feat(analysis/convex/integral): strict Jensen's inequality (#11552)

src/analysis/convex/integral.lean

@@ -4,6 +4,8 @@ Released under Apache 2.0 license as described in the file LICENSE.
 Authors: Yury G. Kudryashov
 -/
 import analysis.convex.function
+import analysis.convex.strict
+import measure_theory.function.ae_eq_of_integral
 import measure_theory.integral.average
 
 /-!
@@ -16,6 +18,17 @@ In this file we prove several forms of Jensen's inequality for integrals.
 - for convex functions: `convex.on.average_mem_epigraph`, `convex_on.map_average_le`,
   `convex_on.set_average_mem_epigraph`, `convex_on.map_set_average_le`, `convex_on.map_integral_le`;
 
+- for strictly convex sets: `strict_convex.ae_eq_const_or_average_mem_interior`;
+
+- for a closed ball in a strictly convex normed space:
+  `strict_convex.ae_eq_const_or_norm_integral_lt_of_norm_le_const`
+
+- for strictly convex functions: `strict_convex_on.ae_eq_const_or_map_average_lt`.
+
+## TODO
+
+- Use a typeclass for strict convexity of a closed ball.
+
 ## Tags
 
 convex, integral, center mass, average value, Jensen's inequality
@@ -31,6 +44,10 @@ variables {α E F : Type*} {m0 : measurable_space α}
   [topological_space.second_countable_topology F] [measurable_space F] [borel_space F]
   {μ : measure α} {s : set E}
 
+/-!
+### Non-strict Jensen's inequality
+-/
+
 /-- If `μ` is a probability measure on `α`, `s` is a convex closed set in `E`, and `f` is an
 integrable function sending `μ`-a.e. points to `s`, then the expected value of `f` belongs to `s`:
 `∫ x, f x ∂μ ∈ s`. See also `convex.sum_mem` for a finite sum version of this lemma. -/
@@ -195,3 +212,125 @@ lemma concave_on.le_map_integral [is_probability_measure μ] {s : set E} {g : E
   ∫ x, g (f x) ∂μ ≤ g (∫ x, f x ∂μ) :=
 by simpa only [average_eq_integral]
   using hg.le_map_average hgc hsc (is_probability_measure.ne_zero μ) hfs hfi hgi
+
+/-!
+### Strict Jensen's inequality
+-/
+
+/-- If `f : α → E` is an integrable function, then either it is a.e. equal to the constant
+`⨍ x, f x ∂μ` or there exists a measurable set such that `μ s ≠ 0`, `μ sᶜ ≠ 0`, and the average
+values of `f` over `s` and `sᶜ` are different. -/
+lemma measure_theory.integrable.ae_eq_const_or_exists_average_ne_compl [is_finite_measure μ]
+  {f : α → E} (hfi : integrable f μ) :
+  (f =ᵐ[μ] const α (⨍ x, f x ∂μ)) ∨ ∃ s, measurable_set s ∧ μ s ≠ 0 ∧ μ sᶜ ≠ 0 ∧
+    ⨍ x in s, f x ∂μ ≠ ⨍ x in sᶜ, f x ∂μ :=
+begin
+  refine or_iff_not_imp_right.mpr (λ H, _), push_neg at H,
+  refine hfi.ae_eq_of_forall_set_integral_eq _ _ (integrable_const _) (λ s hs hs', _), clear hs',
+  simp only [const_apply, set_integral_const],
+  by_cases h₀ : μ s = 0,
+  { rw [restrict_eq_zero.2 h₀, integral_zero_measure, h₀, ennreal.zero_to_real, zero_smul] },
+  by_cases h₀' : μ sᶜ = 0,
+  { rw ← ae_eq_univ at h₀',
+    rw [restrict_congr_set h₀', restrict_univ, measure_congr h₀', measure_smul_average] },
+  have := average_mem_open_segment_compl_self hs.null_measurable_set h₀ h₀' hfi,
+  rw [← H s hs h₀ h₀', open_segment_same, mem_singleton_iff] at this,
+  rw [this, measure_smul_set_average _ (measure_ne_top μ _)]
+end
+
+/-- If an integrable function `f : α → E` takes values in a convex closed set `s` and for some set
+`t` of positive measure, the average value of `f` over `t` belongs to the interior of `s`, then the
+average of `f` over the whole space belongs to the interior of `s`. -/
+lemma convex.average_mem_interior_of_set [is_finite_measure μ] {t : set α} {s : set E}
+  (hs : convex ℝ s) (hsc : is_closed s) (h0 : μ t ≠ 0) {f : α → E} (hfs : ∀ᵐ x ∂μ, f x ∈ s)
+  (hfi : integrable f μ) (ht : ⨍ x in t, f x ∂μ ∈ interior s) :
+  ⨍ x, f x ∂μ ∈ interior s :=
+begin
+  rw ← measure_to_measurable at h0, rw ← restrict_to_measurable (measure_ne_top μ t) at ht,
+  by_cases h0' : μ (to_measurable μ t)ᶜ = 0,
+  { rw ← ae_eq_univ at h0',
+    rwa [restrict_congr_set h0', restrict_univ] at ht },
+  exact hs.open_segment_subset_interior_left ht
+    (hs.set_average_mem hsc h0' (measure_ne_top _ _) (ae_restrict_of_ae hfs) hfi.integrable_on)
+    (average_mem_open_segment_compl_self (measurable_set_to_measurable μ t).null_measurable_set
+      h0 h0' hfi)
+end
+
+/-- If an integrable function `f : α → E` takes values in a strictly convex closed set `s`, then
+either it is a.e. equal to its average value, or its average value belongs to the interior of
+`s`. -/
+lemma strict_convex.ae_eq_const_or_average_mem_interior [is_finite_measure μ] {s : set E}
+  (hs : strict_convex ℝ s) (hsc : is_closed s) {f : α → E} (hfs : ∀ᵐ x ∂μ, f x ∈ s)
+  (hfi : integrable f μ) :
+  f =ᵐ[μ] const α (⨍ x, f x ∂μ) ∨ ⨍ x, f x ∂μ ∈ interior s :=
+begin
+  have : ∀ {t}, μ t ≠ 0 → ⨍ x in t, f x ∂μ ∈ s,
+    from λ t ht, hs.convex.set_average_mem hsc ht (measure_ne_top _ _) (ae_restrict_of_ae hfs)
+      hfi.integrable_on,
+  refine hfi.ae_eq_const_or_exists_average_ne_compl.imp_right _,
+  rintro ⟨t, hm, h₀, h₀', hne⟩,
+  exact hs.open_segment_subset (this h₀) (this h₀') hne
+    (average_mem_open_segment_compl_self hm.null_measurable_set h₀ h₀' hfi)
+end
+
+/-- **Jensen's inequality**, strict version: if an integrable function `f : α → E` takes values in a
+convex closed set `s`, and `g : E → ℝ` is continuous and strictly convex on `s`, then
+either `f` is a.e. equal to its average value, or `g (⨍ x, f x ∂μ) < ⨍ x, g (f x) ∂μ`. -/
+lemma strict_convex_on.ae_eq_const_or_map_average_lt [is_finite_measure μ] {s : set E} {g : E → ℝ}
+  (hg : strict_convex_on ℝ s g) (hgc : continuous_on g s) (hsc : is_closed s) {f : α → E}
+  (hfs : ∀ᵐ x ∂μ, f x ∈ s) (hfi : integrable f μ) (hgi : integrable (g ∘ f) μ) :
+  f =ᵐ[μ] const α (⨍ x, f x ∂μ) ∨ g (⨍ x, f x ∂μ) < ⨍ x, g (f x) ∂μ :=
+begin
+  have : ∀ {t}, μ t ≠ 0 → ⨍ x in t, f x ∂μ ∈ s ∧ g (⨍ x in t, f x ∂μ) ≤ ⨍ x in t, g (f x) ∂μ,
+    from λ t ht, hg.convex_on.set_average_mem_epigraph hgc hsc ht (measure_ne_top _ _)
+      (ae_restrict_of_ae hfs) hfi.integrable_on hgi.integrable_on,
+  refine hfi.ae_eq_const_or_exists_average_ne_compl.imp_right _,
+  rintro ⟨t, hm, h₀, h₀', hne⟩,
+  rcases average_mem_open_segment_compl_self hm.null_measurable_set h₀ h₀' (hfi.prod_mk hgi)
+    with ⟨a, b, ha, hb, hab, h_avg⟩,
+  simp only [average_pair hfi hgi, average_pair hfi.integrable_on hgi.integrable_on, prod.smul_mk,
+    prod.mk_add_mk, prod.mk.inj_iff, (∘)] at h_avg,
+  rw [← h_avg.1, ← h_avg.2],
+  calc g (a • ⨍ x in t, f x ∂μ + b • ⨍ x in tᶜ, f x ∂μ)
+      < a * g (⨍ x in t, f x ∂μ) + b * g (⨍ x in tᶜ, f x ∂μ) :
+    hg.2 (this h₀).1 (this h₀').1 hne ha hb hab
+  ... ≤ a * ⨍ x in t, g (f x) ∂μ + b * ⨍ x in tᶜ, g (f x) ∂μ :
+    add_le_add (mul_le_mul_of_nonneg_left (this h₀).2 ha.le)
+      (mul_le_mul_of_nonneg_left (this h₀').2 hb.le)
+end
+
+/-- **Jensen's inequality**, strict version: if an integrable function `f : α → E` takes values in a
+convex closed set `s`, and `g : E → ℝ` is continuous and strictly concave on `s`, then
+either `f` is a.e. equal to its average value, or `⨍ x, g (f x) ∂μ < g (⨍ x, f x ∂μ)`. -/
+lemma strict_concave_on.ae_eq_const_or_lt_map_average [is_finite_measure μ] {s : set E} {g : E → ℝ}
+  (hg : strict_concave_on ℝ s g) (hgc : continuous_on g s) (hsc : is_closed s) {f : α → E}
+  (hfs : ∀ᵐ x ∂μ, f x ∈ s) (hfi : integrable f μ) (hgi : integrable (g ∘ f) μ) :
+  f =ᵐ[μ] const α (⨍ x, f x ∂μ) ∨ ⨍ x, g (f x) ∂μ < g (⨍ x, f x ∂μ) :=
+by simpa only [pi.neg_apply, average_neg, neg_lt_neg_iff]
+  using hg.neg.ae_eq_const_or_map_average_lt hgc.neg hsc hfs hfi hgi.neg
+
+/-- If the closed ball of radius `C` in a normed space `E` is strictly convex and `f : α → E` is
+a function such that `∥f x∥ ≤ C` a.e., then either either this function is a.e. equal to its
+average value, or the norm of its integral is strictly less than `(μ univ).to_real * C`. -/
+lemma strict_convex.ae_eq_const_or_norm_integral_lt_of_norm_le_const [is_finite_measure μ]
+  {f : α → E} {C : ℝ} (h_convex : strict_convex ℝ (closed_ball (0 : E) C))
+  (h_le : ∀ᵐ x ∂μ, ∥f x∥ ≤ C) :
+  (f =ᵐ[μ] const α ⨍ x, f x ∂μ) ∨ ∥∫ x, f x ∂μ∥ < (μ univ).to_real * C :=
+begin
+  cases le_or_lt C 0 with hC0 hC0,
+  { have : f =ᵐ[μ] 0, from h_le.mono (λ x hx, norm_le_zero_iff.1 (hx.trans hC0)),
+    simp only [average_congr this, pi.zero_apply, average_zero],
+    exact or.inl this },
+  cases eq_or_ne μ 0 with hμ hμ,
+  { rw hμ, exact or.inl rfl },
+  by_cases hfi : integrable f μ, swap,
+  { right,
+    simpa [integral_undef hfi, hC0, measure_lt_top, ennreal.to_real_pos_iff, pos_iff_ne_zero]
+      using hμ },
+  replace h_le : ∀ᵐ x ∂μ, f x ∈ closed_ball (0 : E) C, by simpa only [mem_closed_ball_zero_iff],
+  have hμ' : 0 < (μ univ).to_real,
+    from ennreal.to_real_pos (mt measure_univ_eq_zero.1 hμ) (measure_ne_top _ _),
+  simpa only [interior_closed_ball _ hC0.ne', mem_ball_zero_iff, average_def', norm_smul,
+    real.norm_eq_abs, abs_inv, abs_of_pos hμ', ← div_eq_inv_mul, div_lt_iff' hμ']
+    using h_convex.ae_eq_const_or_average_mem_interior is_closed_ball h_le hfi,
+end