-- Mathlib/Analysis/Fourier/FourierTransformDeriv.lean
/-- The iterated derivative of a function multiplied by `(L v ⬝) ^ n` can be controlled in terms
of the iterated derivatives of the initial function. -/
lemma norm_iteratedFDeriv_fourierPowSMulRight
    {f : V → E} {K : WithTop ℕ∞} {C : ℝ} (hf : ContDiff ℝ K f) {n : ℕ} {k : ℕ} (hk : k ≤ K)
    {v : V} (hv : ∀ i ≤ k, ∀ j ≤ n, ‖v‖ ^ j * ‖iteratedFDeriv ℝ i f v‖ ≤ C) :
    ‖iteratedFDeriv ℝ k (fun v ↦ fourierPowSMulRight L f v n) v‖ ≤
      (2 * π) ^ n * (2 * n + 2) ^ k * ‖L‖ ^ n * C

Proof state:
E : Type u_1
inst✝⁵ : NormedAddCommGroup E
inst✝⁴ : NormedSpace ℂ E
V : Type u_2
W : Type u_3
inst✝³ : NormedAddCommGroup V
inst✝² : NormedSpace ℝ V
inst✝¹ : NormedAddCommGroup W
inst✝ : NormedSpace ℝ W
L : V →L[ℝ] W →L[ℝ] ℝ
f : V → E
K : WithTop ℕ∞
C : ℝ
hf : ContDiff ℝ K f
n k : ℕ
hk : ↑k ≤ K
v : V
hv : ∀ i ≤ k, ∀ j ≤ n, ‖v‖ ^ j * ‖iteratedFDeriv ℝ i f v‖ ≤ C
T : ContinuousMultilinearMap ℝ (fun i => W →L[ℝ] ℝ) (ContinuousMultilinearMap ℝ (fun i => W) ℝ) :=
  (ContinuousMultilinearMap.mkPiAlgebra ℝ (Fin n) ℝ).compContinuousLinearMapLRight
I₁ : ∀ (m : ℕ), ‖iteratedFDeriv ℝ m ⇑T fun x => L v‖ ≤ ↑(n.descFactorial m) * 1 * (‖L‖ * ‖v‖) ^ (n - m)
I₂ :
  ∀ (m : ℕ),
    ‖iteratedFDeriv ℝ m (⇑T ∘ ⇑(ContinuousLinearMap.pi fun x => L)) v‖ ≤
      ↑(n.descFactorial m) * 1 * (‖L‖ * ‖v‖) ^ (n - m) * ‖L‖ ^ m
m : ℕ
⊢ ↑(n.descFactorial m) * 1 * (‖L‖ * ‖v‖) ^ (n - m) * ‖L‖ ^ m = ↑(n.descFactorial m) * ‖L‖ ^ n * ‖v‖ ^ (n - m)

Tactic:
rcases le_or_lt m n with hm | hm

Result:
case inl
E : Type u_1
inst✝⁵ : NormedAddCommGroup E
inst✝⁴ : NormedSpace ℂ E
V : Type u_2
W : Type u_3
inst✝³ : NormedAddCommGroup V
inst✝² : NormedSpace ℝ V
inst✝¹ : NormedAddCommGroup W
inst✝ : NormedSpace ℝ W
L : V →L[ℝ] W →L[ℝ] ℝ
f : V → E
K : WithTop ℕ∞
C : ℝ
hf : ContDiff ℝ K f
n k : ℕ
hk : ↑k ≤ K
v : V
hv : ∀ i ≤ k, ∀ j ≤ n, ‖v‖ ^ j * ‖iteratedFDeriv ℝ i f v‖ ≤ C
T : ContinuousMultilinearMap ℝ (fun i => W →L[ℝ] ℝ) (ContinuousMultilinearMap ℝ (fun i => W) ℝ) :=
  (ContinuousMultilinearMap.mkPiAlgebra ℝ (Fin n) ℝ).compContinuousLinearMapLRight
I₁ : ∀ (m : ℕ), ‖iteratedFDeriv ℝ m ⇑T fun x => L v‖ ≤ ↑(n.descFactorial m) * 1 * (‖L‖ * ‖v‖) ^ (n - m)
I₂ :
  ∀ (m : ℕ),
    ‖iteratedFDeriv ℝ m (⇑T ∘ ⇑(ContinuousLinearMap.pi fun x => L)) v‖ ≤
      ↑(n.descFactorial m) * 1 * (‖L‖ * ‖v‖) ^ (n - m) * ‖L‖ ^ m
m : ℕ
hm : m ≤ n
⊢ ↑(n.descFactorial m) * 1 * (‖L‖ * ‖v‖) ^ (n - m) * ‖L‖ ^ m = ↑(n.descFactorial m) * ‖L‖ ^ n * ‖v‖ ^ (n - m)

case inr
E : Type u_1
inst✝⁵ : NormedAddCommGroup E
inst✝⁴ : NormedSpace ℂ E
V : Type u_2
W : Type u_3
inst✝³ : NormedAddCommGroup V
inst✝² : NormedSpace ℝ V
inst✝¹ : NormedAddCommGroup W
inst✝ : NormedSpace ℝ W
L : V →L[ℝ] W →L[ℝ] ℝ
f : V → E
K : WithTop ℕ∞
C : ℝ
hf : ContDiff ℝ K f
n k : ℕ
hk : ↑k ≤ K
v : V
hv : ∀ i ≤ k, ∀ j ≤ n, ‖v‖ ^ j * ‖iteratedFDeriv ℝ i f v‖ ≤ C
T : ContinuousMultilinearMap ℝ (fun i => W →L[ℝ] ℝ) (ContinuousMultilinearMap ℝ (fun i => W) ℝ) :=
  (ContinuousMultilinearMap.mkPiAlgebra ℝ (Fin n) ℝ).compContinuousLinearMapLRight
I₁ : ∀ (m : ℕ), ‖iteratedFDeriv ℝ m ⇑T fun x => L v‖ ≤ ↑(n.descFactorial m) * 1 * (‖L‖ * ‖v‖) ^ (n - m)
I₂ :
  ∀ (m : ℕ),
    ‖iteratedFDeriv ℝ m (⇑T ∘ ⇑(ContinuousLinearMap.pi fun x => L)) v‖ ≤
      ↑(n.descFactorial m) * 1 * (‖L‖ * ‖v‖) ^ (n - m) * ‖L‖ ^ m
m : ℕ
hm : n < m
⊢ ↑(n.descFactorial m) * 1 * (‖L‖ * ‖v‖) ^ (n - m) * ‖L‖ ^ m = ↑(n.descFactorial m) * ‖L‖ ^ n * ‖v‖ ^ (n - m)

Proof state:
E : Type u_1
inst✝⁵ : NormedAddCommGroup E
inst✝⁴ : NormedSpace ℂ E
V : Type u_2
W : Type u_3
inst✝³ : NormedAddCommGroup V
inst✝² : NormedSpace ℝ V
inst✝¹ : NormedAddCommGroup W
inst✝ : NormedSpace ℝ W
L : V →L[ℝ] W →L[ℝ] ℝ
f : V → E
K : WithTop ℕ∞
C : ℝ
hf : ContDiff ℝ K f
n k : ℕ
hk : ↑k ≤ K
v : V
hv : ∀ i ≤ k, ∀ j ≤ n, ‖v‖ ^ j * ‖iteratedFDeriv ℝ i f v‖ ≤ C
T : ContinuousMultilinearMap ℝ (fun i => W →L[ℝ] ℝ) (ContinuousMultilinearMap ℝ (fun i => W) ℝ) :=
  (ContinuousMultilinearMap.mkPiAlgebra ℝ (Fin n) ℝ).compContinuousLinearMapLRight
I₁ : ∀ (m : ℕ), ‖iteratedFDeriv ℝ m ⇑T fun x => L v‖ ≤ ↑(n.descFactorial m) * 1 * (‖L‖ * ‖v‖) ^ (n - m)
I₂ :
  ∀ (m : ℕ),
    ‖iteratedFDeriv ℝ m (⇑T ∘ ⇑(ContinuousLinearMap.pi fun x => L)) v‖ ≤
      ↑(n.descFactorial m) * 1 * (‖L‖ * ‖v‖) ^ (n - m) * ‖L‖ ^ m
I₃ :
  ∀ (m : ℕ),
    ‖iteratedFDeriv ℝ m (⇑T ∘ ⇑(ContinuousLinearMap.pi fun x => L)) v‖ ≤ ↑(n.descFactorial m) * ‖L‖ ^ n * ‖v‖ ^ (n - m)
A : ContDiff ℝ K fun y => T fun x => L y
⊢ ‖iteratedFDeriv ℝ k
        (fun v =>
          (-(2 * ↑π * I)) ^ n •
            ((smulRightL ℝ (fun x => W) E)
                ((ContinuousMultilinearMap.mkPiAlgebra ℝ (Fin n) ℝ).compContinuousLinearMapLRight fun x => L v))
              (f v))
        v‖ ≤
    (2 * π) ^ n * (2 * ↑n + 2) ^ k * ‖L‖ ^ n * C

Tactic:
rw [iteratedFDeriv_const_smul_apply' (hf := ((smulRightL ℝ (fun _ ↦ W)
  E).isBoundedBilinearMap.contDiff.comp₂ (A.of_le hk) (hf.of_le hk)).contDiffAt),
  norm_smul (β := V [×k]→L[ℝ] (W [×n]→L[ℝ] E))]

Result:
E : Type u_1
inst✝⁵ : NormedAddCommGroup E
inst✝⁴ : NormedSpace ℂ E
V : Type u_2
W : Type u_3
inst✝³ : NormedAddCommGroup V
inst✝² : NormedSpace ℝ V
inst✝¹ : NormedAddCommGroup W
inst✝ : NormedSpace ℝ W
L : V →L[ℝ] W →L[ℝ] ℝ
f : V → E
K : WithTop ℕ∞
C : ℝ
hf : ContDiff ℝ K f
n k : ℕ
hk : ↑k ≤ K
v : V
hv : ∀ i ≤ k, ∀ j ≤ n, ‖v‖ ^ j * ‖iteratedFDeriv ℝ i f v‖ ≤ C
T : ContinuousMultilinearMap ℝ (fun i => W →L[ℝ] ℝ) (ContinuousMultilinearMap ℝ (fun i => W) ℝ) :=
  (ContinuousMultilinearMap.mkPiAlgebra ℝ (Fin n) ℝ).compContinuousLinearMapLRight
I₁ : ∀ (m : ℕ), ‖iteratedFDeriv ℝ m ⇑T fun x => L v‖ ≤ ↑(n.descFactorial m) * 1 * (‖L‖ * ‖v‖) ^ (n - m)
I₂ :
  ∀ (m : ℕ),
    ‖iteratedFDeriv ℝ m (⇑T ∘ ⇑(ContinuousLinearMap.pi fun x => L)) v‖ ≤
      ↑(n.descFactorial m) * 1 * (‖L‖ * ‖v‖) ^ (n - m) * ‖L‖ ^ m
I₃ :
  ∀ (m : ℕ),
    ‖iteratedFDeriv ℝ m (⇑T ∘ ⇑(ContinuousLinearMap.pi fun x => L)) v‖ ≤ ↑(n.descFactorial m) * ‖L‖ ^ n * ‖v‖ ^ (n - m)
A : ContDiff ℝ K fun y => T fun x => L y
⊢ ‖(-(2 * ↑π * I)) ^ n •
        iteratedFDeriv ℝ k
          (fun x => ((smulRightL ℝ (fun x => W) E) (T fun x_1 => L x, f x).1) (T fun x_1 => L x, f x).2) v‖ ≤
    (2 * π) ^ n * (2 * ↑n + 2) ^ k * ‖L‖ ^ n * C