Low Acceptance Agreement Tests via Bounded-Degree Symplectic HDXs
CoRR(2024)
摘要
We solve the derandomized direct product testing question in the low
acceptance regime, by constructing new high dimensional expanders that have no
small connected covers. We show that our complexes have swap cocycle expansion,
which allows us to deduce the agreement theorem by relying on previous work.
Derandomized direct product testing, also known as agreement testing, is the
following problem. Let X be a family of k-element subsets of [n] and let
{f_s:s→Σ}_s∈ X be an ensemble of local functions, each defined
over a subset s⊂ [n]. Suppose that we run the following so-called
agreement test: choose a random pair of sets s_1,s_2∈ X that intersect on
√(k) elements, and accept if f_s_1,f_s_2 agree on the elements in
s_1∩ s_2. We denote the success probability of this test by
Agr({f_s}). Given that Agr({f_s})=ϵ>0, is there a global
function G:[n]→Σ such that f_s = G|_s for a non-negligible fraction
of s∈ X ?
We construct a family X of k-subsets of [n] such that |X| = O(n) and such
that it satisfies the low acceptance agreement theorem. Namely,
Agr ({f_s}) > ϵ ⟶ there is a function
G:[n]→Σ such that _s[f_s0.99≈ G|_s]≥
poly(ϵ).
A key idea is to replace the well-studied LSV complexes by symplectic high
dimensional expanders (HDXs). The family X is just the k-faces of the new
symplectic HDXs. The later serve our needs better since their fundamental group
satisfies the congruence subgroup property, which implies that they lack small
covers.
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