@@ -63,10 +63,12 @@ lemma fourierCoeffOn_bound {f : ℝ → ℂ} (f_continuous : Continuous f) :
6363 ∃ C, ∀ n, ‖fourierCoeffOn Real.two_pi_pos f n‖ ≤ C := by
6464 obtain ⟨C, f_bounded⟩ := continuous_bounded f_continuous.continuousOn
6565 refine ⟨C, fun n ↦ ?_⟩
66- simp only [fourierCoeffOn_eq_integral, sub_zero, one_div, mul_inv_rev]
66+ simp only [fourierCoeffOn_eq_integral, sub_zero, one_div, mul_inv_rev, Complex.real_smul,
67+ Complex.norm_real, Complex.norm_mul, norm_eq_abs, abs_mul, abs_inv, Nat.abs_ofNat]
6768 field_simp
68- rw [abs_of_nonneg pi_pos.le, mul_comm π, div_le_iff₀ Real.two_pi_pos]
69- calc ‖∫ (x : ℝ) in (0 : ℝ)..(2 * π), (starRingEnd ℂ) (Complex.exp (2 * π * Complex.I * n * x / (2 * π))) * f x‖
69+ rw [abs_of_nonneg pi_pos.le, mul_comm π]
70+ calc
71+ _ = ‖∫ (x : ℝ) in (0 : ℝ)..(2 * π), (starRingEnd ℂ) (Complex.exp (2 * π * Complex.I * n * x / (2 * π))) * f x‖ := by simp
7072 _ = ‖∫ (x : ℝ) in (0 : ℝ)..(2 * π), (starRingEnd ℂ) (Complex.exp (Complex.I * n * x)) * f x‖ := by
7173 congr with x
7274 congr
@@ -88,7 +90,7 @@ lemma fourierCoeffOn_bound {f : ℝ → ℂ} (f_continuous : Continuous f) :
8890 /-Could specify `aestronglyMeasurable` and `intervalIntegrable` intead of `f_continuous`. -/
8991 exact IntervalIntegrable.intervalIntegrable_norm_iff f_continuous.aestronglyMeasurable |>.mpr
9092 (f_continuous.intervalIntegrable ..)
91- _ = C * ( 2 * π) := by simp; ring
93+ _ = _ := by simp
9294
9395/-TODO: Assumptions might be weakened. -/
9496lemma periodic_deriv {𝕜 : Type } [NontriviallyNormedField 𝕜] {F : Type } [NormedAddCommGroup F] [NormedSpace 𝕜 F]
@@ -150,9 +152,10 @@ lemma int_sum_nat {β : Type*} [AddCommGroup β] [TopologicalSpace β] [Continuo
150152 rw [←tendsto_add_atTop_iff_nat 1 ] at this
151153 convert this using 1
152154 ext N
153- induction' N with N ih
154- · simp
155- · have : Icc (- Int.ofNat (N.succ)) (N.succ) = insert (↑(N.succ)) (insert (-Int.ofNat (N.succ)) (Icc (-Int.ofNat N) N)) := by
155+ induction N with
156+ | zero => simp
157+ | succ N ih =>
158+ have : Icc (- Int.ofNat (N.succ)) (N.succ) = insert (↑(N.succ)) (insert (-Int.ofNat (N.succ)) (Icc (-Int.ofNat N) N)) := by
156159 rw [←Ico_insert_right, ←Ioo_insert_left]
157160 · congr 2 with n
158161 simp only [Int.ofNat_eq_coe, mem_Ioo, mem_Icc]
@@ -194,7 +197,7 @@ lemma fourierConv_ofTwiceDifferentiable {f : ℝ → ℂ} (periodicf : f.Periodi
194197 rw [summable_congr @fourierCoeff_correspondence, ←summable_norm_iff]
195198 apply summable_of_le_on_nonzero _ _ summable_maj <;> intro i
196199 · simp
197- · intro ine0; field_simp [maj_def, hC i ine0]
200+ · intro ine0; simpa only [maj_def, one_div_mul_eq_div] using hC i ine0
198201 have := int_sum_nat function_sum
199202 rw [ContinuousMap.tendsto_iff_tendstoUniformly, Metric.tendstoUniformly_iff] at this
200203 have := this ε εpos
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