-- Mathlib/Analysis/Calculus/LogDeriv.lean
lemma logDeriv_fun_zpow {f : 𝕜 → 𝕜'} {x : 𝕜} (hdf : DifferentiableAt 𝕜 f x) (n : ℤ) :
    logDeriv (f · ^ n) x = n * logDeriv f x

Proof state:
𝕜 : Type u_1
𝕜' : Type u_2
inst✝² : NontriviallyNormedField 𝕜
inst✝¹ : NontriviallyNormedField 𝕜'
inst✝ : NormedAlgebra 𝕜 𝕜'
f : 𝕜 → 𝕜'
x : 𝕜
hdf : DifferentiableAt 𝕜 f x
n : ℤ
⊢ logDeriv (fun x => f x ^ n) x = ↑n * logDeriv f x

Tactic:
rcases eq_or_ne n 0 with rfl | hn

Result:
case inl
𝕜 : Type u_1
𝕜' : Type u_2
inst✝² : NontriviallyNormedField 𝕜
inst✝¹ : NontriviallyNormedField 𝕜'
inst✝ : NormedAlgebra 𝕜 𝕜'
f : 𝕜 → 𝕜'
x : 𝕜
hdf : DifferentiableAt 𝕜 f x
⊢ logDeriv (fun x => f x ^ 0) x = ↑0 * logDeriv f x

case inr
𝕜 : Type u_1
𝕜' : Type u_2
inst✝² : NontriviallyNormedField 𝕜
inst✝¹ : NontriviallyNormedField 𝕜'
inst✝ : NormedAlgebra 𝕜 𝕜'
f : 𝕜 → 𝕜'
x : 𝕜
hdf : DifferentiableAt 𝕜 f x
n : ℤ
hn : n ≠ 0
⊢ logDeriv (fun x => f x ^ n) x = ↑n * logDeriv f x

Proof state:
case inr.inr
𝕜 : Type u_1
𝕜' : Type u_2
inst✝² : NontriviallyNormedField 𝕜
inst✝¹ : NontriviallyNormedField 𝕜'
inst✝ : NormedAlgebra 𝕜 𝕜'
f : 𝕜 → 𝕜'
x : 𝕜
hdf : DifferentiableAt 𝕜 f x
n : ℤ
hn : n ≠ 0
hf : f x ≠ 0
⊢ ↑n * f x ^ (n - 1) * deriv f x / f x ^ n = ↑n * (deriv f x / f x)

Tactic:
field_simp [zpow_ne_zero, zpow_sub_one₀ hf]

Result:
case inr.inr
𝕜 : Type u_1
𝕜' : Type u_2
inst✝² : NontriviallyNormedField 𝕜
inst✝¹ : NontriviallyNormedField 𝕜'
inst✝ : NormedAlgebra 𝕜 𝕜'
f : 𝕜 → 𝕜'
x : 𝕜
hdf : DifferentiableAt 𝕜 f x
n : ℤ
hn : n ≠ 0
hf : f x ≠ 0
⊢ ↑n * f x ^ n * deriv f x * f x = ↑n * deriv f x * (f x * f x ^ n)

Proof state:
case inl
𝕜 : Type u_1
𝕜' : Type u_2
inst✝² : NontriviallyNormedField 𝕜
inst✝¹ : NontriviallyNormedField 𝕜'
inst✝ : NormedAlgebra 𝕜 𝕜'
f : 𝕜 → 𝕜'
x : 𝕜
hdf : DifferentiableAt 𝕜 f x
⊢ logDeriv (fun x => f x ^ 0) x = ↑0 * logDeriv f x

Tactic:
simp

Result:
no goals

Proof state:
case inr
𝕜 : Type u_1
𝕜' : Type u_2
inst✝² : NontriviallyNormedField 𝕜
inst✝¹ : NontriviallyNormedField 𝕜'
inst✝ : NormedAlgebra 𝕜 𝕜'
f : 𝕜 → 𝕜'
x : 𝕜
hdf : DifferentiableAt 𝕜 f x
n : ℤ
hn : n ≠ 0
⊢ logDeriv (fun x => f x ^ n) x = ↑n * logDeriv f x

Tactic:
rcases eq_or_ne (f x) 0 with hf | hf

Result:
case inr.inl
𝕜 : Type u_1
𝕜' : Type u_2
inst✝² : NontriviallyNormedField 𝕜
inst✝¹ : NontriviallyNormedField 𝕜'
inst✝ : NormedAlgebra 𝕜 𝕜'
f : 𝕜 → 𝕜'
x : 𝕜
hdf : DifferentiableAt 𝕜 f x
n : ℤ
hn : n ≠ 0
hf : f x = 0
⊢ logDeriv (fun x => f x ^ n) x = ↑n * logDeriv f x

case inr.inr
𝕜 : Type u_1
𝕜' : Type u_2
inst✝² : NontriviallyNormedField 𝕜
inst✝¹ : NontriviallyNormedField 𝕜'
inst✝ : NormedAlgebra 𝕜 𝕜'
f : 𝕜 → 𝕜'
x : 𝕜
hdf : DifferentiableAt 𝕜 f x
n : ℤ
hn : n ≠ 0
hf : f x ≠ 0
⊢ logDeriv (fun x => f x ^ n) x = ↑n * logDeriv f x

Proof state:
case inr.inr
𝕜 : Type u_1
𝕜' : Type u_2
inst✝² : NontriviallyNormedField 𝕜
inst✝¹ : NontriviallyNormedField 𝕜'
inst✝ : NormedAlgebra 𝕜 𝕜'
f : 𝕜 → 𝕜'
x : 𝕜
hdf : DifferentiableAt 𝕜 f x
n : ℤ
hn : n ≠ 0
hf : f x ≠ 0
⊢ ↑n * f x ^ n * deriv f x * f x = ↑n * deriv f x * (f x * f x ^ n)

Tactic:
ring

Result:
no goals

Proof state:
case inr.inl
𝕜 : Type u_1
𝕜' : Type u_2
inst✝² : NontriviallyNormedField 𝕜
inst✝¹ : NontriviallyNormedField 𝕜'
inst✝ : NormedAlgebra 𝕜 𝕜'
f : 𝕜 → 𝕜'
x : 𝕜
hdf : DifferentiableAt 𝕜 f x
n : ℤ
hn : n ≠ 0
hf : f x = 0
⊢ logDeriv (fun x => f x ^ n) x = ↑n * logDeriv f x

Tactic:
simp [logDeriv_apply, zero_zpow, *]

Result:
no goals