-- Mathlib/MeasureTheory/Integral/SetToL1.lean
theorem setToFun_zero_left' (hT : DominatedFinMeasAdditive μ T C)
    (h_zero : ∀ s, MeasurableSet s → μ s < ∞ → T s = 0) : setToFun μ T hT f = 0

Proof state:
case neg
α : Type u_1
E : Type u_2
F : Type u_3
inst✝⁴ : NormedAddCommGroup E
inst✝³ : NormedSpace ℝ E
inst✝² : NormedAddCommGroup F
inst✝¹ : NormedSpace ℝ F
m : MeasurableSpace α
μ : Measure α
inst✝ : CompleteSpace F
T : Set α → E →L[ℝ] F
C : ℝ
f : α → E
hT : DominatedFinMeasAdditive μ T C
h_zero : ∀ (s : Set α), MeasurableSet s → μ s < ⊤ → T s = 0
hf : ¬Integrable f μ
⊢ setToFun μ T hT f = 0

Tactic:
exact setToFun_undef hT hf

Result:
no goals

Proof state:
case pos
α : Type u_1
E : Type u_2
F : Type u_3
inst✝⁴ : NormedAddCommGroup E
inst✝³ : NormedSpace ℝ E
inst✝² : NormedAddCommGroup F
inst✝¹ : NormedSpace ℝ F
m : MeasurableSpace α
μ : Measure α
inst✝ : CompleteSpace F
T : Set α → E →L[ℝ] F
C : ℝ
f : α → E
hT : DominatedFinMeasAdditive μ T C
h_zero : ∀ (s : Set α), MeasurableSet s → μ s < ⊤ → T s = 0
hf : Integrable f μ
⊢ setToFun μ T hT f = 0

Tactic:
rw [setToFun_eq hT hf]

Result:
case pos
α : Type u_1
E : Type u_2
F : Type u_3
inst✝⁴ : NormedAddCommGroup E
inst✝³ : NormedSpace ℝ E
inst✝² : NormedAddCommGroup F
inst✝¹ : NormedSpace ℝ F
m : MeasurableSpace α
μ : Measure α
inst✝ : CompleteSpace F
T : Set α → E →L[ℝ] F
C : ℝ
f : α → E
hT : DominatedFinMeasAdditive μ T C
h_zero : ∀ (s : Set α), MeasurableSet s → μ s < ⊤ → T s = 0
hf : Integrable f μ
⊢ (L1.setToL1 hT) (Integrable.toL1 f hf) = 0

Proof state:
α : Type u_1
E : Type u_2
F : Type u_3
inst✝⁴ : NormedAddCommGroup E
inst✝³ : NormedSpace ℝ E
inst✝² : NormedAddCommGroup F
inst✝¹ : NormedSpace ℝ F
m : MeasurableSpace α
μ : Measure α
inst✝ : CompleteSpace F
T : Set α → E →L[ℝ] F
C : ℝ
f : α → E
hT : DominatedFinMeasAdditive μ T C
h_zero : ∀ (s : Set α), MeasurableSet s → μ s < ⊤ → T s = 0
⊢ setToFun μ T hT f = 0

Tactic:
by_cases hf : Integrable f μ

Result:
case pos
α : Type u_1
E : Type u_2
F : Type u_3
inst✝⁴ : NormedAddCommGroup E
inst✝³ : NormedSpace ℝ E
inst✝² : NormedAddCommGroup F
inst✝¹ : NormedSpace ℝ F
m : MeasurableSpace α
μ : Measure α
inst✝ : CompleteSpace F
T : Set α → E →L[ℝ] F
C : ℝ
f : α → E
hT : DominatedFinMeasAdditive μ T C
h_zero : ∀ (s : Set α), MeasurableSet s → μ s < ⊤ → T s = 0
hf : Integrable f μ
⊢ setToFun μ T hT f = 0

case neg
α : Type u_1
E : Type u_2
F : Type u_3
inst✝⁴ : NormedAddCommGroup E
inst✝³ : NormedSpace ℝ E
inst✝² : NormedAddCommGroup F
inst✝¹ : NormedSpace ℝ F
m : MeasurableSpace α
μ : Measure α
inst✝ : CompleteSpace F
T : Set α → E →L[ℝ] F
C : ℝ
f : α → E
hT : DominatedFinMeasAdditive μ T C
h_zero : ∀ (s : Set α), MeasurableSet s → μ s < ⊤ → T s = 0
hf : ¬Integrable f μ
⊢ setToFun μ T hT f = 0

Proof state:
case pos
α : Type u_1
E : Type u_2
F : Type u_3
inst✝⁴ : NormedAddCommGroup E
inst✝³ : NormedSpace ℝ E
inst✝² : NormedAddCommGroup F
inst✝¹ : NormedSpace ℝ F
m : MeasurableSpace α
μ : Measure α
inst✝ : CompleteSpace F
T : Set α → E →L[ℝ] F
C : ℝ
f : α → E
hT : DominatedFinMeasAdditive μ T C
h_zero : ∀ (s : Set α), MeasurableSet s → μ s < ⊤ → T s = 0
hf : Integrable f μ
⊢ (L1.setToL1 hT) (Integrable.toL1 f hf) = 0

Tactic:
exact L1.setToL1_zero_left' hT h_zero _

Result:
no goals