-- Mathlib/MeasureTheory/Measure/LogLikelihoodRatio.lean
lemma llr_tilted_left [SigmaFinite μ] [SigmaFinite ν] (hμν : μ ≪ ν)
    (hf : Integrable (fun x ↦ exp (f x)) μ) (hfν : AEMeasurable f ν) :
    (llr (μ.tilted f) ν) =ᵐ[μ] fun x ↦ f x - log (∫ z, exp (f z) ∂μ) + llr μ ν x

Proof state:
case inl
α : Type u_1
mα : MeasurableSpace α
μ ν : Measure α
f : α → ℝ
inst✝¹ : SigmaFinite μ
inst✝ : SigmaFinite ν
hμν : μ ≪ ν
hf : Integrable (fun x => rexp (f x)) μ
hfν : AEMeasurable f ν
hμ : μ = 0
⊢ ∀ᶠ (x : α) in ⊥, llr (Measure.tilted 0 f) ν x = f x - log (∫ (z : α), rexp (f z) ∂0) + llr 0 ν x

Tactic:
exact Filter.eventually_bot

Result:
no goals

Proof state:
case h.hx
α : Type u_1
mα : MeasurableSpace α
μ ν : Measure α
f : α → ℝ
inst✝¹ : SigmaFinite μ
inst✝ : SigmaFinite ν
hμν : μ ≪ ν
hf : Integrable (fun x => rexp (f x)) μ
hfν : AEMeasurable f ν
h0 : NeZero μ
x : α
hx : ((μ.tilted f).rnDeriv ν x).toReal = (rexp (f x) / ∫ (x : α), rexp (f x) ∂μ) * (μ.rnDeriv ν x).toReal
hx_pos : 0 < μ.rnDeriv ν x
hx_lt_top : μ.rnDeriv ν x < ⊤
⊢ ¬(rexp (f x) = 0 ∨ ∫ (x : α), rexp (f x) ∂μ = 0)

Tactic:
push_neg

Result:
case h.hx
α : Type u_1
mα : MeasurableSpace α
μ ν : Measure α
f : α → ℝ
inst✝¹ : SigmaFinite μ
inst✝ : SigmaFinite ν
hμν : μ ≪ ν
hf : Integrable (fun x => rexp (f x)) μ
hfν : AEMeasurable f ν
h0 : NeZero μ
x : α
hx : ((μ.tilted f).rnDeriv ν x).toReal = (rexp (f x) / ∫ (x : α), rexp (f x) ∂μ) * (μ.rnDeriv ν x).toReal
hx_pos : 0 < μ.rnDeriv ν x
hx_lt_top : μ.rnDeriv ν x < ⊤
⊢ rexp (f x) ≠ 0 ∧ ∫ (x : α), rexp (f x) ∂μ ≠ 0

Proof state:
case h
α : Type u_1
mα : MeasurableSpace α
μ ν : Measure α
f : α → ℝ
inst✝¹ : SigmaFinite μ
inst✝ : SigmaFinite ν
hμν : μ ≪ ν
hf : Integrable (fun x => rexp (f x)) μ
hfν : AEMeasurable f ν
h0 : NeZero μ
x : α
hx : ((μ.tilted f).rnDeriv ν x).toReal = (rexp (f x) / ∫ (x : α), rexp (f x) ∂μ) * (μ.rnDeriv ν x).toReal
hx_pos : 0 < μ.rnDeriv ν x
hx_lt_top : μ.rnDeriv ν x < ⊤
⊢ (∫ (x : α), rexp (f x) ∂μ)⁻¹ ≠ 0

Tactic:
simp only [ne_eq, inv_eq_zero]

Result:
case h
α : Type u_1
mα : MeasurableSpace α
μ ν : Measure α
f : α → ℝ
inst✝¹ : SigmaFinite μ
inst✝ : SigmaFinite ν
hμν : μ ≪ ν
hf : Integrable (fun x => rexp (f x)) μ
hfν : AEMeasurable f ν
h0 : NeZero μ
x : α
hx : ((μ.tilted f).rnDeriv ν x).toReal = (rexp (f x) / ∫ (x : α), rexp (f x) ∂μ) * (μ.rnDeriv ν x).toReal
hx_pos : 0 < μ.rnDeriv ν x
hx_lt_top : μ.rnDeriv ν x < ⊤
⊢ ¬∫ (x : α), rexp (f x) ∂μ = 0

Proof state:
case h.hy
α : Type u_1
mα : MeasurableSpace α
μ ν : Measure α
f : α → ℝ
inst✝¹ : SigmaFinite μ
inst✝ : SigmaFinite ν
hμν : μ ≪ ν
hf : Integrable (fun x => rexp (f x)) μ
hfν : AEMeasurable f ν
h0 : NeZero μ
x : α
hx : ((μ.tilted f).rnDeriv ν x).toReal = (rexp (f x) / ∫ (x : α), rexp (f x) ∂μ) * (μ.rnDeriv ν x).toReal
hx_pos : 0 < μ.rnDeriv ν x
hx_lt_top : μ.rnDeriv ν x < ⊤
⊢ (μ.rnDeriv ν x).toReal ≠ 0

Tactic:
simp [ENNReal.toReal_eq_zero_iff, hx_lt_top.ne, hx_pos.ne']

Result:
no goals

Proof state:
case h
α : Type u_1
mα : MeasurableSpace α
μ ν : Measure α
f : α → ℝ
inst✝¹ : SigmaFinite μ
inst✝ : SigmaFinite ν
hμν : μ ≪ ν
hf : Integrable (fun x => rexp (f x)) μ
hfν : AEMeasurable f ν
h0 : NeZero μ
x : α
hx : ((μ.tilted f).rnDeriv ν x).toReal = (rexp (f x) / ∫ (x : α), rexp (f x) ∂μ) * (μ.rnDeriv ν x).toReal
hx_pos : 0 < μ.rnDeriv ν x
hx_lt_top : μ.rnDeriv ν x < ⊤
⊢ ¬∫ (x : α), rexp (f x) ∂μ = 0

Tactic:
exact (integral_exp_pos hf).ne'

Result:
no goals

Proof state:
case inl
α : Type u_1
mα : MeasurableSpace α
μ ν : Measure α
f : α → ℝ
inst✝¹ : SigmaFinite μ
inst✝ : SigmaFinite ν
hμν : μ ≪ ν
hf : Integrable (fun x => rexp (f x)) μ
hfν : AEMeasurable f ν
hμ : μ = 0
⊢ llr (μ.tilted f) ν =ᶠ[ae μ] fun x => f x - log (∫ (z : α), rexp (f z) ∂μ) + llr μ ν x

Tactic:
simp only [hμ, ae_zero, Filter.EventuallyEq]

Result:
case inl
α : Type u_1
mα : MeasurableSpace α
μ ν : Measure α
f : α → ℝ
inst✝¹ : SigmaFinite μ
inst✝ : SigmaFinite ν
hμν : μ ≪ ν
hf : Integrable (fun x => rexp (f x)) μ
hfν : AEMeasurable f ν
hμ : μ = 0
⊢ ∀ᶠ (x : α) in ⊥, llr (Measure.tilted 0 f) ν x = f x - log (∫ (z : α), rexp (f z) ∂0) + llr 0 ν x

Proof state:
case h.hx
α : Type u_1
mα : MeasurableSpace α
μ ν : Measure α
f : α → ℝ
inst✝¹ : SigmaFinite μ
inst✝ : SigmaFinite ν
hμν : μ ≪ ν
hf : Integrable (fun x => rexp (f x)) μ
hfν : AEMeasurable f ν
h0 : NeZero μ
x : α
hx : ((μ.tilted f).rnDeriv ν x).toReal = (rexp (f x) / ∫ (x : α), rexp (f x) ∂μ) * (μ.rnDeriv ν x).toReal
hx_pos : 0 < μ.rnDeriv ν x
hx_lt_top : μ.rnDeriv ν x < ⊤
⊢ rexp (f x) / ∫ (x : α), rexp (f x) ∂μ ≠ 0

Tactic:
simp only [ne_eq, div_eq_zero_iff]

Result:
case h.hx
α : Type u_1
mα : MeasurableSpace α
μ ν : Measure α
f : α → ℝ
inst✝¹ : SigmaFinite μ
inst✝ : SigmaFinite ν
hμν : μ ≪ ν
hf : Integrable (fun x => rexp (f x)) μ
hfν : AEMeasurable f ν
h0 : NeZero μ
x : α
hx : ((μ.tilted f).rnDeriv ν x).toReal = (rexp (f x) / ∫ (x : α), rexp (f x) ∂μ) * (μ.rnDeriv ν x).toReal
hx_pos : 0 < μ.rnDeriv ν x
hx_lt_top : μ.rnDeriv ν x < ⊤
⊢ ¬(rexp (f x) = 0 ∨ ∫ (x : α), rexp (f x) ∂μ = 0)

Proof state:
case inr
α : Type u_1
mα : MeasurableSpace α
μ ν : Measure α
f : α → ℝ
inst✝¹ : SigmaFinite μ
inst✝ : SigmaFinite ν
hμν : μ ≪ ν
hf : Integrable (fun x => rexp (f x)) μ
hfν : AEMeasurable f ν
h0 : NeZero μ
⊢ llr (μ.tilted f) ν =ᶠ[ae μ] fun x => f x - log (∫ (z : α), rexp (f z) ∂μ) + llr μ ν x

Tactic:
filter_upwards [hμν.ae_le (toReal_rnDeriv_tilted_left μ hfν), Measure.rnDeriv_pos hμν,
  hμν.ae_le (Measure.rnDeriv_lt_top μ ν)] with x hx hx_pos hx_lt_top

Result:
case h
α : Type u_1
mα : MeasurableSpace α
μ ν : Measure α
f : α → ℝ
inst✝¹ : SigmaFinite μ
inst✝ : SigmaFinite ν
hμν : μ ≪ ν
hf : Integrable (fun x => rexp (f x)) μ
hfν : AEMeasurable f ν
h0 : NeZero μ
x : α
hx : ((μ.tilted f).rnDeriv ν x).toReal = (rexp (f x) / ∫ (x : α), rexp (f x) ∂μ) * (μ.rnDeriv ν x).toReal
hx_pos : 0 < μ.rnDeriv ν x
hx_lt_top : μ.rnDeriv ν x < ⊤
⊢ llr (μ.tilted f) ν x = f x - log (∫ (z : α), rexp (f z) ∂μ) + llr μ ν x

Proof state:
case h
α : Type u_1
mα : MeasurableSpace α
μ ν : Measure α
f : α → ℝ
inst✝¹ : SigmaFinite μ
inst✝ : SigmaFinite ν
hμν : μ ≪ ν
hf : Integrable (fun x => rexp (f x)) μ
hfν : AEMeasurable f ν
h0 : NeZero μ
x : α
hx : ((μ.tilted f).rnDeriv ν x).toReal = (rexp (f x) / ∫ (x : α), rexp (f x) ∂μ) * (μ.rnDeriv ν x).toReal
hx_pos : 0 < μ.rnDeriv ν x
hx_lt_top : μ.rnDeriv ν x < ⊤
⊢ llr (μ.tilted f) ν x = f x - log (∫ (z : α), rexp (f z) ∂μ) + llr μ ν x

Tactic:
rw [llr, hx, log_mul, div_eq_mul_inv, log_mul (exp_pos _).ne', log_exp, log_inv, llr,
  ← sub_eq_add_neg]

Result:
case h
α : Type u_1
mα : MeasurableSpace α
μ ν : Measure α
f : α → ℝ
inst✝¹ : SigmaFinite μ
inst✝ : SigmaFinite ν
hμν : μ ≪ ν
hf : Integrable (fun x => rexp (f x)) μ
hfν : AEMeasurable f ν
h0 : NeZero μ
x : α
hx : ((μ.tilted f).rnDeriv ν x).toReal = (rexp (f x) / ∫ (x : α), rexp (f x) ∂μ) * (μ.rnDeriv ν x).toReal
hx_pos : 0 < μ.rnDeriv ν x
hx_lt_top : μ.rnDeriv ν x < ⊤
⊢ (∫ (x : α), rexp (f x) ∂μ)⁻¹ ≠ 0

case h.hx
α : Type u_1
mα : MeasurableSpace α
μ ν : Measure α
f : α → ℝ
inst✝¹ : SigmaFinite μ
inst✝ : SigmaFinite ν
hμν : μ ≪ ν
hf : Integrable (fun x => rexp (f x)) μ
hfν : AEMeasurable f ν
h0 : NeZero μ
x : α
hx : ((μ.tilted f).rnDeriv ν x).toReal = (rexp (f x) / ∫ (x : α), rexp (f x) ∂μ) * (μ.rnDeriv ν x).toReal
hx_pos : 0 < μ.rnDeriv ν x
hx_lt_top : μ.rnDeriv ν x < ⊤
⊢ rexp (f x) / ∫ (x : α), rexp (f x) ∂μ ≠ 0

case h.hy
α : Type u_1
mα : MeasurableSpace α
μ ν : Measure α
f : α → ℝ
inst✝¹ : SigmaFinite μ
inst✝ : SigmaFinite ν
hμν : μ ≪ ν
hf : Integrable (fun x => rexp (f x)) μ
hfν : AEMeasurable f ν
h0 : NeZero μ
x : α
hx : ((μ.tilted f).rnDeriv ν x).toReal = (rexp (f x) / ∫ (x : α), rexp (f x) ∂μ) * (μ.rnDeriv ν x).toReal
hx_pos : 0 < μ.rnDeriv ν x
hx_lt_top : μ.rnDeriv ν x < ⊤
⊢ (μ.rnDeriv ν x).toReal ≠ 0