# Imperfect Gaps in Gap-ETH and PCPs

## TL;DR

This paper explores transforming PCPs with imperfect completeness into perfect completeness, introduces a robust circuit construction, and relates the complexities of approximating 3SAT with and without perfect completeness.

## Contribution

It provides a new method for converting imperfect to perfect completeness in PCPs and establishes equivalences in the complexity of approximating 3SAT with different completeness levels.

## Key findings

- Conversion of imperfect to perfect PCPs with linear size and minimal query loss.
- Equivalence of Gap-ETH with and without perfect completeness.
- Fine-grained complexity relationship between approximating MAX 3SAT with perfect and imperfect completeness.

## Abstract

We study the role of perfect completeness in probabilistically checkable proof systems (PCPs) and give a new way to transform a PCP with imperfect completeness to a PCP with perfect completeness when the initial gap is a constant. In particular, we show that $\text{PCP}_{c,s}[r,q] \subseteq \text{PCP}_{1,1-\Omega(1)}[r+O(1),q+O(r)]$, for $c-s=\Omega(1)$. This implies that one can convert imperfect completeness to perfect in linear-sized PCPs for $NTIME[O(n)]$ with a $O(\log n)$ additive loss in the query complexity $q$. We show our result by constructing a "robust circuit" using threshold gates. These results are a gap amplification procedure for PCPs (when completeness is imperfect), analogous to questions studied in parallel repetition and pseudorandomness. We also investigate the time complexity of approximating perfectly satisfiable instances of 3SAT versus those with imperfect completeness. We show that the Gap-ETH conjecture without perfect completeness is equivalent to Gap-ETH with perfect completeness, i.e. we show that Gap-3SAT, where the gap is not around 1, has a subexponential algorithm, if and only if, Gap-3SAT with perfect completeness has subexponential algorithms. We also relate the time complexities of these two problems in a more fine-grained way, to show that $T_2(n) \leq T_1(n(\log\log n)^{O(1)})$, where $T_1(n),T_2(n)$ denote the randomized time-complexity of approximating MAX 3SAT with perfect and imperfect completeness, respectively.