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import OAI.MathematicalPhysics.RapidForcing.Detector
namespace OAI
open scoped BigOperators ENNReal Topology
open Set MeasureTheory
namespace RapidForcing
section UniformBounds
variable {P E F : Type} [NormedAddCommGroup P] [NormedSpace ℝ P]
[NormedAddCommGroup E] [NormedSpace ℝ E]
[NormedAddCommGroup F] [NormedSpace ℝ F]
lemma norm_iteratedFDeriv_slice_le {f : P × E → F}
(hf : ContDiff ℝ (⊤ : ℕ∞) f) (a : P) (x : E) (k : ℕ) :
‖iteratedFDeriv ℝ k (fun y => f (a, y)) x‖ ≤
‖iteratedFDeriv ℝ k f (a, x)‖ := by
let L := ContinuousLinearMap.inr ℝ P E
have hL : ‖L‖ ≤ 1 := ContinuousLinearMap.opNorm_le_bound _ zero_le_one (by
intro y
simp [L])
have hf' : ContDiff ℝ (⊤ : ℕ∞) (fun z : P × E => f (z + (a, 0))) :=
hf.comp (contDiff_id.add contDiff_const)
have hd := L.iteratedFDeriv_comp_right hf' x (i := k) (by exact_mod_cast (le_top : (k : ℕ∞) ≤ ⊤))
have hfun : (fun y => f (a, y)) = (fun z : P × E => f (z + (a, 0))) ∘ L := by
ext y
simp [L]
rw [hfun, hd, iteratedFDeriv_comp_add_right]
have hb := (iteratedFDeriv ℝ k f (L x + (a, 0))).norm_compContinuousLinearMap_le
(fun _ : Fin k => L)
simp only [Finset.prod_const, Finset.card_univ, Fintype.card_fin] at hb
calc
_ ≤ ‖iteratedFDeriv ℝ k f (L x + (a, 0))‖ * ‖L‖ ^ k := hb
_ ≤ ‖iteratedFDeriv ℝ k f (L x + (a, 0))‖ * 1 :=
mul_le_mul_of_nonneg_left (by simpa using pow_le_pow_left₀ (norm_nonneg L) hL k)
(norm_nonneg _)
_ = _ := by simp [L]
lemma uniform_derivative_bound {f : P × E → F}
(hf : ContDiff ℝ (⊤ : ℕ∞) f) {A : Set P} {B : Set E}
(hA : IsCompact A) (hB : IsCompact B)
(hs : ∀ a ∈ A, tsupport (fun x => f (a, x)) ⊆ B) (k : ℕ) :
∃ C : ℝ, 0 ≤ C ∧ ∀ a ∈ A, ∀ x,
‖iteratedFDeriv ℝ k (fun y => f (a, y)) x‖ ≤ C := by
obtain ⟨C, hC⟩ := (hA.prod hB).exists_bound_of_continuousOn
((hf.continuous_iteratedFDeriv (m := k) (by exact_mod_cast (le_top : (k : ℕ∞) ≤ ⊤))).continuousOn)
refine ⟨max 0 C, le_max_left _ _, ?_⟩
intro a ha x
by_cases hx : x ∈ B
· exact (norm_iteratedFDeriv_slice_le hf a x k).trans ((hC (a, x) ⟨ha, hx⟩).trans
(le_max_right _ _))
· have hz : iteratedFDeriv ℝ k (fun y => f (a, y)) x = 0 :=
image_eq_zero_of_notMem_tsupport (fun h => hx (hs a ha
(tsupport_iteratedFDeriv_subset k h)))
rw [hz, norm_zero]
exact le_max_left _ _
end UniformBounds
noncomputable def scaledCurl (η ξ : Space) : Field Space := fun σ y =>
curl (fun z => ζ (z - θ σ • η) •
((1 / 2 : ℝ) • cross (deriv θ σ • ξ) (z - θ σ • η))) y
lemma scaledCurl_joint_smooth : ContDiff ℝ (⊤ : ℕ∞)
(fun p : (Space × Space) × (ℝ × Space) => scaledCurl p.1.1 p.1.2 p.2.1 p.2.2) := by
have hθ := theta_smooth
have hdθ : ContDiff ℝ (⊤ : ℕ∞) (deriv θ) :=
(contDiff_infty_iff_deriv.mp theta_smooth).2
have h : ContDiff ℝ (⊤ : ℕ∞)
(fun p : ((Space × Space) × (ℝ × Space)) × Space =>
ζ (p.2 - θ p.1.2.1 • p.1.1.1) •
((1 / 2 : ℝ) • cross (deriv θ p.1.2.1 • p.1.1.2)
(p.2 - θ p.1.2.1 • p.1.1.1))) := by
apply (contDiff_piLp 2).mpr
intro i
fin_cases i <;> dsimp [cross, vec] <;> unfold ζ <;> fun_prop (disch := assumption)
have hD : ContDiff ℝ (⊤ : ℕ∞)
(fun p : (Space × Space) × (ℝ × Space) =>
fderiv ℝ (fun z => ζ (z - θ p.2.1 • p.1.1) •
((1 / 2 : ℝ) • cross (deriv θ p.2.1 • p.1.2) (z - θ p.2.1 • p.1.1))) p.2.2) :=
h.fderiv (contDiff_snd.snd) (by simp)
apply (contDiff_piLp 2).mpr
intro i
fin_cases i <;> dsimp [scaledCurl, curl, vec] <;> fun_prop
lemma deriv_theta_left {σ : ℝ} (hσ : σ < 1 / 4) : deriv θ σ = 0 := by
have he : θ =ᶠ[𝓝 σ] fun _ => 0 := by
filter_upwards [Iio_mem_nhds hσ] with s hs
exact theta_zero hs.le
simpa using he.deriv_eq
lemma deriv_theta_right {σ : ℝ} (hσ : 3 / 4 < σ) : deriv θ σ = 0 := by
have he : θ =ᶠ[𝓝 σ] fun _ => 1 := by
filter_upwards [Ioi_mem_nhds hσ] with s hs
exact theta_one hs.le
simpa using he.deriv_eq
lemma scaledCurl_eq_zero_of_deriv_theta (η ξ : Space) {σ : ℝ} (hσ : deriv θ σ = 0) :
scaledCurl η ξ σ = 0 := by
funext y
have hc : ∀ z : Space, cross (0 : Space) z = 0 := by
intro z
ext i
fin_cases i <;> simp [cross, vec]
simp [scaledCurl, hσ, hc, curl, vec]
lemma norm_le_two_mul_of_coord_le {z : Space} {r : ℝ} (hr : 0 ≤ r)
(hz : ∀ i : Fin 3, |z i| ≤ r) : ‖z‖ ≤ 2 * r := by
have hs (i : Fin 3) : (z i) ^ 2 ≤ r ^ 2 := by
simpa only [sq_abs] using (sq_le_sq₀ (abs_nonneg _) hr).mpr (hz i)
have hnorm : ‖z‖ ^ 2 = (z 0) ^ 2 + (z 1) ^ 2 + (z 2) ^ 2 := by
simpa [Fin.sum_univ_three, Real.norm_eq_abs, sq_abs] using EuclideanSpace.norm_sq_eq z
nlinarith [hs 0, hs 1, hs 2, norm_nonneg z, sq_nonneg r]
lemma scaledCurl_tsupport_subset (η ξ : Space) (hη : ‖η‖ ≤ 2) :
tsupport (Function.uncurry (scaledCurl η ξ)) ⊆
Icc (0 : ℝ) 1 ×ˢ Metric.closedBall (0 : Space) 3 := by
apply closure_minimal _ (isClosed_Icc.prod Metric.isClosed_closedBall)
intro p hp
have ht : p.1 ∈ Icc (0 : ℝ) 1 := by
by_contra h
have hz : scaledCurl η ξ p.1 = 0 := by
have hor : p.1 < 0 ∨ 1 < p.1 := by
by_cases hh : 0 ≤ p.1
· exact Or.inr (lt_of_not_ge (fun hh' => h ⟨hh, hh'⟩))
· exact Or.inl (lt_of_not_ge hh)
rcases hor with h | h
· exact scaledCurl_eq_zero_of_deriv_theta η ξ (deriv_theta_left (by linarith))
· exact scaledCurl_eq_zero_of_deriv_theta η ξ (deriv_theta_right (by linarith))
exact hp (congrFun hz p.2)
refine ⟨ht, ?_⟩
have hz : p.2 ∈ tsupport (fun z => ζ (z - θ p.1 • η) •
((1 / 2 : ℝ) • cross (deriv θ p.1 • ξ) (z - θ p.1 • η))) :=
curl_tsupport_subset _ (subset_closure hp)
have hs : tsupport (fun z => ζ (z - θ p.1 • η) •
((1 / 2 : ℝ) • cross (deriv θ p.1 • ξ) (z - θ p.1 • η))) ⊆
supportBox (θ p.1 • η) (1 / 8) := by
apply closure_minimal _ (supportBox_closed _ _)
intro z hz i
by_contra hi
have hv := zeta_zero (i := i) (v := z - θ p.1 • η) (le_of_lt (not_le.mp hi))
exact hz (by dsimp; rw [hv, zero_smul])
have hc := hs hz
have hn : ‖p.2 - θ p.1 • η‖ ≤ 1 := by
have hcoord := norm_le_two_mul_of_coord_le (z := p.2 - θ p.1 • η) (by norm_num : (0 : ℝ) ≤ 1 / 8) (fun i => hc i)
exact hcoord.trans (by norm_num)
have hθη : ‖θ p.1 • η‖ ≤ 2 := by
rw [norm_smul, Real.norm_eq_abs, abs_of_nonneg (theta_nonneg _)]
exact (mul_le_of_le_one_left (norm_nonneg η) (theta_le_one _)).trans hη
rw [Metric.mem_closedBall, dist_zero_right]
calc
‖p.2‖ = ‖(p.2 - θ p.1 • η) + θ p.1 • η‖ := by rw [sub_add_cancel]
_ ≤ ‖p.2 - θ p.1 • η‖ + ‖θ p.1 • η‖ := norm_add_le _ _
_ ≤ 3 := by linarith
lemma scaledCurl_derivative_bound (k : ℕ) : ∃ C : ℝ, 0 ≤ C ∧
∀ η ξ : Space, ‖η‖ ≤ 2 → ‖ξ‖ ≤ 2 → ∀ p : ℝ × Space,
‖iteratedFDeriv ℝ k (Function.uncurry (scaledCurl η ξ)) p‖ ≤ C := by
obtain ⟨C, hC, hb⟩ := uniform_derivative_bound scaledCurl_joint_smooth
((isCompact_closedBall (0 : Space) 2).prod (isCompact_closedBall (0 : Space) 2))
(isCompact_Icc.prod (isCompact_closedBall (0 : Space) 3))
(fun a ha => scaledCurl_tsupport_subset a.1 a.2 (by simpa using ha.1)) k
refine ⟨C, hC, ?_⟩
intro η ξ hη hξ p
exact hb (η, ξ) (by simpa using And.intro hη hξ) p
lemma cross_smul_left (r : ℝ) (a b : Space) : cross (r • a) b = r • cross a b := by
ext i
fin_cases i <;> dsimp [cross, vec] <;> ring
lemma cross_smul_right (r : ℝ) (a b : Space) : cross a (r • b) = r • cross a b := by
ext i
fin_cases i <;> dsimp [cross, vec] <;> ring
lemma curl_const_smul {V : Space → Space} (hV : Differentiable ℝ V) (r : ℝ) (x : Space) :
curl (fun y => r • V y) x = r • curl V x := by
have hd : fderiv ℝ (fun y => r • V y) x = r • fderiv ℝ V x :=
fderiv_const_smul (hV x) r
ext i
fin_cases i <;> simp only [curl, hd, vec, smul_apply] <;>
dsimp <;> ring
lemma curl_affine_scale {V : Space → Space} (hV : Differentiable ℝ V)
(a : Space) (r : ℝ) (x : Space) :
curl (fun y => V (r • (y - a))) x = r • curl V (r • (x - a)) := by
have ha : HasFDerivAt (fun y : Space => r • (y - a))
(r • ContinuousLinearMap.id ℝ Space) x :=
((hasFDerivAt_id x).sub_const a).const_smul r
have hd : fderiv ℝ (fun y => V (r • (y - a))) x =
(fderiv ℝ V (r • (x - a))).comp (r • ContinuousLinearMap.id ℝ Space) :=
((hV (r • (x - a))).hasFDerivAt.comp x ha).fderiv
ext i
fin_cases i <;> simp only [curl, hd, vec, ContinuousLinearMap.comp_apply,
smul_apply, ContinuousLinearMap.id_apply, map_smul] <;>
dsimp <;> ring
lemma movingCurl_normalized (a v : Space) {δ b : ℝ} (hδ : δ ≠ 0) (hb : b ≠ 0)
(σ : ℝ) (x : Space) :
movingCurl (fun s => a + θ s • v) δ σ x =
b • scaledCurl (δ⁻¹ • v) (b⁻¹ • v) σ (δ⁻¹ • (x - a)) := by
let η := δ⁻¹ • v
let ξ := b⁻¹ • v
let V : Space → Space := fun z => ζ (z - θ σ • η) •
((1 / 2 : ℝ) • cross (deriv θ σ • ξ) (z - θ σ • η))
have hθ := theta_smooth
have hVsm : ContDiff ℝ (⊤ : ℕ∞) V := by
apply (contDiff_piLp 2).mpr
intro i
fin_cases i <;> dsimp [V, cross, vec] <;> unfold ζ <;> fun_prop
have hV : Differentiable ℝ V := hVsm.differentiable (by simp)
have hc : deriv (fun s => a + θ s • v) σ = deriv θ σ • v := by
rw [deriv_const_add, deriv_smul_const]
exact theta_smooth.differentiable (by simp) σ
have hz (y : Space) : δ⁻¹ • (y - (a + θ σ • v)) = δ⁻¹ • (y - a) - θ σ • η := by
dsimp [η]
module
have hz' (y : Space) : y - (a + θ σ • v) =
δ • (δ⁻¹ • (y - a) - θ σ • η) := by
rw [← hz, smul_inv_smul₀ hδ]
have hv : deriv θ σ • v = b • (deriv θ σ • ξ) := by
dsimp [ξ]
rw [smul_comm (deriv θ σ), smul_inv_smul₀ hb]
have hpot : (fun y => ζ (δ⁻¹ • (y - (a + θ σ • v))) •
((1 / 2 : ℝ) • cross (deriv (fun s => a + θ s • v) σ) (y - (a + θ σ • v)))) =
(fun y => (b * δ) • V (δ⁻¹ • (y - a))) := by
funext y
rw [hc, hz, hv, hz' y, cross_smul_left, cross_smul_right]
dsimp [V]
module
change curl _ x = _
have hcomp : Differentiable ℝ (fun y => V (δ⁻¹ • (y - a))) :=
hV.comp (((differentiable_id : Differentiable ℝ (fun y : Space => y)).sub_const a).const_smul δ⁻¹)
rw [hpot, curl_const_smul hcomp, curl_affine_scale hV]
change (b * δ) • (δ⁻¹ • _) = _
rw [smul_smul, mul_assoc, mul_inv_cancel₀ hδ, mul_one]
rfl
lemma address_parameters_bound (M : Machine) (w : M.Input) (n m : ℕ) :
‖((M.scaleData w).δ n)⁻¹ • addressDisplacement M (M.scaleData w) n m‖ ≤ 2 ∧
‖((M.scaleData w).b n)⁻¹ • addressDisplacement M (M.scaleData w) n m‖ ≤ 2 := by
let d := M.scaleData w
have hv := addressDisplacement_bound M w n m
have hη (i : Fin 3) : |(d.δ n)⁻¹ * addressDisplacement M d n m i| ≤ 1 := by
rw [abs_mul, abs_of_pos (inv_pos.mpr (d.delta_pos n))]
exact (mul_le_mul_of_nonneg_left (hv i) (inv_nonneg.mpr (d.delta_nonneg n))).trans
((inv_mul_le_one₀ (d.delta_pos n)).mpr <| (d.b_lt_delta_div_eight n).le.trans
(by linarith [d.delta_nonneg n]))
have hξ (i : Fin 3) : |(d.b n)⁻¹ * addressDisplacement M d n m i| ≤ 1 := by
rw [abs_mul, abs_of_pos (inv_pos.mpr (d.b_pos n))]
exact (mul_le_mul_of_nonneg_left (hv i) (inv_nonneg.mpr (d.b_pos n).le)).trans_eq
(inv_mul_cancel₀ (d.b_pos n).ne')
constructor
· simpa only [mul_one] using norm_le_two_mul_of_coord_le (r := 1) (by norm_num) hη
· simpa only [mul_one] using norm_le_two_mul_of_coord_le (r := 1) (by norm_num) hξ
section ScaledBounds
variable {E : Type} [NormedAddCommGroup E] [NormedSpace ℝ E]
lemma norm_iteratedFDeriv_scaled {g : ℝ × Space → E}
(hg : ContDiff ℝ (⊤ : ℕ∞) g) {k : ℕ} {C r : ℝ}
(hC : ∀ p, ‖iteratedFDeriv ℝ k g p‖ ≤ C) (hr : 1 ≤ r)
(b τ : ℝ) (a : Space) (p : ℝ × Space) :
‖iteratedFDeriv ℝ k (fun q : ℝ × Space =>
b • g (q.1 - τ, r • (q.2 - a))) p‖ ≤ |b| * C * r ^ k := by
let L : (ℝ × Space) →L[ℝ] (ℝ × Space) :=
(ContinuousLinearMap.fst ℝ ℝ Space).prod (r • ContinuousLinearMap.snd ℝ ℝ Space)
have hL : ‖L‖ ≤ r := by
apply ContinuousLinearMap.opNorm_le_bound _ (by linarith)
intro q
change max ‖q.1‖ ‖r • q.2‖ ≤ r * ‖q‖
apply max_le
· exact (norm_fst_le q).trans (le_mul_of_one_le_left (norm_nonneg _) hr)
· rw [norm_smul, Real.norm_eq_abs, abs_of_nonneg (by linarith : 0 ≤ r)]
exact mul_le_mul_of_nonneg_left (norm_snd_le q) (by linarith)
have heq : (fun q : ℝ × Space => g (q.1 - τ, r • (q.2 - a))) =
(fun q => (g ∘ L) (q - (τ, a))) := rfl
have hsm : ContDiff ℝ (⊤ : ℕ∞) (fun q : ℝ × Space =>
g (q.1 - τ, r • (q.2 - a))) := by
exact hg.comp ((contDiff_fst.sub contDiff_const).prodMk
((contDiff_snd.sub contDiff_const).const_smul r))
rw [iteratedFDeriv_const_smul_apply' (hsm.of_le
(by exact_mod_cast (le_top : (k : ℕ∞) ≤ ⊤))).contDiffAt, heq,
iteratedFDeriv_comp_sub, L.iteratedFDeriv_comp_right hg _
(by exact_mod_cast (le_top : (k : ℕ∞) ≤ ⊤)), norm_smul, Real.norm_eq_abs]
calc
_ ≤ |b| * (‖iteratedFDeriv ℝ k g (L (p - (τ, a)))‖ * ‖L‖ ^ k) := by
apply mul_le_mul_of_nonneg_left _ (abs_nonneg _)
simpa using (iteratedFDeriv ℝ k g (L (p - (τ, a)))).norm_compContinuousLinearMap_le
(fun _ : Fin k => L)
_ ≤ |b| * (C * r ^ k) := by
apply mul_le_mul_of_nonneg_left _ (abs_nonneg _)
exact mul_le_mul (hC _) (pow_le_pow_left₀ (norm_nonneg _) hL _)
(pow_nonneg (norm_nonneg _) _) ((norm_nonneg _).trans (hC (0, 0)))
_ = _ := by ring
end ScaledBounds
lemma addressCurl_derivative_bound (M : Machine) (w : M.Input) (k : ℕ) :
∃ C : ℝ, 0 ≤ C ∧ ∀ n m : ℕ, ∀ p : ℝ × Space,
‖iteratedFDeriv ℝ k (fun q : ℝ × Space =>
movingCurl (addressPath M (M.scaleData w) n m) ((M.scaleData w).δ n)
(q.1 - 1 - (n : ℝ)) q.2) p‖ ≤
C * ((M.scaleData w).b n / (M.scaleData w).δ n ^ k) := by
obtain ⟨C, hC, hbound⟩ := scaledCurl_derivative_bound k
refine ⟨C, hC, ?_⟩
intro n m p
let d := M.scaleData w
let η := (d.δ n)⁻¹ • addressDisplacement M d n m
let ξ := (d.b n)⁻¹ • addressDisplacement M d n m
have hsm : ContDiff ℝ (⊤ : ℕ∞) (Function.uncurry (scaledCurl η ξ)) := by
change ContDiff ℝ (⊤ : ℕ∞)
((fun p : (Space × Space) × (ℝ × Space) => scaledCurl p.1.1 p.1.2 p.2.1 p.2.2) ∘
(fun q : ℝ × Space => ((η, ξ), q)))
exact scaledCurl_joint_smooth.comp (contDiff_const.prodMk contDiff_id)
have heq : (fun q : ℝ × Space => movingCurl (addressPath M d n m) (d.δ n)
(q.1 - 1 - (n : ℝ)) q.2) = (fun q : ℝ × Space =>
d.b n • scaledCurl η ξ (q.1 - (1 + (n : ℝ)))
((d.δ n)⁻¹ • (q.2 - addressCenter d n m))) := by
funext q
change movingCurl (fun s => addressCenter d n m + θ s • addressDisplacement M d n m)
(d.δ n) _ _ = _
rw [movingCurl_normalized _ _ (d.delta_pos n).ne' (d.b_pos n).ne']
change d.b n • scaledCurl η ξ (q.1 - 1 - (n : ℝ)) _ = _
congr 2
ring
rw [heq]
have hparam := address_parameters_bound M w n m
have hbnd := norm_iteratedFDeriv_scaled hsm (hbound η ξ hparam.1 hparam.2)
((one_le_inv₀ (d.delta_pos n)).mpr (d.delta_le_one n))
(d.b n) (1 + (n : ℝ)) (addressCenter d n m) p
convert hbnd using 1
· rfl
· change C * (d.b n / d.δ n ^ k) = _
rw [abs_of_pos (d.b_pos n), inv_pow]
ring
lemma stepVelocity_derivative_bound (M : Machine) (w : M.Input) (k : ℕ) :
∃ C : ℝ, 0 ≤ C ∧ ∀ n : ℕ, ∀ p : ℝ × Space,
‖iteratedFDeriv ℝ k (Function.uncurry (stepVelocity M w n)) p‖ ≤
C * ((M.scaleData w).b n / (M.scaleData w).δ n ^ (k + 1)) := by
obtain ⟨C, hC, hb⟩ := addressCurl_derivative_bound M w k
refine ⟨2 * C, by positivity, ?_⟩
intro n p
let d := M.scaleData w
have hsm (m : ℕ) : ContDiff ℝ (⊤ : ℕ∞)
(fun q : ℝ × Space => movingCurl (addressPath M d n m) (d.δ n)
(q.1 - 1 - (n : ℝ)) q.2) := by
change ContDiff ℝ (⊤ : ℕ∞)
(Function.uncurry (movingCurl (addressPath M d n m) (d.δ n)) ∘
(fun q : ℝ × Space => (q.1 - 1 - (n : ℝ), q.2)))
exact (movingCurl_joint_smooth (addressPath_smooth M d n m) _).comp
((contDiff_fst.sub contDiff_const |>.sub contDiff_const).prodMk contDiff_snd)
have hsum := iteratedFDeriv_fun_sum_apply (𝕜 := ℝ) (n := k) (x := p)
(u := Finset.range (d.D n + 1)) (fun m _ => (hsm m |>.of_le
(by exact_mod_cast (le_top : (k : ℕ∞) ≤ ⊤))).contDiffAt)
change ‖iteratedFDeriv ℝ k (fun q : ℝ × Space => ∑ m ∈ Finset.range (d.D n + 1),
movingCurl (addressPath M d n m) (d.δ n) (q.1 - 1 - (n : ℝ)) q.2) p‖ ≤ _
rw [hsum]
calc
_ ≤ ∑ m ∈ Finset.range (d.D n + 1),
‖iteratedFDeriv ℝ k (fun q : ℝ × Space => movingCurl (addressPath M d n m)
(d.δ n) (q.1 - 1 - (n : ℝ)) q.2) p‖ := norm_sum_le _ _
_ ≤ ∑ m ∈ Finset.range (d.D n + 1), C * (d.b n / d.δ n ^ k) :=
Finset.sum_le_sum (fun m _ => hb n m p)
_ = ((d.D n : ℝ) + 1) * (C * (d.b n / d.δ n ^ k)) := by simp
_ ≤ (2 * (d.δ n)⁻¹) * (C * (d.b n / d.δ n ^ k)) := by
apply mul_le_mul_of_nonneg_right _ (mul_nonneg hC
(div_nonneg (d.b_pos n).le (pow_nonneg (d.delta_nonneg n) k)))
have hD : (d.D n : ℝ) = (d.δ n)⁻¹ := by rw [d.delta_eq_inv_D, inv_inv]
rw [hD]
have hi := (one_le_inv₀ (d.delta_pos n)).mpr (d.delta_le_one n)
linarith
_ = (2 * C) * (d.b n / d.δ n ^ (k + 1)) := by rw [pow_succ]; field_simp
end RapidForcing
end OAI