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Long codes and the complete nonadaptive test

Lax253009.LongCode · concepts/Lax253009/LongCode.lean · lax-253009

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    Natural Language Statement

    Definition

    The long code of a Boolean word x∈{0,1}wx\in\{0,1\}^w is its evaluation table: at the coordinate indexed by a Boolean function gg, it stores g(x)g(x). A purported long code is an arbitrary table of the same shape.

    The complete nonadaptive test chooses ss Boolean functions fif_i. After reading their table entries ai=A(fi)a_i=A(f_i), it checks A(B∘f)=B(a)A(B\circ f)=B(a) for every Boolean predicate BB of ss bits. Only the initial ss answers are free: they determine every checked answer. These are the long code of Section 3 and the CNA test of Section 4 of Håstad's paper, written with Boolean values instead of signs.

    With a side condition hh, the test additionally checks that each queried answer is unchanged when its function is replaced by any function agreeing with it on the set where hh is true. This is step (3) of the test preceding Theorem 4.17. The definitions below describe a fixed choice of the functions; the probabilistic soundness estimates are separate statements.

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    1import Mathlib.Data.Fintype.Pi
    2
    3/-!
    4---
    5title: Long codes and the complete nonadaptive test
    6type: definition
    7---
    8The long code of a Boolean word x∈{0,1}wx\in\{0,1\}^w is its evaluation table:
    9at the coordinate indexed by a Boolean function gg, it stores g(x)g(x).
    10A purported long code is an arbitrary table of the same shape.
    11
    12The complete nonadaptive test chooses ss Boolean functions fif_i. After
    13reading their table entries ai=A(fi)a_i=A(f_i), it checks
    14A(B∘f)=B(a)A(B\circ f)=B(a) for every Boolean predicate BB of ss bits.
    15Only the initial ss answers are free: they determine every checked answer.
    16These are the long code of Section 3 and the CNA test of Section 4 of
    17Håstad's paper, written with Boolean values instead of signs.
    18
    19With a side condition hh, the test additionally checks that each queried
    20answer is unchanged when its function is replaced by any function agreeing
    21with it on the set where hh is true. This is step (3) of the test preceding
    22Theorem 4.17. The definitions below describe a fixed choice of the functions;
    23the probabilistic soundness estimates are separate statements.
    24-/
    25
    26namespace Lax253009.LongCode
    27
    28abbrev Word (w : ℕ) := Fin w → Bool
    29abbrev Coordinate (w : ℕ) := Word w → Bool
    30abbrev Table (w : ℕ) := Coordinate w → Bool
    31
    32def evaluation {w : ℕ} (x : Word w) : Table w := fun g ↦ g x
    33
    34def compose {w s : ℕ} (f : Fin s → Coordinate w) (B : Word s → Bool) : Coordinate w :=
    35 fun x ↦ B (fun i ↦ f i x)
    36
    37def Accepts {w s : ℕ} (A : Table w) (f : Fin s → Coordinate w) : Prop :=
    38 ∀ B : Word s → Bool, A (compose f B) = B (fun i ↦ A (f i))
    39
    40def LooksLike {w s : ℕ} (A : Table w) (f : Fin s → Coordinate w) (x : Word w) : Prop :=
    41 (∀ i, A (f i) = f i x) ∧
    42 (∀ B : Word s → Bool, A (compose f B) = compose f B x)
    43
    44def AgreesOn {w : ℕ} (h g g' : Coordinate w) : Prop :=
    45 ∀ x, h x = true → g x = g' x
    46
    47def Queried {w s : ℕ} (f : Fin s → Coordinate w) (g : Coordinate w) : Prop :=
    48 (∃ i, g = f i) ∨ ∃ B : Word s → Bool, g = compose f B
    49
    50def AcceptsWithCondition {w s : ℕ} (A : Table w) (f : Fin s → Coordinate w)
    51 (h : Coordinate w) : Prop :=
    52 Accepts A f ∧ ∀ g, Queried f g → ∀ g', AgreesOn h g g' → A g = A g'
    53
    54end Lax253009.LongCode
    55

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