intro.intro.intro.intro
𝕜₁ : Type u_1
𝕜₂ : Type u_2
inst✝¹⁰ : NontriviallyNormedField 𝕜₁
inst✝⁹ : NormedField 𝕜₂
σ₁₂ : 𝕜₁ →+* 𝕜₂
M₁ : Type u_3
M₂ : Type u_4
inst✝⁸ : SeminormedAddCommGroup M₁
inst✝⁷ : AddCommGroup M₂
inst✝⁶ : NormedSpace 𝕜₁ M₁
inst✝⁵ : Module 𝕜₂ M₂
inst✝⁴ : UniformSpace M₂
inst✝³ : UniformAddGroup M₂
inst✝² : ContinuousConstSMul 𝕜₂ M₂
inst✝¹ : T2Space M₂
inst✝ : CompleteSpace M₂
u : M₁ →SL[σ₁₂] M₂
hu : ∀ U ∈ 𝓝 0, ∃ y ∈ {f | IsCompactOperator ⇑f}, y - u ∈ U
U : Set M₂
hU : U ∈ 𝓝 0
V : Set M₂
hV : V ∈ 𝓝 0
hVU : ∀ v ∈ V, ∀ w ∈ V, v + w ∈ U
SV : Set M₁ × Set M₂ := (closedBall 0 1, -V)
v : M₁ →SL[σ₁₂] M₂
hv : v ∈ {f | IsCompactOperator ⇑f}
huv : v - u ∈ {f | ∀ x ∈ SV.1, f x ∈ SV.2}
⊢ ∃ t, t.Finite ∧ ⇑u '' closedBall 0 1 ⊆ ⋃ y ∈ t, y +ᵥ U

Tactic:
rcases totallyBounded_iff_subset_finite_iUnion_nhds_zero.mp
    (hv.isCompact_closure_image_closedBall 1).totallyBounded V hV with
  ⟨T, hT, hTv⟩

Result:
case intro.intro.intro.intro.intro.intro
𝕜₁ : Type u_1
𝕜₂ : Type u_2
inst✝¹⁰ : NontriviallyNormedField 𝕜₁
inst✝⁹ : NormedField 𝕜₂
σ₁₂ : 𝕜₁ →+* 𝕜₂
M₁ : Type u_3
M₂ : Type u_4
inst✝⁸ : SeminormedAddCommGroup M₁
inst✝⁷ : AddCommGroup M₂
inst✝⁶ : NormedSpace 𝕜₁ M₁
inst✝⁵ : Module 𝕜₂ M₂
inst✝⁴ : UniformSpace M₂
inst✝³ : UniformAddGroup M₂
inst✝² : ContinuousConstSMul 𝕜₂ M₂
inst✝¹ : T2Space M₂
inst✝ : CompleteSpace M₂
u : M₁ →SL[σ₁₂] M₂
hu : ∀ U ∈ 𝓝 0, ∃ y ∈ {f | IsCompactOperator ⇑f}, y - u ∈ U
U : Set M₂
hU : U ∈ 𝓝 0
V : Set M₂
hV : V ∈ 𝓝 0
hVU : ∀ v ∈ V, ∀ w ∈ V, v + w ∈ U
SV : Set M₁ × Set M₂ := (closedBall 0 1, -V)
v : M₁ →SL[σ₁₂] M₂
hv : v ∈ {f | IsCompactOperator ⇑f}
huv : v - u ∈ {f | ∀ x ∈ SV.1, f x ∈ SV.2}
T : Set M₂
hT : T.Finite
hTv : closure (⇑↑v '' closedBall 0 1) ⊆ ⋃ y ∈ T, y +ᵥ V
⊢ ∃ t, t.Finite ∧ ⇑u '' closedBall 0 1 ⊆ ⋃ y ∈ t, y +ᵥ U

Proof state:
case intro.intro.intro.intro.intro.intro.intro.intro
𝕜₁ : Type u_1
𝕜₂ : Type u_2
inst✝¹⁰ : NontriviallyNormedField 𝕜₁
inst✝⁹ : NormedField 𝕜₂
σ₁₂ : 𝕜₁ →+* 𝕜₂
M₁ : Type u_3
M₂ : Type u_4
inst✝⁸ : SeminormedAddCommGroup M₁
inst✝⁷ : AddCommGroup M₂
inst✝⁶ : NormedSpace 𝕜₁ M₁
inst✝⁵ : Module 𝕜₂ M₂
inst✝⁴ : UniformSpace M₂
inst✝³ : UniformAddGroup M₂
inst✝² : ContinuousConstSMul 𝕜₂ M₂
inst✝¹ : T2Space M₂
inst✝ : CompleteSpace M₂
u : M₁ →SL[σ₁₂] M₂
hu : ∀ U ∈ 𝓝 0, ∃ y ∈ {f | IsCompactOperator ⇑f}, y - u ∈ U
U : Set M₂
hU : U ∈ 𝓝 0
V : Set M₂
hV : V ∈ 𝓝 0
hVU : ∀ v ∈ V, ∀ w ∈ V, v + w ∈ U
SV : Set M₁ × Set M₂ := (closedBall 0 1, -V)
v : M₁ →SL[σ₁₂] M₂
hv : v ∈ {f | IsCompactOperator ⇑f}
huv : v - u ∈ {f | ∀ x ∈ SV.1, f x ∈ SV.2}
T : Set M₂
hT : T.Finite
hTv : closure (⇑↑v '' closedBall 0 1) ⊆ ⋃ y ∈ T, y +ᵥ V
x : M₁
hx : x ∈ closedBall 0 1
t : M₂
ht : t ∈ T
htx : v x - t ∈ V
⊢ u x - t ∈ U

Tactic:
convert hVU _ htx _ (huv x hx) using 1

Result:
case h.e'_5
𝕜₁ : Type u_1
𝕜₂ : Type u_2
inst✝¹⁰ : NontriviallyNormedField 𝕜₁
inst✝⁹ : NormedField 𝕜₂
σ₁₂ : 𝕜₁ →+* 𝕜₂
M₁ : Type u_3
M₂ : Type u_4
inst✝⁸ : SeminormedAddCommGroup M₁
inst✝⁷ : AddCommGroup M₂
inst✝⁶ : NormedSpace 𝕜₁ M₁
inst✝⁵ : Module 𝕜₂ M₂
inst✝⁴ : UniformSpace M₂
inst✝³ : UniformAddGroup M₂
inst✝² : ContinuousConstSMul 𝕜₂ M₂
inst✝¹ : T2Space M₂
inst✝ : CompleteSpace M₂
u : M₁ →SL[σ₁₂] M₂
hu : ∀ U ∈ 𝓝 0, ∃ y ∈ {f | IsCompactOperator ⇑f}, y - u ∈ U
U : Set M₂
hU : U ∈ 𝓝 0
V : Set M₂
hV : V ∈ 𝓝 0
hVU : ∀ v ∈ V, ∀ w ∈ V, v + w ∈ U
SV : Set M₁ × Set M₂ := (closedBall 0 1, -V)
v : M₁ →SL[σ₁₂] M₂
hv : v ∈ {f | IsCompactOperator ⇑f}
huv : v - u ∈ {f | ∀ x ∈ SV.1, f x ∈ SV.2}
T : Set M₂
hT : T.Finite
hTv : closure (⇑↑v '' closedBall 0 1) ⊆ ⋃ y ∈ T, y +ᵥ V
x : M₁
hx : x ∈ closedBall 0 1
t : M₂
ht : t ∈ T
htx : v x - t ∈ V
⊢ u x - t = v x - t + -(v - u) x

Proof state:
𝕜₁ : Type u_1
𝕜₂ : Type u_2
inst✝¹⁰ : NontriviallyNormedField 𝕜₁
inst✝⁹ : NormedField 𝕜₂
σ₁₂ : 𝕜₁ →+* 𝕜₂
M₁ : Type u_3
M₂ : Type u_4
inst✝⁸ : SeminormedAddCommGroup M₁
inst✝⁷ : AddCommGroup M₂
inst✝⁶ : NormedSpace 𝕜₁ M₁
inst✝⁵ : Module 𝕜₂ M₂
inst✝⁴ : UniformSpace M₂
inst✝³ : UniformAddGroup M₂
inst✝² : ContinuousConstSMul 𝕜₂ M₂
inst✝¹ : T2Space M₂
inst✝ : CompleteSpace M₂
u : M₁ →SL[σ₁₂] M₂
hu : ∀ U ∈ 𝓝 0, ∃ y ∈ {f | IsCompactOperator ⇑f}, y - u ∈ U
this : TotallyBounded (⇑u '' closedBall 0 1)
⊢ IsCompact (closure (⇑↑u '' closedBall 0 1))

Tactic:
exact isCompact_of_totallyBounded_isClosed this.closure isClosed_closure

Result:
no goals