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ArkLib.Data.CodingTheory.ProximityGap

Definitions and Theorems about Proximity Gaps #

We define the proximity gap properties of linear codes over finite fields.

[BCIKS20] refers to the paper "Proximity Gaps for Reed-Solomon Codes" by Eli Ben-Sasson, Dan Carmon, Yuval Ishai, Swastik Kopparty, and Shubhangi Saraf.

Main Definitions #

def ProximityGap.proximityMeasure {n : Type} [Fintype n] [DecidableEq n] {F : Type} [Field F] [Fintype F] [DecidableEq F] (C : Submodule F (nF)) [DecidablePred fun (x : nF) => x C] (u v : nF) (d : ) :

The proximity measure of two vectors u and v from a code C at distance d is the number of vectors at distance at most d from the linear combination of u and v with coefficients r in F.

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      def ProximityGap.proximityGap {n : Type} [Fintype n] [DecidableEq n] {F : Type} [Field F] [Fintype F] [DecidableEq F] (C : Submodule F (nF)) [DecidablePred fun (x : nF) => x C] (d bound : ) :

      A code C exhibits proximity gap at distance d and cardinality bound bound if for every pair of vectors u and v, whenever the proximity measure for C u v d is greater than bound, then the distance of [u | v] from the interleaved code C ^⊗ 2 is at most d.

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          def ProximityGap.correlatedAgreement {n : Type} [Fintype n] {F : Type} (C : Set (nF)) (δ : NNReal) {k : } (W : Fin knF) :

          A code C exhibits δ-correlated agreement with respect to a tuple of vectors W_1, ..., W_k if there exists a set S of coordinates such that the size of S is at least (1 - δ) * |n|, and there exists a tuple of codewords v_1, ..., v_k such that v_i agrees with W_i on S for all i.

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              noncomputable def ProximityGap.generalProximityGap {ι : Type} [Fintype ι] [Nonempty ι] {α : Type} [DecidableEq α] [Nonempty α] (P : Finset (ια)) (C : Set (Finset (ια))) (δ ε : NNReal) :

              Definition 1.1 in [BCIKS20].

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                  noncomputable def ProximityGap.errorBound {ι : Type} [Fintype ι] {F : Type} [Field F] [Fintype F] (δ : NNReal) (deg : ) (domain : ι F) :

                  The error bound ε in the pair of proximity and error parameters (δ,ε) for Reed-Solomon codes defined up to the Johnson bound. More precisely, let ρ be the rate of the Reed-Solomon code. Then for δ ∈ (0, 1 - √ρ), we define the relevant error parameter ε for the unique decoding bound, i.e. δ ∈ [0, (1-ρ)/2] and Johnson bound, i.e. δ ∈ [(1-ρ)/2 , 1 - √ρ]. Otherwise, we set ε = 0.

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                      theorem ProximityGap.proximity_gap_RSCodes {ι : Type} [Fintype ι] [Nonempty ι] {F : Type} [Field F] [Fintype F] [DecidableEq F] {k t : } [NeZero k] [NeZero t] {deg : } {domain : ι F} (C : Fin tFin kιF) {δ : NNReal} ( : δ 1 - ReedSolomonCode.sqrtRate deg domain) :

                      Theorem 1.2 (Proximity Gaps for Reed-Solomon Codes) in [BCIKS20].

                      theorem ProximityGap.correlatedAgreement_lines {ι : Type} [Fintype ι] [Nonempty ι] {F : Type} [Field F] [Fintype F] [DecidableEq F] {u : Fin 2ιF} {deg : } {domain : ι F} {δ : NNReal} ( : δ 1 - ReedSolomonCode.sqrtRate deg domain) (hproximity : (do let zPMF.uniformOfFintype F pure (δᵣ(u 0 + z u 1, (ReedSolomon.code domain deg)) δ)) True > (errorBound δ deg domain)) :
                      correlatedAgreement (↑(ReedSolomon.code domain deg)) δ u

                      Theorem 1.4 (Main Theorem — Correlated agreement over lines) in [BCIKS20].

                      theorem ProximityGap.correlatedAgreement_affine_curves {ι : Type} [Fintype ι] [Nonempty ι] {F : Type} [Field F] [Fintype F] [DecidableEq F] [DecidableEq ι] {k : } {u : Fin kιF} {deg : } {domain : ι F} {δ : NNReal} ( : δ 1 - ReedSolomonCode.sqrtRate deg domain) (hproximity : (do let yPMF.uniformOfFintype { x : ιF // x Curve.parametrisedCurveFinite u } pure (δᵣ(y, (ReedSolomon.code domain deg)) δ)) True > k * (errorBound δ deg domain)) :
                      correlatedAgreement (↑(ReedSolomon.code domain deg)) δ u

                      Theorem 1.5 (Correlated agreement for low-degree parameterised curves) in [BCIKS20].

                      theorem ProximityGap.correlatedAgreement_affine_spaces {ι : Type} [Fintype ι] [Nonempty ι] {F : Type} [Field F] [Fintype F] [DecidableEq F] {k : } [NeZero k] {u : Fin (k + 1)ιF} {deg : } {domain : ι F} {δ : NNReal} ( : δ 1 - ReedSolomonCode.sqrtRate deg domain) (hproximity : (do let yPMF.uniformOfFintype ↥(u 0 +ᵥ affineSpan F (Finset.image (Fin.tail u) Finset.univ)) pure (δᵣ(y, (ReedSolomon.code domain deg)) δ)) True > (errorBound δ deg domain)) :
                      correlatedAgreement (↑(ReedSolomon.code domain deg)) δ u

                      Theorem 1.6 (Correlated agreement over affine spaces) in [BCIKS20].