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arXiv 2107.09423cs.CCcs.LO
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组合间隙定理与Promise CSP约简

Combinatorial Gap Theorem and Reductions between Promise CSPs

Libor Barto, Marcin Kozik

摘要:

CSP实例的值通常被定义为可以同时满足的约束的一部分。我们提出了另一种定义的一个实例的值,并表明,纯粹的组合的原因,一个不可解的实例的值是有界远离一个,我们称之为这一事实的间隙定理。我们发现,间隙定理意味着NP-硬度的间隙版本的分层标签覆盖问题。从PCP定理可以得到同样的结果,但是我们的简化的完整的、自包含的证明是相当短的,并且结果仍然可以为许多问题提供无PCP的NP-困难证明。我们的推理的简单性也表明,较弱版本的独特游戏类型的结构,例如,d到1猜想,可能是可访问的,并作为一个中间步骤,证明这些猜想在他们的全部力量。作为第二个主要应用,我们给出了一个充分条件,在此条件下,一个固定模板的承诺约束满足问题(PCSP)减少到另一个PCSP。归约的正确性取决于间隙定理,但归约本身非常简单。因此,我们得到每个CSP可以规范地减少到大多数已知的NP-困难的PCSP,如近似超图着色问题。

英文摘要:

A value of a CSP instance is typically defined as a fraction of constraints that can be simultaneously met. We propose an alternative definition of a value of an instance and show that, for purely combinatorial reasons, a value of an unsolvable instance is bounded away from one; we call this fact a gap theorem. We show that the gap theorem implies NP-hardness of a gap version of the Layered Label Cover Problem. The same result can be derived from the PCP Theorem, but a full, self-contained proof of our reduction is quite short and the result can still provide PCP-free NP-hardness proofs for numerous problems. The simplicity of our reasoning also suggests that weaker versions of Unique-Games-type conjectures, e.g., the d-to-1 conjecture, might be accessible and serve as an intermediate step for proving these conjectures in their full strength. As the second, main application we provide a sufficient condition under which a fixed template Promise Constraint Satisfaction Problem (PCSP) reduces to another PCSP. The correctness of the reduction hinges on the gap theorem, but the reduction itself is very simple. As a consequence, we obtain that every CSP can be canonically reduced to most of the known NP-hard PCSPs, such as the approximate hypergraph coloring problem.

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