@@ -19,7 +19,6 @@ theorem exceptional_set_carleson {f : ℝ → ℂ}
1919 set ε' := ε / 4 / C_control_approximation_effect ε with ε'def
2020 have ε'pos : ε' > 0 := div_pos (div_pos εpos (by norm_num))
2121 (C_control_approximation_effect_pos εpos)
22-
2322 /- Approximate f by a smooth f₀. -/
2423 have unicont_f : UniformContinuous f := periodic_f.uniformContinuous_of_continuous
2524 Real.two_pi_pos cont_f.continuousOn
@@ -33,19 +32,17 @@ theorem exceptional_set_carleson {f : ℝ → ℂ}
3332 have h_bound : ∀ x, ‖h x‖ ≤ ε' := by
3433 intro x
3534 simpa only [hdef, Pi.sub_apply, norm_sub_rev] using hf₀ x
36-
3735 /- Control approximation effect: Get a bound on the partial Fourier sums of h. -/
3836 obtain ⟨E, Esubset, Emeasurable, Evolume, hE⟩ := control_approximation_effect εpos ε'pos
3937 h_measurable h_periodic h_bound
40-
4138 /- This is a classical "epsilon third" argument. -/
4239 use E, Esubset, Emeasurable, Evolume, N₀
4340 intro x hx N NgtN₀
4441 calc ‖f x - S_ N f x‖
4542 _ = ‖(f x - f₀ x) + (f₀ x - S_ N f₀ x) + (S_ N f₀ x - S_ N f x)‖ := by ring_nf
4643 _ ≤ ‖(f x - f₀ x) + (f₀ x - S_ N f₀ x)‖ + ‖S_ N f₀ x - S_ N f x‖ := norm_add_le ..
4744 _ ≤ ‖f x - f₀ x‖ + ‖f₀ x - S_ N f₀ x‖ + ‖S_ N f₀ x - S_ N f x‖ :=
48- add_le_add_right (norm_add_le ..) _
45+ add_le_add_left (norm_add_le ..) _
4946 _ ≤ ε' + (ε / 4 ) + (ε / 4 ) := by
5047 gcongr
5148 · exact hf₀ x
@@ -66,17 +63,14 @@ theorem exceptional_set_carleson {f : ℝ → ℂ}
6663theorem carleson_interval {f : ℝ → ℂ} (cont_f : Continuous f) (periodic_f : f.Periodic (2 * π)) :
6764 ∀ᵐ x ∂volume.restrict (Set.Icc 0 (2 * π)),
6865 Filter.Tendsto (S_ · f x) Filter.atTop (nhds (f x)) := by
69-
7066 let δ (k : ℕ) : ℝ := 1 / (k + 1 ) --arbitrary sequence tending to zero
7167 have δconv : Filter.Tendsto δ Filter.atTop (nhds 0 ) := tendsto_one_div_add_atTop_nhds_zero_nat
7268 have δpos (k : ℕ) : 0 < δ k := by apply div_pos zero_lt_one (by linarith)
73-
7469 -- ENNReal version to be comparable to volumes
7570 let δ' (k : ℕ) := ENNReal.ofReal (δ k)
7671 have δ'conv : Filter.Tendsto δ' Filter.atTop (nhds 0 ) := by
7772 rw [← ENNReal.ofReal_zero]
7873 exact ENNReal.tendsto_ofReal δconv
79-
8074 set ε := fun k n ↦ (1 / 2 ) ^ n * 2 ⁻¹ * δ k with εdef
8175 have εpos (k n : ℕ) : 0 < ε k n := by positivity
8276 have εsmall (k : ℕ) {e : ℝ} (epos : 0 < e) : ∃ n, ε k n < e := by
@@ -91,7 +85,6 @@ theorem carleson_interval {f : ℝ → ℂ} (cont_f : Continuous f) (periodic_f
9185 use n
9286 convert (hn n (by simp))
9387 simp_rw [dist_zero_right, Real.norm_eq_abs, abs_of_nonneg (εpos k n).le]
94-
9588 have δ'_eq {k : ℕ} : δ' k = ∑' n, ENNReal.ofReal (ε k n) := by
9689 rw [εdef ]
9790 conv => rhs; pattern ENNReal.ofReal _; rw [ENNReal.ofReal_mul' (δpos k).le,
@@ -102,43 +95,35 @@ theorem carleson_interval {f : ℝ → ℂ} (cont_f : Continuous f) (periodic_f
10295 conv => pattern ENNReal.ofReal _; ring_nf; rw [ENNReal.ofReal_one]
10396 · rw [one_mul]
10497 norm_num
105-
10698 -- Main step: Apply exceptional_set_carleson to get a family of exceptional sets parameterized by ε.
10799 choose Eε hEε_subset _ hEε_measure hEε using (@exceptional_set_carleson f cont_f periodic_f)
108-
109100 have Eεmeasure {ε : ℝ} (hε : 0 < ε) : volume (Eε hε) ≤ ENNReal.ofReal ε := by
110101 rw [ENNReal.le_ofReal_iff_toReal_le _ hε.le]
111102 · exact hEε_measure hε
112103 · rw [← lt_top_iff_ne_top]
113104 apply lt_of_le_of_lt (measure_mono (hEε_subset hε)) measure_Icc_lt_top
114-
115105 -- Define exceptional sets parameterized by δ.
116106 let Eδ (k : ℕ) := ⋃ (n : ℕ), Eε (εpos k n)
117107 have Eδmeasure (k : ℕ) : volume (Eδ k) ≤ δ' k := by
118108 apply le_trans (measure_iUnion_le _)
119109 rw [δ'_eq]
120110 exact ENNReal.tsum_le_tsum (fun n ↦ Eεmeasure (εpos k n))
121-
122111 -- Define final exceptional set.
123112 let E := ⋂ (k : ℕ), Eδ k
124-
125113 -- Show that it has the desired property.
126114 have hE : ∀ x ∈ (Set.Icc 0 (2 * π)) \ E, Filter.Tendsto (S_ · f x) Filter.atTop (nhds (f x)) := by
127115 intro x hx
128116 rw [Set.diff_iInter, Set.mem_iUnion] at hx
129117 rcases hx with ⟨k,hk⟩
130118 rw [Set.diff_iUnion, Set.mem_iInter] at hk
131-
132119 rw [Metric.tendsto_atTop']
133120 intro e epos
134121 rcases (εsmall k epos) with ⟨n, lt_e⟩
135-
136122 rcases (hEε (εpos k n)) with ⟨N₀,hN₀⟩
137123 use N₀
138124 intro N hN
139125 rw [dist_comm, dist_eq_norm]
140126 exact (hN₀ x (hk n) N hN).trans_lt lt_e
141-
142127 -- Show that is has measure zero.
143128 have Emeasure : volume E ≤ 0 := by
144129 have : ∀ k, volume E ≤ δ' k := by
@@ -147,7 +132,6 @@ theorem carleson_interval {f : ℝ → ℂ} (cont_f : Continuous f) (periodic_f
147132 apply measure_mono
148133 apply Set.iInter_subset
149134 exact ge_of_tendsto' δ'conv this
150-
151135 -- Use results to prove the statement.
152136 rw [ae_restrict_iff' measurableSet_Icc]
153137 apply le_antisymm _ (zero_le _)
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