On Parallel Repetition of PCPs

Authors Alessandro Chiesa, Ziyi Guan, Burcu Yıldız

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Alessandro Chiesa
  • EPFL, Lausanne, Switzerland
Ziyi Guan
  • EPFL, Lausanne, Switzerland
Burcu Yıldız
  • EPFL, Lausanne, Switzerland


The authors thank Ngoc Khanh Nguyen, Guy Weissenberg, Eylon Yogev, and Mingnan Zhao for valuable feedback and comments on earlier drafts of this paper. The authors thank anonymous reviewers of ITCS for valuable comments and suggestions that have improved this paper.

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Alessandro Chiesa, Ziyi Guan, and Burcu Yıldız. On Parallel Repetition of PCPs. In 15th Innovations in Theoretical Computer Science Conference (ITCS 2024). Leibniz International Proceedings in Informatics (LIPIcs), Volume 287, pp. 34:1-34:14, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2024)


Parallel repetition refers to a set of valuable techniques used to reduce soundness error of probabilistic proofs while saving on certain efficiency measures. Parallel repetition has been studied for interactive proofs (IPs) and multi-prover interactive proofs (MIPs). In this paper we initiate the study of parallel repetition for probabilistically checkable proofs (PCPs). We show that, perhaps surprisingly, parallel repetition of a PCP can increase soundness error, in fact bringing the soundness error to one as the number of repetitions tends to infinity. This "failure" of parallel repetition is common: we find that it occurs for a wide class of natural PCPs for NP-complete languages. We explain this unexpected phenomenon by providing a characterization result: the parallel repetition of a PCP brings the soundness error to zero if and only if a certain "MIP projection" of the PCP has soundness error strictly less than one. We show that our characterization is tight via a suitable example. Moreover, for those cases where parallel repetition of a PCP does bring the soundness error to zero, the aforementioned connection to MIPs offers preliminary results on the rate of decay of the soundness error. Finally, we propose a simple variant of parallel repetition, called consistent parallel repetition (CPR), which has the same randomness complexity and query complexity as the plain variant of parallel repetition. We show that CPR brings the soundness error to zero for every PCP (with non-trivial soundness error). In fact, we show that CPR decreases the soundness error at an exponential rate in the repetition parameter.

Subject Classification

ACM Subject Classification
  • Theory of computation → Computational complexity and cryptography
  • probabilistically checkable proofs
  • parallel repetition


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