import Mathlib import OAI.Probability.LogConcave.LowerBounds.DensityCoefficient import OAI.Probability.LogConcave.Sampling.Map namespace OAI section section noncomputable section open MeasureTheory Filter open scoped ENNReal NNReal Topology section LowerProof open Matrix Topology TopologicalSpace ProbabilityTheory Classical WithLp open scoped Matrix.Norms.Elementwise open WithLp open MeasureTheory ProbabilityTheory open scoped ENNReal NNReal open Matrix open Polynomial open scoped BigOperators namespace LogConcaveSampling.LowerBound open WithLp open scoped RealInnerProductSpace lemma spectrum_iterate_apply {d : ℕ} (j : ℕ) (x : Point d) (k : Fin d) : ((spectrumOperator d).toLinearMap^[j]) x k = x k * eigenvalue d k ^ j := by induction j with | zero => simp | succ j ih => rw [Function.iterate_succ_apply'] change spectrumOperator d (((spectrumOperator d).toLinearMap^[j]) x) k = _ rw [spectrumOperator_apply,ih,pow_succ] ring lemma orbitHull_in_krylov {d r : ℕ} (a : Fin (r+1) → Point d) {u : Point d} (hu : u ∈ orbitHull (spectrumOperator d).toLinearMap r a) : ofLp u ∈ krylovSpace (fun k i => a i k) := by let coord : Point d →ₗ[ℝ] (Fin d → ℝ) := {toFun := ofLp, map_add' := by intros; rfl, map_smul' := by intros; rfl} have h : orbitHull (spectrumOperator d).toLinearMap r a ≤ (krylovSpace (fun k i => a i k)).comap coord := by apply Submodule.span_le.mpr rintro v ⟨i,j,hj,rfl⟩ apply Submodule.subset_span refine ⟨i,j,hj,?_⟩ funext k exact spectrum_iterate_apply j (a i) k exact h hu lemma linearReply_eq_quadratic {d : ℕ} (O : Rotations d) (x : Point d) : linearReply (spectrumOperator d) O x = firstOrderReply (quadraticPotential O) x := by rw [quadratic_firstOrderReply] apply Prod.ext · change ⟪(rotationIsometry O).symm x, spectrumOperator d ((rotationIsometry O).symm x)⟫ / 2 = _ have h := (rotationIsometry O).inner_map_map ((rotationIsometry O).symm x) (spectrumOperator d ((rotationIsometry O).symm x)) simpa only [LinearIsometryEquiv.apply_symm_apply, quadraticOperator, ContinuousLinearMap.comp_apply, LinearIsometryEquiv.coe_toContinuousLinearEquiv, ContinuousLinearEquiv.coe_coe] using congrArg (fun z : ℝ => z/2) h.symm · rfl lemma linearHistory_eq_quadratic {Ω : Type*} [MeasurableSpace Ω] {d q : ℕ} (A : OracleAlgorithm Ω d q) (O : Rotations d) (ω : Ω) (n : ℕ) : linearHistory A (spectrumOperator d) O ω n = A.history (quadraticPotential O) ω n := by induction n with | zero => rfl | succ n ih => simp only [linearHistory,OracleAlgorithm.history,ih,linearReply_eq_quadratic] def hardEigenOutput {Ω : Type*} [MeasurableSpace Ω] {d q : ℕ} (A : OracleAlgorithm Ω d q) (p : Ω × Rotations d) : Fin d → ℝ := ofLp ((rotationIsometry p.2).symm (A.run (quadraticPotential p.2) p.1)) lemma measurable_hardEigenOutput {Ω : Type*} [MeasurableSpace Ω] {d q : ℕ} (A : OracleAlgorithm Ω d q) : Measurable (hardEigenOutput A) := by have hr : Measurable (fun p : Ω × Rotations d => A.run (quadraticPotential p.2) p.1) := by simpa only [OracleAlgorithm.run,← linearHistory_eq_quadratic,Function.comp_def] using A.output_measurable.comp (measurable_fst.prodMk (measurable_linearHistory A (spectrumOperator d) q)) have haction : Measurable (fun p : Rotations d × Point d => (rotationIsometry p.1).symm p.2) := by have hh := (continuous_rotation_apply (d := d)).measurable.comp (f := fun p : Rotations d × Point d => (p.1⁻¹,p.2)) (measurable_fst.inv.prodMk measurable_snd) simpa only [Function.comp_def,rotationIsometry_inv_apply] using hh exact (measurable_ofLp 2 _).comp (f := fun p : Ω × Rotations d => (rotationIsometry p.2).symm (A.run (quadraticPotential p.2) p.1)) (haction.comp (f := fun p : Ω × Rotations d => (p.2,A.run (quadraticPotential p.2) p.1)) (measurable_snd.prodMk hr)) theorem hard_algorithm_separation {Ω : Type*} [MeasurableSpace Ω] {d q : ℕ} (μ : Measure Ω) [IsProbabilityMeasure μ] (A : OracleAlgorithm Ω d q) (hd : 2*q+1 ≤ d) (hqd : 4*(q+q)+1 < 2*d) (ht : blockTolerance d (q+q) ≤ 1/2) : (μ.prod (rotationHaar d)).real {p | hardEigenOutput A p ∈ separationEvent d (q+q)} ≤ 1/100 := by let P := couplingLaw μ d q let U : CouplingNoise Ω d q → Fin d → ℝ := fun p => ofLp (finishCoordinate A (by omega) (couplingState A (spectrumOperator d) hd p).1) let rows : CouplingNoise Ω d q → Fin d → Fin (q+q+1) → ℝ := fun p k i => couplingColumns p i k have hrows : HasLaw rows (gaussianRows d (q+q+1)) P := by have he : MeasurePreserving (fun a : Fin (q+q+1) → Point d => fun k i => a i k) (Measure.pi (fun _ : Fin (q+q+1) => stdGaussian (Point d))) (gaussianRows d (q+q+1)) := ⟨by fun_prop,gaussianColumns_rows_law d (q+q+1)⟩ exact (he.comp (couplingColumns_law μ)).hasLaw have hU : ∀ᵐ p ∂P, U p ∈ krylovSpace (rows p) := by filter_upwards [coupling_output_span μ A (spectrumOperator d) hd] with p hp exact orbitHull_in_krylov (couplingColumns p) hp have hb := algorithm_span_separation_bound hqd ht hrows U hU have he : ∀ᵐ p ∂P, U p = hardEigenOutput A (finishComplete A (by omega) (couplingState A (spectrumOperator d) hd p)) := by filter_upwards [coupling_output_correct μ A (spectrumOperator d) hd] with p hp simpa only [hardEigenOutput,OracleAlgorithm.run,linearHistory_eq_quadratic] using congrArg ofLp hp have hmeas : MeasurableSet {p | hardEigenOutput A p ∈ separationEvent d (q+q)} := (separationEvent_measurable d (q+q)).preimage (measurable_hardEigenOutput A) rw [← coupling_complete_law μ A (spectrumOperator d) hd, Measure.real,Measure.map_apply ((measurable_finishComplete A (by omega)).comp (measurable_couplingState A (spectrumOperator d) hd)) hmeas] change P.real {p | hardEigenOutput A (finishComplete A (by omega) (couplingState A (spectrumOperator d) hd p)) ∈ separationEvent d (q+q)} ≤ _ have hs : {p | hardEigenOutput A (finishComplete A (by omega) (couplingState A (spectrumOperator d) hd p)) ∈ separationEvent d (q+q)} =ᵐ[P] {p | U p ∈ separationEvent d (q+q)} := by filter_upwards [he] with p hp exact congrArg (fun z : Fin d → ℝ => z ∈ separationEvent d (q+q)) hp.symm rwa [measureReal_congr hs] end LogConcaveSampling.LowerBound namespace LogConcaveSampling.LowerBound lemma quadratic_gibbs_eigenframe {d : ℕ} (O : Rotations d) : (gibbs (quadraticPotential O)).map (fun x => ofLp ((rotationIsometry O).symm x)) = targetGaussian d := by rw [← quadratic_gibbs_eq_gaussian O,Measure.map_map (by fun_prop) (by fun_prop)] simp only [Function.comp_def,LinearIsometryEquiv.symm_apply_apply,WithLp.ofLp_toLp] exact Measure.map_id lemma hardEigenOutput_tv {Ω : Type*} [MeasurableSpace Ω] {d q : ℕ} (μ : Measure Ω) [IsProbabilityMeasure μ] (A : OracleAlgorithm Ω d q) (h : ∀ O : Rotations d, TVAtMost (μ.map (A.run (quadraticPotential O))) (gibbs (quadraticPotential O)) (1/10)) (O : Rotations d) : TVAtMost (μ.map (fun ω => hardEigenOutput A (ω,O))) (targetGaussian d) (1/10) := by have hr : Measurable (A.run (quadraticPotential O)) := by change Measurable (fun ω : Ω => A.output (ω,A.history (quadraticPotential O) ω q)) simp_rw [← linearHistory_eq_quadratic A O] exact A.output_measurable.comp (f := fun ω : Ω => (ω,linearHistory A (spectrumOperator d) O ω q)) (measurable_id.prodMk ((measurable_linearHistory A (spectrumOperator d) q).comp (f := fun ω : Ω => (ω,O)) (measurable_id.prodMk measurable_const))) have ht := (h O).map (f := fun x => ofLp ((rotationIsometry O).symm x)) (by fun_prop) rwa [quadratic_gibbs_eigenframe,Measure.map_map (by fun_prop) hr] at ht theorem canSample_target_separation_bound {d q : ℕ} (hd : 2*q+1 ≤ d) (hqd : 4*(q+q)+1 < 2*d) (ht : blockTolerance d (q+q) ≤ 1/2) (h : CanSample d q) : (targetGaussian d).real (separationEvent d (q+q)) ≤ 11/100 := by obtain ⟨Ω,mΩ,μ,hμ,A,hA⟩ := h have hTV := hardEigenOutput_tv μ A (fun O => (hA _ (quadraticPotential_admissible O)).2) have hevent := hard_algorithm_separation μ A hd hqd ht have hm : MeasurableSet {p | hardEigenOutput A p ∈ separationEvent d (q+q)} := (separationEvent_measurable d (q+q)).preimage (measurable_hardEigenOutput A) have hlower := prob_prod_event_lower μ (rotationHaar d) hm (a := (targetGaussian d).real (separationEvent d (q+q)) - 1/10) (fun O => by have hh := hTV O _ (separationEvent_measurable d (q+q)) have hb : Measurable (fun ω : Ω => hardEigenOutput A (ω,O)) := (measurable_hardEigenOutput A).comp (f := fun ω : Ω => (ω,O)) (measurable_id.prodMk measurable_const) rw [Measure.real,Measure.map_apply hb (separationEvent_measurable d (q+q))] at hh have hh' := (abs_le.mp hh).1 change -(1/10 : ℝ) ≤ μ.real {ω | hardEigenOutput A (ω,O) ∈ separationEvent d (q+q)} - _ at hh' change _ ≤ μ.real {ω | hardEigenOutput A (ω,O) ∈ separationEvent d (q+q)} linarith) linarith end LogConcaveSampling.LowerBound namespace LogConcaveSampling.LowerBound lemma spectralDecay_bounds : (1/8 : ℝ) ≤ spectralDecay ∧ spectralDecay ≤ 1/4 := by have h := Real.sq_sqrt (show (0 : ℝ) ≤ 2 by norm_num) have h' := Real.sqrt_nonneg (2 : ℝ) dsimp [spectralDecay] constructor <;> nlinarith theorem target_mean_lower {d r : ℕ} (hn : 2*r+1 < d) : (d : ℝ) / (4 * 8^(2*r+1)) ≤ |∫ u, diagonalStatistic (fun k : Fin d => weight d (2*r+1) k) u ∂targetGaussian d| := by let n := 2*r+1 let m := ∫ u, diagonalStatistic (fun k : Fin d => weight d (2*r+1) k) u ∂targetGaussian d have hr := (lower_target_statistic hn).1 have hp : 0 < spectralDecay := spectralDecay_pos have hb := spectralDecay_bounds have hd : (0 : ℝ) ≤ d := Nat.cast_nonneg d have hs : 0 < Real.sqrt (2 : ℝ) := Real.sqrt_pos.mpr (by norm_num) have hs2 : Real.sqrt (2 : ℝ) ≤ 2 := by nlinarith [Real.sq_sqrt (show (0 : ℝ) ≤ 2 by norm_num)] have hp2 : spectralDecay^2 ≤ 1/16 := by nlinarith have hpow : spectralDecay^(2*d-n) ≤ spectralDecay^(n+2) := pow_le_pow_of_le_one hp.le spectralDecay_lt_one.le (by dsimp [n]; omega) have hrem : Real.sqrt 2 * d * spectralDecay^(2*d-n) / (1-spectralDecay) ≤ (d : ℝ) * spectralDecay^n / 4 := by calc _ ≤ 2 * d * spectralDecay^(n+2) / (1/2) := by apply div_le_div₀ (by positivity) · gcongr · norm_num · linarith _ = 4 * d * spectralDecay^n * spectralDecay^2 := by rw [pow_add]; ring _ ≤ (d : ℝ) * spectralDecay^n / 4 := by have hh := mul_le_mul_of_nonneg_left hp2 (mul_nonneg hd (pow_nonneg hp.le n)) nlinarith have hlead : (d : ℝ) * spectralDecay^n / 2 ≤ |(d : ℝ) / Real.sqrt 2 * (-spectralDecay)^n| := by rw [abs_mul,abs_div,abs_pow,abs_neg,abs_of_nonneg hd,abs_of_pos hs,abs_of_pos hp] calc _ = ((d : ℝ)/2) * spectralDecay^n := by ring _ ≤ _ := by gcongr have htri := abs_sub_abs_le_abs_sub ((d : ℝ) / Real.sqrt 2 * (-spectralDecay)^n) m rw [abs_sub_comm] at htri have hmean : (d : ℝ) * spectralDecay^n / 4 ≤ |m| := by change |m - (d : ℝ) / Real.sqrt 2 * (-spectralDecay)^n| ≤ _ at hr linarith only [hr,hrem,hlead,htri] calc _ = (d : ℝ) * (1/8)^n / 4 := by rw [div_pow]; simp only [one_pow]; field_simp; rfl _ ≤ (d : ℝ) * spectralDecay^n / 4 := by gcongr; exact hb.1 _ ≤ _ := hmean lemma lower_denominator (q : ℕ) : (4 : ℝ) * 8^(2*(q+q)+1) = 2^(12*q+5) := by rw [show (4 : ℝ) = 2^2 by norm_num, show (8 : ℝ) = 2^3 by norm_num, ← pow_mul,← pow_add] congr 1 omega lemma polynomial_threshold_le_pow (q : ℕ) : 160 * ((q+q : ℕ)+1 : ℝ)^2 + 20 ≤ (2 : ℝ)^(2*q+10) := by have hq : ((q+q : ℕ)+1 : ℝ) ≤ (2 : ℝ)^(q+1) := by have h := (Nat.lt_two_pow_self (n := q)).succ_le have hh : (q : ℝ)+1 ≤ (2 : ℝ)^q := by exact_mod_cast h push_cast rw [pow_succ] linarith have hp : (1 : ℝ) ≤ 2^(2*q+2) := one_le_pow₀ (by norm_num) have hsq : (((q+q : ℕ)+1 : ℝ)^2) ≤ (2 : ℝ)^(2*q+2) := by calc _ ≤ ((2 : ℝ)^(q+1))^2 := by gcongr _ = _ := by rw [← pow_mul]; congr 1; omega calc _ ≤ 180 * (2 : ℝ)^(2*q+2) := by nlinarith _ ≤ 256 * (2 : ℝ)^(2*q+2) := by gcongr; norm_num _ = _ := by rw [show 256 = (2:ℝ)^8 by norm_num,← pow_add]; congr 1; omega lemma sqrt_exponential_lower {d q : ℕ} (hd : 2^(64*(q+1)) ≤ d) : (2 : ℝ)^(32*(q+1)) ≤ Real.sqrt d := by apply (Real.le_sqrt (by positivity) (by positivity)).mpr have hh : (2 : ℝ)^(64*(q+1)) ≤ d := by exact_mod_cast hd convert hh using 1 rw [← pow_mul] congr 1 omega lemma target_large_mean {d q : ℕ} (hd : 2^(64*(q+1)) ≤ d) (hn : 2*(q+q)+1 < d) : separationThreshold d (q+q) + 20 * Real.sqrt d ≤ |∫ u, diagonalStatistic (fun k : Fin d => weight d (2*(q+q)+1) k) u ∂targetGaussian d| := by have hm := target_mean_lower hn rw [lower_denominator] at hm have hs := sqrt_exponential_lower hd have hcoeff := polynomial_threshold_le_pow q have hp : (2 : ℝ)^(2*q+10) * 2^(12*q+5) ≤ Real.sqrt d := by calc _ = (2 : ℝ)^(14*q+15) := by rw [← pow_add]; congr 1; omega _ ≤ 2^(32*(q+1)) := by apply pow_le_pow_right₀ (by norm_num); omega _ ≤ _ := hs have hh : (160 * (((q+q : ℕ)+1 : ℝ))^2 + 20) * 2^(12*q+5) ≤ Real.sqrt d := le_trans (mul_le_mul_of_nonneg_right hcoeff (by positivity)) hp have hh' := mul_le_mul_of_nonneg_right hh (Real.sqrt_nonneg (d : ℝ)) rw [← sq,Real.sq_sqrt (Nat.cast_nonneg d)] at hh' calc _ = (160 * (((q+q : ℕ)+1 : ℝ))^2 + 20) * Real.sqrt d := by unfold separationThreshold push_cast ring _ ≤ (d : ℝ) / 2^(12*q+5) := by apply (le_div_iff₀ (by positivity)).mpr nlinarith only [hh'] _ ≤ _ := hm end LogConcaveSampling.LowerBound namespace LogConcaveSampling.LowerBound lemma exponential_dimension_bounds {d q : ℕ} (hd : 2^(64*(q+1)) ≤ d) : 100 ≤ d ∧ 2*(q+q)+1 < d ∧ 2*q+1 ≤ d ∧ 4*(q+q)+1 < 2*d := by have hlin := (Nat.lt_two_pow_self (n := 64*(q+1))).trans_le hd have hexp : 2^64 ≤ 2^(64*(q+1)) := pow_le_pow_right₀ (by norm_num) (by omega) have h100 : 100 ≤ d := le_trans (by norm_num : 100 ≤ 2^64) (hexp.trans hd) omega lemma exponential_tolerance {d q : ℕ} (hd : 2^(64*(q+1)) ≤ d) : blockTolerance d (q+q) ≤ 1/2 := by have hpos : 0 < d := by have := (exponential_dimension_bounds hd).1; omega have hs := sqrt_exponential_lower hd have hcoeff := polynomial_threshold_le_pow q have hp : (2 : ℝ)^(2*q+10) ≤ 2^(32*(q+1)) := pow_le_pow_right₀ (by norm_num) (by omega) have hh : 80 * (((q+q : ℕ)+1 : ℝ))^2 ≤ Real.sqrt d := by have hsq := sq_nonneg (((q+q : ℕ)+1 : ℝ)) linarith unfold blockTolerance apply (div_le_iff₀ (Real.sqrt_pos.mpr (by exact_mod_cast hpos))).mpr push_cast at hh ⊢ linarith theorem target_event_large {d q : ℕ} (hd : 2^(64*(q+1)) ≤ d) : 9/10 ≤ (targetGaussian d).real (separationEvent d (q+q)) := by have hb := exponential_dimension_bounds hd have hpos : 0 < d := by omega have hdr : (0 : ℝ) < d := by exact_mod_cast hpos have hsqrt : 0 < Real.sqrt (d : ℝ) := Real.sqrt_pos.mpr hdr have hm := target_large_mean hd hb.2.1 let m := |∫ u, diagonalStatistic (fun k : Fin d => weight d (2*(q+q)+1) k) u ∂targetGaussian d| have hmargin : 20 * Real.sqrt d ≤ m - separationThreshold d (q+q) := by dsimp [m]; linarith only [hm] have hsep : separationThreshold d (q+q) < m := by linarith have htail := target_separation_tail hpos hsep have hden : 400 * (d : ℝ) ≤ (m-separationThreshold d (q+q))^2 := by calc _ = (20 * Real.sqrt (d : ℝ))^2 := by rw [mul_pow,Real.sq_sqrt hdr.le]; ring _ ≤ _ := by gcongr have h₁ : 2/(d : ℝ) ≤ 1/50 := by apply (div_le_iff₀ hdr).mpr have hh : (100 : ℝ) ≤ d := by exact_mod_cast hb.1 linarith have h₂ : 2*d/(m-separationThreshold d (q+q))^2 ≤ 1/200 := by apply (div_le_iff₀ (sq_pos_of_pos (sub_pos.mpr hsep))).mpr linarith rw [measureReal_compl (separationEvent_measurable d (q+q)),probReal_univ] at htail change 1 - (targetGaussian d).real (separationEvent d (q+q)) ≤ 2/(d : ℝ) + 2*d/(m-separationThreshold d (q+q))^2 at htail linarith end LogConcaveSampling.LowerBound namespace LogConcaveSampling.LowerBound theorem not_canSample_exponential {d q : ℕ} (hd : 2^(64*(q+1)) ≤ d) : ¬ CanSample d q := by intro h have hb := exponential_dimension_bounds hd have h₁ := canSample_target_separation_bound hb.2.2.1 hb.2.2.2 (exponential_tolerance hd) h have h₂ := target_event_large hd linarith lemma exponential_of_small_log {d q : ℕ} (hd : 2^128 ≤ d) (hq : (q : ℝ) < (128 * Real.log 2)⁻¹ * Real.log d) : 2^(64*(q+1)) ≤ d := by have hl : 0 < Real.log (2 : ℝ) := Real.log_pos (by norm_num) have hdp : 0 < d := lt_of_lt_of_le (by positivity) hd have hdr : (0 : ℝ) < d := by exact_mod_cast hdp have hd' : (2 : ℝ)^128 ≤ d := by exact_mod_cast hd have hlog : 128 * Real.log (2 : ℝ) ≤ Real.log d := by have hh := Real.log_le_log (by positivity) hd' simpa only [Real.log_pow, Nat.cast_ofNat] using hh have hq' : 128 * Real.log (2 : ℝ) * q < Real.log d := by have hh := mul_lt_mul_of_pos_left hq (show 0 < 128 * Real.log (2 : ℝ) by positivity) rwa [← mul_assoc,mul_inv_cancel₀ (by positivity),one_mul] at hh have hx : Real.log ((2 : ℝ)^(64*(q+1))) ≤ Real.log d := by rw [Real.log_pow] push_cast nlinarith only [hlog,hq'] have hexp := (Real.log_le_log_iff (by positivity) hdr).mp hx exact_mod_cast hexp theorem source_logarithmic_lower : ∃ c : ℝ, 0 < c ∧ ∃ d₀ : ℕ, 2 ≤ d₀ ∧ ∀ d : ℕ, d₀ ≤ d → ENNReal.ofReal (c * Real.log (d : ℝ)) ≤ (queryComplexity d).toENNReal := by refine ⟨(128 * Real.log 2)⁻¹, by positivity, 2^128, by norm_num, ?_⟩ intro d hd have hfeasible : ∀ q : ℕ, CanSample d q → (128 * Real.log 2)⁻¹ * Real.log d ≤ q := by intro q hq by_contra hh exact not_canSample_exponential (exponential_of_small_log hd (lt_of_not_ge hh)) hq unfold queryComplexity rw [sInf_image] simp only [ENat.toENNReal_iInf] apply le_iInf intro q apply le_iInf intro hq simpa only [ENat.toENNReal_coe,ENNReal.ofReal_natCast] using ENNReal.ofReal_le_ofReal (hfeasible q hq) end LogConcaveSampling.LowerBound end LowerProof end end end end OAI