We show that a parallel repetition of any two-prover one-round proof system (MIP(2,1)) decreases the probability of error at an exponential rate. No constructive bound was previously known. The constant in the exponent (in our analysis) depends only on the original probability of error and on the total number of possible answers of the two provers. The dependency on the total number of possible answers is logarithmic, which was recently proved to be almost the best possible [U. Feige and O. Verbitsky, Proc.11th Annual IEEE Conference on Computational Complexity, IEEE Computer Society Press, Los Alamitos, CA, 1996, pp. 70--76].
We show that a parallel repetition of any two-prover one-round proof system (MIP(2,1)) decreases the probability of error at an exponential rate. No constructive bound was previously known. The constant in the exponent (in our analysis) depends only on the original probability of error and on the total number of possible answers of the two provers. The dependency on the total number of possible answers is logarithmic, which was recently proved to be almost the best possible [U. Feige and O. Verbitsky, Proc.11th Annual IEEE Conference on Computational Complexity, IEEE Computer Society Press, Los Alamitos, CA, 1996, pp. 70--76].
We show that a parallel repetition of two-prover oneround proof systems (MIP(2, 1)) reduces the probability of error at an exponential rate.This settles a standing open problem, sometimes called 'The Parallel Repetition Conjecture".Previously, no constructive bound was known.The constant we have in the exponent is logarithmic in the total number of possible answers of the two provers.This is significantly better than what was previously known even in the much stmpler case where the questions to the two provers are chosen independently at random.In the most frequently used cases, our constant is just a global constant.
We show that no fixed number of parallel repetitions suffices in order to reduce the error in two-prover one-round proof systems from one constant to another. Our results imply that the recent bounds proven by Ran Raz, showing that the number of rounds that suffice is inversely proportional to the answer length, are nearly best possible. Our proof technique builds upon an idea of Oleg Verbitsky. We use this opportunity to survey the known results on parallel repetition, and to present the proofs of some previously claimed theorems. 1 Introduction A two prover one round proof system [8], MIP(2,1), is a protocol by which two provers jointly try to convince a computationally limited probabilistic verifier that a common input belongs to a prespecified language. The verifier selects a pair of questions at random. Each prover sees only one of the two questions, and sends back an answer. The verifier evaluates a predicate on the common input and the two questions and answers, and accepts or ...
The Parallel Repetition Theorem says that n-fold parallel execution of a two-prover one-round interactive proof system reduces the error probability exponentially in n. The bound on the error probability of the parallelized system depends on the error probability and the answer size of the single proof system. It is still unknown whether the theorem holds true with a bound depending only on the query size. This kind of a bound may be preferable whenever the query size is considerably smaller than the answer size, what really happens in some cryptographic protocols. Such a bound is only known in the case that queries to the provers are independent. The present paper extends this result to some cases of strong correlation between queries. In particular, a query-based variant of the Parallel Repetition Theorem is proven when the graph of dependence between queries to the provers is a tree and, in a bit weaker form, when this graph is a cycle.
The parallel repetition theorem states that for any two provers one round game with value at most 1 - ∊ (for ∊ < 1/2), the value of the game repeated n times in parallel is at most (1 - ∊3)Ω(n/ log s) where s is the size of the answers set [9, 12]. It is not known how the value of the game decreases when there are three or more players. In this paper we address the problem of the error decrease of parallel repetition game for k-provers where k > 2. We consider a special case of the No-Signaling model and show that the error of the parallel repetition of k-provers one round game, for k > 2, in this model, decreases exponentially depending only on the error of the original game and on the number of repetitions. There were no prior results for k-provers parallel repetition for k > 2 in any model.
The parallel repetition theorem states that, for any two-prover game with value 1−ϵ (for, say, ϵ≤1/2), the value of the game repeated in parallel n times is at most (1−ϵc)Ω(n/s), where s is the answer's length (of the original game) and c is a universal constant [R. Raz, SIAM J. Comput., 27 (1998), pp. 763–803]. Several researchers asked whether this bound could be improved to (1−ϵ)Ω(n/s); this question is usually referred to as the strong parallel repetition problem. We show that the answer to this question is negative. More precisely, we consider the odd cycle game of size m, a two-prover game with value 1−1/2m. We show that the value of the odd cycle game repeated in parallel n times is at least 1−(1/m)⋅O(n). This implies that, for large enough n (say, n≥Ω(m2)), the value of the odd cycle game repeated in parallel n times is at least (1−1/4m2)O(n). Thus the following hold. 1. For parallel repetition of general games, the bounds of (1−ϵc)Ω(n/s) given in [R. Raz, SIAM J. Comput., 27 (1998), pp. 763–803; T. Holenstein, in Proceedings of STOC 2002, ACM, New York, 2002, pp. 767–775] are of the right form, up to determining the exact value of the constant c≥2. 2. For parallel repetition of XOR games, unique games, and projection games, the bounds of (1−ϵ2)Ω(n) given in [U. Feige, G. Kindler, and R. O'Donnell, in Proceedings of CCC 2007, IEEE Computer Society, Washington, DC, 2007, pp. 179–192] (for XOR games) and in [A. Rao, in Proceedings of STOC 2008, ACM, New York, 2008, pp. 1–10] (for unique and projection games) are tight. 3. For parallel repetition of the odd cycle game, the bound of 1−(1/m)⋅Ω~(n) given in [U. Feige, G. Kindler, and R. O'Donnell, in Proceedings of CCC 2007, IEEE Computer Society, Washington, DC, 2007, pp. 179–192] is almost tight. A major motivation for the recent interest in the strong parallel repetition problem is that a strong parallel repetition theorem would have implied that the unique game conjecture is equivalent to the NP hardness of distinguishing between instances of Max-Cut that are at least 1−ϵ2 satisfiable from instances that are at most 1−(2/π)⋅ϵ satisfiable. Our results suggest that this cannot be proved just by improving the known bounds on parallel repetition.